GB2502202A - Drying building construction panels using gas pressure differential - Google Patents

Abstract

A method of forming a construction panel, comprising: forming a frame structure of the panel; filling the frame structure with a filling material; and drying the filling material by creating a gas pressure difference across the panel so as to cause gas to flow through the filling material. This reduces drying time of the panel, thus improving the rate of manufacture. This also extends to larger structures such as whole buildings. Also, a method of forming a construction panel comprising: forming a frame structure of the panel; casting slabs of a filler material; drying said slabs of filler material; and placing said slabs in said frame structure. Drying smaller slabs separately is faster than drying a single large panel. Additionally, a method of forming a construction panel comprising: forming a frame structure of the panel; attaching a perforated board to one face of the frame structure; filling the frame structure with filler material; and drying said filler material. The perforated board facilitates air flow through the material. Also, a method of forming a filler material comprising: mixing a vegetable based aggregate with a mineral binder in a continuous action mixer so as to create particles of said vegetable based aggregate coated with said mineral binder. Such filler material can be generated quickly andused in loose fill applications.

Description

Method of drying panels and other building structures The invention relates to the drying of panels or other building structures in the construction industry.

Construction panels need structural strength and need to be capable of supporting the weight of panels above them as well as the weight of intermediate floor structures and/or roots. Some panels are designed for structural capabilities like these, whilst others are designed as cladding for an existing building structure or building framework. Cladding panels require less structural strength than structural panels.

Construction panels and cladding panels are generally rectangular units which are connected together along their edges so as to form a larger surface or wall. To position the panels in the correct locations on site, they need to be lifted and manoeuvred into place, typically by a crane. The panels therefore also need to have the structural stability required for such lifting.

One particular type of panel uses a rigid frame structure (formed e.g. from timber) which is then filled with a thermally insulating material which provides the panel with the desired thermal properties as well as adding to the structural strength and stability of the panel.

Environmentally friendly thermal insulation materials have typically been applied to building structures in a wet application process, e.g. by casting or spraying on site.

This has often been the case for example for Tradical® Hemcrete® which is a mixture of lime binder and hemp shiv among other ingredients. The material provides high levels of insulation (low U-values) and also has a low carbon footprint. Additionally, the hemp is a sustainable material that locks up carbon and the product is recyclable. Using Tradical® Hemcrete®, it is possible to build houses conforming to the UK Code for Sustainable Homes, levels 4, 5 and 6.

Buildings constructed on site typically involve first building a timber frame with interior walls, then spraying or filling this structure with Tradical® Hemcrete® and then applying an exterior render to finish. This wet application process has certain disadvantages as follows. Working in the cold, especially in winter can lead to longer setting and drying times which can be inefficient, As the mixture dries it can shrink. As the mixture shrinks, in some extreme cases cracks may appear in the walls. These then need to be repaired before the render is applied. If the render is applied before the underlying mixture is dry, it can extend overall drying times and increase the risk of staining from tannin in the hemp. The thermal performance of the product is not maximised until the mixture is fully dry, i.e. until it has reached an equilibrium moisture content (typically around 5-10%). Drying to this level can take as much as 2-3 years in some cases.

To address these problems, construction panels were developed which could be castoff site, dried faster (e.g. by circulating warm and/or dry air) around the panels) in a drying chamber and then transported to site for construction. With such panels, the drying time of the filler material can be decreased to a few days or weeks. Such panels and the drying process are described in International Patent Application Number FCT/GB2O1 2/050426.

As described in PCT/0B2012/050428, the chemicals in many binders are dependent on water for their reactions, so force drying the panel too early can reduce the strength of the panel by reducing the set of the chemicals. According to the drying methods described therein, heat andlor moving dry air is therefore preferably not applied until after a certain degree of set has been attained. If a sufficient degree of set is not attained, then the filler material is liable to lose structural integrity and may crumble, particularly under the stresses involved in moving the panel from the warehouse to the construction site and subsequently moving the panel into place in the construction.

There is however still a need for further reductions in drying times to decrease the production time of a panel and to increase the rate at which panels can be produced. Equally, in cases where on site casting methods are to be used, there still exists a need for decreasing the drying time so as to decrease the time taken for the materials to attain the desired strength and thermal properties.

According to a first aspect of the invention there s provided a method of forming a construction panel comprising: forming a frame structure of the panel; casting slabs of a filler material; drying said slabs of filler material; and placing said slabs in said frame structure.

Forming slabs of filler material separately from the panel allows each slab to be formed with a smaller size and thus with a greater surface to volume ratio. This allows faster drying of the slabs and thus faster drying of a sufficient volume of filler material to fill a panel.

In preferred embodiments, the filler material may be a solidifiable liquid or plastic state material, preferably a mixture comprising lime and hemp, more preferably bio-composite material such as Hemcrete®. In preferred embodiments the filler material is a mixture comprising organic aggregate(s), e.g. vegetable based aggregate(s) and mineral or organic binders e.g. hemp (or other cellulosic material), cement and lime. The mixture may in preferred embodiments be Tradical® Hemcrete®.

The filler material is typically a wet mixture which sets as it dries. As the material dries and sets, it gains structural strength and integrity. The filler material mixture preferably also contains a material with good thermal insulating properties (such as organic aggregates as mentioned above). As the mixture dries, the thermal insulating properties of the slab (and therefore of the final panel) also increase.

Preferably, the frame structure has one or more parallel studs separating the panel into a plurality of elongate channels, and the slabs are cast in a cuboid shape, having a first dimension, a second dimension and a third dimension; wherein said first dimension is equal to the thickness of the frame structure and wherein said second and third dimensions are common inter-stud distances. Studs are used in many building constructions and, although the spacing between studs may be selected from a wide range, there are some spacings which have become fairly common on standard. In preferred embodiments, the construction panels will use these standard stud separations for compatibility with other construction products.

* Two common stud centres (i.e. the distance between the centres of two adjacent * studs) are 400 mm and 600 mm.

By forming the slabs of filler material with dimensions which match common stud centres, the slabs will neatly fit between the studs of the frame structure. Within one elongate channel (i.e. between adjacent studs), several slabs may be stacked end to end so as to fill the channel. If the channel length is not a multiple of the length of the slabs, then the final slab in a channel will need to be cut to size.

Similarly, if a panel has a width which is not a multiple of the standard stud centres, then a thinner channel will have to be formed at one side. All slabs within that thinner channel will need to be cut to size.

In one particularly preferred embodiment, the slabs have a second dimension of about 400 mm and a third dimension of about 600 mm. These slabs are suitable for fitting between the two common stud centres mentioned above. The slabs are placed in the panel with the dimension which matches the inter-stud distance perpendicular to the studs, while the other dimension is laid parallel to the studs, i.e. in the stacking direction.

According to a second aspect of the invention there is provided a method of forming a construction panel comprising: forming a frame structure of the panel; attaching a perforated board to one face of the frame structure; filling the frame structure with filler material; and drying said filler material.

In some preferred embodiments, a second perforated board may be attached to a second face of the frame structure for extra structural strength. The second board may be attached before drying, e.g. if the panel is to be moved and/or structural strength is needed, but the second board may also be applied after the drying step.

The use of boards on the faces of the panel hinders the contact of the filler material with air during the drying process. However, by using perforated boards, i.e. boards with a plurality of holes (perforations) therethrough, air can still contact the mixture and evaporation of excess moisture from the panel can still take place. The boards provide containment for the filler material so that the panel can be manoeuvred before the filler material has set. This facilitates the workflow within the factory as the drying can take place away from the casting site. It has been found that the use of perforated boards allows sufficient air contact with the filler material that the panels can still be dried in a short time.

The size of perforations will depend on the choice of filler material being used in the panel. The perforations should be small enough that the filler material does not exude through the perforations and thus remains contained within the panel. The perforations should however, be as large as possible to provide the greatest surface contact between the filler material and the air. In some preferred embodiments, the perforated boards contain perforations with a diameter of greater than 5 mm. Preferably the diameter is less than 10 mm, and most preferably the diameter is about 7mm.

The perforations may be arranged in any pattern, but in some preferred embodiments, the perforated boards contain perforations in an array with the distance between adjacent perforations being in the range 20mm to 50 mm, most preferably about 25 mm.

The array may be an irregular array, with an average separation as above, but preferably, the array is regular. The array may for example be a square array or a triangular array.

Although the drying step may simply be a resting step, preferably the drying step comprises blowing air around the panel so as to increase the evaporation rate from the filler material, thus decreasing the drying time. The blown air may be heated and/or dried (to have a reduced relative humidity) so as to increase the evaporation rate from the tiller material.

As described above, the filler material typically includes a binder material to bind aggregate together. There are a number of different binders which may be used, depending on the desired properties. For example, Tradical® HB is a binder which is proven to work will with hemp aggregate, lime-cement mixes are common inexpensive binders, lime-pozzolan mixes have a low carbon impact, and calcium hydroxide Ca(OH)2 is a simple and inexpensive binder. Other special cements may also be used with other properties such as fast setting compounds.

In some particularly preferred embodiments, the binder material used in the filler material includes Calcium Oxide (also known as quicklime). Calcium Oxide reacts with water when it is added to the mixture to produce Calcium Hydroxide. This is an exothermic reaction. Calcium Oxide has generally been avoided previously due to this exothermic reaction being strongly exothermic and poorly understood.

However, there are two beneficial effects from this reaction. Firstly, the reaction consumes the water, thus leading to a faster set, with less evaporation needed to reach the required equilibrium moisture content. Secondly, the heat produced by the exothermic reaction increases the evaporation rate of any remaining water, again decreasing the drying time.

According to a third aspect of the invention, there is provided a method of forming a filler material comprising: mixing a vegetable based aggregate with a mineral binder in a continuous action mixer so as to create particles of said vegetable based aggregate coated with said mineral binder.

A continuous action mixer is any mixer which operates continuously during the drying process so as to keep the particles of aggregate moving and preventing them from binding to each other. Preferably, the mixer is a rotary mixer, i.e. one having a rotary action, e.g. a drum mixer, a pan mixer or an auger mixer.

The use such particles of vegetable based aggregate coated with mineral binder can be used as a loose fill material in building structures, e.g. in panels as described above. Vegetable based aggregates on their own have not been usable as a loose fill material because they do not meet certain buildings requirements such as fire resistance, fungal resistance and pest resistance. When the vegetable based aggregate is combined together with mineral binder in a wet mix as described above, these requirements are met. However, according to the third aspect of the invention, it has been found that vegetable aggregate can be coated with the mineral binder and that the particles so formed have the required properties for use in the construction industry. Therefore such particles can advantageously be used as a loose fill insulation material.

Preferably the particles of aggregate are in the range of 0 to 25 mm diameter.

As described above, the mineral binder is typically mixed with water and the vegetable aggregate to form a wet mix as before, but it is then mixed under constant movement so that, as it dries, the particles of aggregate do not stick together into a large mass. It will be appreciated that the large surface to volume ratio of such particles means that drying can be accomplished extremely quickly.

Therefore the loose fill material can be produced rapidly and is ready for immediate use.

Heat may be provided in the mixing step to increase the evaporation and thus decrease the drying time further.

The invention extends to a method of forming a construction panel, comprising: forming a frame structure of the panel; attaching a board to one face of the frame structure; filling the frame structure with particles of vegetable based aggregate coated with mineral binder; and attaching a second board to a second face of the frame structure.

It will be appreciated that the order of these steps is flexible. For example, the boards can both be attached to the frame work prior to filling provided that an opening remains for pouring in the particles. This facilitates filling of a panel in a vertical orientation. Alternatively, the panel may be laid flat with one board attached on the lower side. The second (upper) board may then be attached after the panel has been filled from the upper side.

The coated particles of filler material are preferably formed according to the method described above.

According to a fourth aspect, there is provided a method of drying a structure, wherein the structure comprises a framework and a filler material, the method comprising: drying the filler material by creating a gas pressure difference across the filler material so as to cause gas to flow through the filler material.

The framework may be the frame of a construction panel (of either the structural or cladding type) or it may be a framework of a building constructed on site. Filler material can be applied to the framework in the normal fashion (e.g. by casting, or by spraying). Applying a pressure difference across the filler material causes gas to flow through the filler material in the direction of the lower pressure region.

Moisture within the filler material is carried with the gas and thus is carried through and out of the tiller material.

As the moisture is carried in one direction, the filler material begins to dry first at the high pressure side, The boundary between dry filler material and wet filler material (i.e. a dry front) gradually moves through the tiller material from the high pressure side to the low pressure side. When the pressure difference is applied, the low pressure side of the tiller material quickly becomes very wet and it remains so until the dry front reaches the low pressure side of the filler material. This provides an excellent visual indicator as to when drying has been completed. When drying has been completed, no moisture will be visible on the low pressure side.

As mentioned, the structure may be a panel, but in some preferred embodiments the structure is a building and the pressure difference is created by sealing all openings in the building structure and forcing gas into the interior of the building.

Gas is thus driven from the interior of the building structure to the exterior of the building structure through the filler material, drying it in the process, from the inside out.

In preferred embodiments the gas used is simply air. However, it will be appreciated that other gases can in principle be used.

The air (or other gas) being passed through the filling material may be heated and/or dried (to a low relative humidity). This can be achieved either by heating/drying air outside the building and pumping it in to the building interior.

Alternatively, the heating/drying equipment could be located within the building. In that case, ambient air is pumped into the building, heated and/or dried inside.and forced out through the filler material by the increased interior air pressure.

As mentioned, the structure may be a panel. Therefore according to another aspect of the invention, there is provided a method of forming a construction panel, comprising: forming a frame structure of the panel; filling the frame structure with a filling material; and drying the filling material by creating a as pressure difference across the panel so as to cause gas to flow through the filling material.

The pressure difference across the filler material may be created in any suitable way. However, in some preferred embodiments, the pressure difference is created by sealing a volume of air (or other gas) against one face of the panel and forcing air (or other gas) into said volume to increase the pressure therein. As the volume is sealed around the panel, the only way for air to escape the volume is through the panel, i.e. through the filler material. An alternative arrangement is to draw air out of the volume so as to lower the pressure therein. As the volume is sealed around the panel, the only way for air to flow into the volume is through the panel, i.e. through the filler material.

The volume may be formed by sealing a flexible sheet around the perimeter of the panel. The flexible sheet may be a polythene sheet or any other airtight membrane.

The sheet may be sealed to the panel in any suitable way. For example, adhesive tape may be used to attach the sheet to the panel around the entire circumference of the face of the panel. An air inlet will also be required to allow air to be pumped into (or out of) the volume. The air inlet could be another opening in the sheet or it could be located at the boundary between the sheet and the panel. The air inlet must be similarly sealed so as to allow the elevated (or reduced) pressure to be created and maintained within the volume.

In an alternative arrangement, the volume may be formed as a box with one side of the box comprising the panel. This arrangement is particularly well suited to drying panels in a horizontal orientation. The box can be created on the floor of a factory, with the panel forming the upper face of the box. Air is then pumped into the box underneath the panel, creating high pressure beneath the panel and thus driving air upwards through the filling material. Many such arrangements can be located next to each other on the floor, possibly with interconnected air inlets. In this way, many panels can be dried simultaneously in an easy fashion. Thus one side of the box may comprises more than one panel. Again, alternatively a low pressure region may be created beneath the panel, thus drawing air downwards through the filling material.

In some preferred embodiments, more than one side of the box comprises a panel.

With such an arrangement, many panels can be dried simultaneously from the same pressurized (or depressurized) volume. For example, a cuboid volume could be formed with a ring of four adjacent side panels all being dried simultaneously by air driven from the pressurized (or depressurized) interior of the box to the non-pressurized (or non-depressurized) exterior. A lid would of course be required on top of the four panels to complete the box. The floor could be used to form the bottom of the box. A fifth panel could also be used as a lid or as pad of a lid, thus to be dried at the same time as the other four.

Again, each side of the box could comprises more than one panel. The most efficient drying arrangements will depend on the size of the panels, the size of the drying space and the air pumping system available. Therefore the most appropriate drying arrangements may be selected in accordance with the available facilities.

In preferred embodiments, the air driven through the panel has a temperature of greater than 20 degrees centigrade. It will be appreciated that ambient air temperature may often be at or above this temperature. Therefore, heating is not always required to attain this temperature. However, in cold condition, heating of the air may be required to ensure that this temperature is achieved. In some arrangements, the air may be heated to a temperature of greater than 50 degrees centigrade. This will provide a faster drying process, but the energy required will most likely be greater than for lower temperatures.

In preferred embodiments, suitable temperature ranges include temperatures from to 50 degrees centigrade, but the optimum and thus most preferred temperature for speed and cost is 20 to 30 degrees centigrade.

The air driven through the panel(s) may have a relative humidity of anywhere between 5% and 95%, but the higher the relative humidity, the slower the drying.

Therefore, preferably the air has a relative humidity of less than 50%. Again, it will be appreciated that ambient air may already have a low enough relative humidity and thus no drying will necessarily be required. However, in cold and/or wet conditions, the relative humidity will most likely be higher and thus artificial drying of the air may be required to achieve this relative humidity level. Again, there is an increased energy consumption associated with the artificial drying, so the skilled person will be able to make a decision as to the best drying method and target humidity level to aim for.

Preferably the pressure difference across the filling material is at least 100 Pa. If the pressure difference is too low, then the drying process is not uniform across the filling material, instead drying preferentially in the vicinity of the air inlet. When sufficient pressure is maintained, the drying process becomes more uniform. Extra pressure will increase the rate of drying, but also uses more energy. The skilled person will therefore be able to select a suitable pressure at which drying is uniform and sufficiently fast, but without undue (or unnecessarily excessive) energy consumption. Preferably the pressure difference across the panel is at least 100 Pa per 100 mm of thickness, i.e. a pressure gradient of at least 1000 Pa per metre.

As described above, the filler material is preferably a mixture comprising a vegetable based aggregate, a mineral binder and water. The vegetable based aggregate is preferably a hemp based aggregate. In some embodiments, the mineral binder is Tradical® HB or Calcium Oxide. The advantages of these binders are provided above.

Preferably the filler material is left for a period of time between the initial casting or spraying of the filler material onto the frame structure and the application of the drying step. This initial period of time allows sufficient setting and curing of the wet mixture that it is firm and has gained strength through chemical reactions of the binder material. The mixture may typically take 24-48 hours to solidify and a further 3-5 days are preferably allowed for curing so that the filler material gains strength before drying. Thus a total initial period of time between casting/spraying and the beginning of the drying step is preferably 5-7 days. This represents a significant reduction in time compared with previous drying methods which allowed 2-3 weeks for the initial setting and curing to take place before drying. It has been found that pressure drying the filler material makes the filler material more robust than the previous drying method. Therefore the curing time can be reduced, while still achieving sufficient strength in a fully dried panel. The pressure drying step itself is typically less than 24 hours, so the time from casting/spraying of the filler material onto the frame structure to the fully dried structure is typically 6-8 days.

Although the invention has been described above in relation to drying methods, it will be appreciated that the invention also extends to panels or structures formed by such methods.

Preferred embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which: Fig. 1 shows a typical construction panel; Fig. 2a shows a first arrangement of precast slabs in accordance with a first embodiment of the invention; Fig. 2b shows a second arrangement of precast slabs in accordance with a first embodiment of the invention; Fig. 2c shows a single precast slab; Fig. 3a shows a side view of a panel with perforated face boards in accordance with a second embodiment of the invention; Fig. Sb shows a partial view of a perforated face board; Fig. 4 shows a filling process in accordance with the second embodiment of the invention; Fig. 5 is a flow diagram illustrating a filling process; Fig. 6 is a schematic illustration of a process of making a loose fill material in accordance with a third embodiment of the invention; Fig. 7 shows a set up for force drying panels using a pressure gradient in accordance with a fourth embodiment of the invention; Fig. 8 shows a cross-section of a panel being force dried according to the fourth embodiment of the invention; Fig. 9 illustrates a method of drying multiple panels at once; and Fig. 10 illustrates a method of drying a building on site.

The basic construction of an exemplary construction panel 100 is shown in Fig. 1.

The panel 100 comprises two frame members 102 and 104. Each frame member 102, 104 is rectangular in shape and is constructed from four engineered timber beams joined with mitred joints at their ends (although it will be appreciated that in other embodiments, more or fewer beams could be used and they may be joined in any way). The two rectangular frame members 102, 104 are positioned adjacent to -13-one another and in parallel so that the two frames together define a cuboid box frame for the panel 100.

The frame members 102, 104 are joined together around their perimeters by plates (made of plywood or other composite board material) 106 which extend perpendicular to both frame members 102, 104 and extend along the length of the side of the panel 100. Plates 106 are provided on each of the four long, narrow sides of the panel 100, thereby forming a rectangular perimeter of the panel 100 with its two largest faces still open.

The plates 106 are not located at the outermost position between the frame members 102, 104. That is, they are not flush with the outermost edge of the frame members 102, 104. Instead, the plates 106 are set back from the edge so as to form a recess in each of the long, narrow sides of the panel 100. The profile of the recess is a square U shape with its sides formed from the opposing faces of the engineered timber beams of frame members 102 and 104 and with its bottom formed from the outer face of the plate 106. The plates 106 shown in the embodiment of Fig. 1 are symmetrically located with respect to the studs of the frame members, that is the plates 106 extend from the mid-point of the side of one timber stud (e.g. of frame member 102) to the mid-point of the opposing side of the other timber stud (e.g. of frame member 104). Therefore together the timber studs and the plate 106 form an I-beam (that is the side of the panel framework has a cross-section in the shape of an I). In other variations, the plate 106 may be located off-centre, that is the profile of the I-beam may be asymmetrical with the plate 106 located more towards the inside of the panel 100 for a deeper recess or more towards the outside of the panel 100 for a shallower recess. A corresponding recess is formed by the opposite part of the I-beam, i.e. on the interior of the panel.

This recess provides a key to be filled by filler material which thereby provides greatly increased rigidity and resistance to buckling. Hence the panel as a whole has greatly increased strength. In yet other variations, the timber studs and the plate 106 may form a C-beam, i.e. having a cross-section in the shape of a letter C. This is just an extreme case of an I-beam, but with no recess on the interior side. It is simpler to manufacture.

As shown in Fig. 1, a number (three in this case) of additional support struts 108 are provided in each frame member 102, 104. The support struts 108 of the first frame member 102 are parallel to and adjacent to those of the second frame member 104 so that they may be readily joined together along their length by perpendicular connecting members 110. The struts 108 extend parallel to one of the sides of the rectangular frame members 102, 104, The struts 108 provide additional structural support and stability to the panel 100 as a whole. The struts 108 also provide attachment points for further structural or decorative components to be attached to the panel. For example, further insulation boards, render carrier boards, plasterboard or other interior decorative board or additional stud work can be attached to the struts 108. The struts 108 are provided at regular intervals along the largest faces of the panel 100. The connecting members 110 which join the struts 108 of one frame member 102 to those of the opposite frame member 104 are made from a material with low thermal conductivity so as to hinder the transfer of heat across the panel. The connecting members shown in Fig. 1 are connecting rods. An alternative to connecting rods is connecting gusset plates, i.e. short lengths of connecting plate, each connecting between the struts 108 of one frame member 102 and the struts 108 of the other frame member 104. A plurality of such rods (or gusset plates) 110 are provided along the length of each pair of struts 108 so as to connect them together in a plurality of places.

The structure described above forms a framework of a panel 100 which is to be filled with tiller material 112. Fig. I shows the panel with filler material 112 filling the framework. The filler material 112 may be a variety of materials, but is typically a thermally insulating material. In some preferred embodiments, the filler material 112 is Tradical® Hemcrete® which is a mixture of predominantly hemp shiv and lime binder. In other embodiments the filler material 112 is other blends of organic aggregates and mineral or organic binders, The filler material provides additional structural support to the panel 100. The filler material 112 may be applied in a wet form, e.g. filled or sprayed into the panel 100 and then allowed to set and dry. The filler material 112 thus binds to the framework of the panel 100 (i.e. the frame members 102, 104, the plates 106, the struts 108 and the connecting rods or gusset plates 110), thus binding the whole panel structure together. The filler material also makes the panel 100 solid. The filler material 112 provides rigidity to the framework and prevents it from buckling. This provides a large increase in structural strength. The filler material 112 is thermally insulating and preferably has a low thermal conductivity. The filler material 112 also has a large mass and therefore a large thermal inertia which slows down the changes in temperature which may occur within a building constructed from such panels 100.

On the exterior of the panel 100 shown in Fig. 1 (i.e. the side which will face the exterior of a building formed from such panels and which will be open to the elements), various additional layers are illustrated that may be applied for additional insulation, weather protection and for improved appearance. First, a plurality of wood fibre boards 114 are fixed onto the frame member 102 and its struts 108. The wood fibre boards 114 may be fixed by nails, screws, staples etc. or they may be attached with adhesive. On top of the wood fibre boards 114, a layer of render basecoat 116 is applied. The render basecoat 116 is applied with an alkali resistant glass fibre mesh 118 for additional structural support. On top of the render basecoat 116, a render topcoat 120 is applied for the final exterior finish. If desired, the topcoat 120 could also be painted. The render coatings 116, 120 are breathable so as to allow moisture within the panel to escape to the outside.

Preferably the render coatings contain lime among other ingredients.

On the interior of the panel 100 of Fig. I (i.e. the side which will face the interior of a building formed from such panels 100 and can therefore form the interior walls thereof), additional structures may be formed as illustrated. Internal studs 122 are fixed to the struts 108 of the frame member 104. Plasterboard 124 is then fixed to the studs 122. The studs 122 serve to space the plasterboard 124 from the filler material 112 and thus create a service void therebetween which can be used for electrical work or pipe work.

Additionally, breathable vapour control layers (membranes) may be fitted to either or both of the interior and exterior sides of the panel 100 so as to control the flow of moisture through the panel 100. On the interior side, a vapour control layer 128 (not shown) may be placed adjacent to the filler material 112, between the struts 108 and the studs 122. On the exterior side of the panel 100, a vapour control layer (not shown) may be placed adjacent to the filler material 112, between the struts 108 and the wood fibre board 114. The exterior vapour control layer must have a diffusion resistance less than that of the interior vapour control layer so that moisture entering the panel from the inside is not significantly impeded from escaping to the outside.

It will be appreciated that, although the panel 100 shown in Fig. us shown in a fully pre-constructed form, i.e. with all layers applied in a factory before transport to the construction site, the panels 100 could be formed in a more basic form in the factory, e.g. with just the framework 102, 104, 106, 108, 110 and the filler material 112. The additional layers of insulation, render and interior and exterior surface layers may be applied on site after the panels 100 have been assembled together to form a building. In this way these interior and exterior layers may bridge the gap between panels 100, thus adding to the weatherproofing and creating a smoother finish. The layers need not then be restricted in extent to a single panel size, but may be applied across whole building walls at once. For example, the external render layers may be sprayed over the whole building in a single application, bridging all joins in the process. Likewise, the internal walls of rooms of the building may be bridged by the internal boards and plaster may be applied over all joins for a smooth finish. This may be a full depth plaster layer or just a skim finish. If the vapour control layers are applied in the factory, they provide some protection to the panel 100 during transport. At the site, the vapour control layers of adjacent panels 100 may be joined with tape to seal the gap between the panels.

Figs. 2a, 2b and 2c illustrate a first embodiment of the invention. In this embodiment, the drying of the filler material 112 is expedited by casting smaller slabs 200 of filler material 112 which can be dried separately and faster. The drying time for a small slab 200 is less than for a full panel 100 as the slab 200 has a much greater surface to volume ratio, thus allowing the moisture within the slab 200 to evaporate faster, thus drying the filler material 112 faster. The drying process is the most time consuming pad of panel production and therefore reducing the drying time to produce a completed panel results in faster production and greater throughput.

Once dried, the slabs 200 are inserted into the framework of a panel 100 so as to complete the panel 100. Additional layers may be applied to the panel 100 as described in relation to Fig. 1.

As described above, panels 100 typically involve a framework with intermediate struts 108 for additional strength and support. These struts 108 are preferably spaced at a common inter-stud spacing such as 400 mm or 600 mm. This allows the panels 100 to interoperate well with other construction systems or features that make use of such a common inter-stud spacing. To fit between the struts 108 of the panel 100, the slabs 200 of precast, predried filler material 112 need to have a dimension equal to the spacing of the struts 108. Thus for the more common applications, the slabs 200 should have a dimension of 400 mm or 600 mm.

For a particularly versatile slab 200, the slab 200 can be formed with one dimension being 400 mm and another dimension being 600 mm. Such a slab 200 can then be fitted between struts 108 at either of the common spacings. Such a slab 200 will not be large enough to fill the whole height of the panel 100, so many slabs 200 are simply stacked one on top of the other to fill the space. If the height of the panel does not correspond to an exact multiple of slabs 200, then a small gap will remain at the top. This gap can either be filled with a smaller slab, cut to size from a standard slab 200, or it can be filled with wet-fill material. The drying time for small amounts of wet-fill material will be small and will not significantly affect the throughput. In a completed panel, the slabs 200 are held in place, e.g. by an adhesive/mortar, screws, nails or clips so as to fix the slabs 200 relative to the panel frame.

Fig. 2a shows a front view of a panel framework 210 with struts 108 spaced 600 mm apart. The framework width is not an exact multiple of the strut-spacing, 50 the right hand strut 212 is only 300 mm from the outer frame member 102 of framework 210.

Slabs 200 are placed in between struts 108 with their long dimension (600 mm) filling the space between struts 108. The slabs 200 are formed with one dimension * 30 matching the panel depth (i.e. the distance between the two parallel rectangular outer frames). The third dimension (400 mm) thus extends in the direction of the height of the framework 210. Several slabs 200 are stacked adjacent to one another to fill the height of the framework 210. As shown in Fig. 2a, the height of the panel is not an exact multiple of slab heights (i.e. it is not a multiple of 400 mm). -18-

Therefore, the topmost slab 200 in the stack must be cut down to size or the gap formed at the top of the stack must be filled with wet-fill material.

In Fig. 2a, the slabs indicated by reference number 201 are full-sized slabs 200.

Those indicated by reference number 202 have been cut down to size to fit the top of a stack. Those indicated by reference number 203 have been cut down to size to fill the narrow space between strut 212 and frame member 102, and the slab indicated by reference number 204 has been cut down to size in two dimensions to fit the top of that narrow space.

Fig. 2b shows another panel framework 220 with a slightly different size and with struts 108 spaced 400mm apart. As in Fig. 2a, the right hand strut 108 is only 300 mm from the framework outer member 102, forming a narrow channel at one side of the framework 220.

The slabs 200 are placed in framework 220 with their 400 mm side positioned to fill the space between struts 108. The 600 mm side thus extends in the height direction of the framework 220. Slabs 200 are stacked on top of each other, as in Fig. 2a. In this embodiment, the height of the framework 220 matches the height of three full-size slabs 201, so there is no gap at the top of each stack to be filled with cut-down slabs or wet-fill material. The narrow channel at the right hand side is filled with cut-down slabs 203 as in Fig. 2a.

Fig. 2c shows a slab 200 with dimensions LI, L2 and L3. In the embodiments described above, Li may be 600 mm, L2 may be 400 mm and L3 may for example be 120 mm, 200 mm, 450 mm or any other typical panel thickness. It will be appreciated that panel thickness will vary according to the application, e.g. the size of building being constructed, whether the panels are structural (i.e. weight bearing) or just cladding, and how much thermal insulation is required.

Figs. 3a, 3b and 4 relate to an alternative method of decreasing the drying time of the panel. Unlike the first embodiment, this second embodiment uses a wet-fill process to fill the panel framework, i.e. pouring or spraying the wet filler mixture into the panel framework and allowing it to set.

As shown in Fig. 3a, the panel 300 formed using this process of the second embodiment has a perforated board 310 on the two main faces of the panel framework (the framework being formed by the frame members 102, 104, 106, 108, as described in relation to Fig. 1). The two main faces are the ones that are designed to form the interior and the exterior of the building in use, i.e. the interior face being designed to face the interior of the building or to receive further internal panelling or interior decorative structures such as interior studwork and/or plasterboard, and the exterior face being designed to face the exterior of the building or to receive exterior panelling or coatings such as renders.

The two perforated boards 310 retain the wet filler material within the volume of the framework, i.e. they prevent the mixture from slumping out of the framework, even if the panel is stood vertically (i.e. with the main faces vertical). The perforated board has many perforations 315 formed therethrough. These perforations extend substantially perpendicular to the main face of the panel 300 and boards 310 and provide passages for air to contact the filler material 112 and for moisture within the filler material 112 to pass out. The size of the perforations 315 is important as they determine the rate at which moisture can evaporate from the panel, but they also must be small enough that the wet mixture does not pass through them. The maximum size of perforation that can be used will depend on the type of filler material being used, in particular, the size of the aggregate particles and the viscosity of the binder will determine how big the perforations can be before the filler material is capable of exuding out of the perforations 315. In some preferred embodiment, such as that shown in Figs. 3a and 3b, the perforations are circular holes with a diameter of 7 mm. The perforations 315 are arranged in a square array, with the centres of the perforations 315 being separated by 25mm. It will be appreciated that a triangular array or any other regular or irregular arrangement of perforations 315 may also be used.

The combination of the distance between perforations 315 and the size of the perforations 315 will affect the strength of the board 310. As the board 310 adds to the structural strength of the panel as a whole, this can be an important factor. In particular, another way of reducing the drying time of the filler material 112 is simply to reduce the amount of water added to the aggregate and binder to make the wet mixture in the first place. As described above, the water is an important factor in -20 -the reactions which take place to solidify and strengthen the filler material 112.

Therefore, reducing the amount of water in the mixture will reduce the final strength of the mixture and thus of the panel as a whole. However, it has been found that the panel framework can provide sufficient structural strength that a reduction in the strength of the filler material 112 can be accommodated without compromising the strength of the panel as a whole. In such circumstances, the strength of the perforated boards 310 and their contribution to overall panel strength may be an important factor in determining the starting water content of the tiller material mixture and thus the final strength of the filler material 112.

Fig. 4 illustrates an example filling process for forming a panel 300 according to the second embodiment. First (in step (a)), the framework, with one perforated board 310 attached to one face, is laid flat, with the perforated board 310 at the bottom.

Filler material 112 is poured or sprayed into the framework to fill the framework level with the upper face of the framework. Second (in step (b)), the second perforated board 310 is fixed to the framework to enclose the filler material 112 within a closed structure. Third (in step (c)), the panel 300 is rotated from a horizontal orientation to a vertical orientation. The second and third steps (b) and (c) are optional, but are sometimes preferred for transport and storage. If panels are left horizontal for drying, then the upper face can be left open for faster drying.

During drying, horizontal panels are raised off the floor so as to allow air to circulate underneath the panel, contacting the filler material via the perforations in the lower board.

Fig. 5 is a flow chart showing an example of a process for constructing a panel according to the second embodiment described above.

In step Sal, the framework is constructed, e.g. of engineered timber beams and interconnecting rods and/or boards. This provides an empty structure which is to be filled with filler material 112. The large faces (i.e. those that form the interior and exterior faces when the panel is in use) of the framework are both open at this point in the process, while the smaller edge faces are preferably closed. The terms open and closed are used here to mean respectively allowing or disallowing the passage of wet-mix filler material 112 to pass through the face in question. In step S02, one of those open faces is closed by fitting a perforated board to the framework -21 -structure. At this point, the framework and the perforated panel essentially form a box structure, open on one face (i.e. a cuboid shape with five closed faces and one open face).

6 In step S03, the frame structure (including one perforated board) is laid flat on the ground, with the perforated panel lowermost, thus presenting the open face uppermost. Filler material 112 is prepared (from aggregate, binder and water) in step S03a. The framework structure is filled with filler material 112. The choice of filler material may vary according to the circumstances.

In step S04, after the panel has been filled, the second perforated board is attached to the open face, thus forming a fully enclosed box with the filler material 112 contained within it.

16 In step 305, the panel is moved to a storage or drying area. As the panel is now contained, it may be rotated from a horizontal orientation to a vertical orientation without the filler material 112 escaping from within the panel.

The use of perforated panels increases the contact area of the filler material with the air, thus increasing the rate of evaporation of the moisture within the panel and decreasing the drying time.

In a particularly preferred form of this embodiment, the binder used in the filler material is Calcium Oxide (CaO). As CaO binds with the water in the wet-mix, it reduces the amount of water that needs to be evaporated out of the panel in the drying process. Additionally, as CaO reacts exothermically with water, the heat generated by that reaction increases the evaporation rate for the remaining unbound water. Thus drying time is even more reduced when CaO is used as the binder for the filler material 112.

Fig. 6 illustrates a third embodiment for reducing the drying time of a panel. This embodiment differs from the first and second embodiments in that it forms a loose-fill filler material 112 that can then be poured into the panel in its dried form. Thus the panel, once filled, is already fully dried and ready for use.

-22 -The filler aggregate particles 410, the binder 420 and water 430 are all added to a rotary mixing apparatus 440 which continuously moves the ingredients throughout the drying process. This results in the aggregate particles being coated with the binder. The coated particles 450 have resistance to fire, fungus and pests as required by building regulations and thus the coated particles can be used as a loose-fill material whereas the uncoated aggregate particles would not meet the regulations.

The loose fill particles 450 can be added to a panel using the same process as described above with reference to Fig. 5 for the wet-mix filler material 112. The only differences in that process are that the dry loose-fill material is used in place of the wet-mix filler material and the perforated boards are not required (although they may be used if desired) as no further drying of the filler material 112 is required.

Instead, simple non-perforated boards may be used in their place, these providing greater structural strength and stability and being less expensive than perforated boards.

Figs. 7 and 8 illustrate a method of drying a panel with reduced drying time according to a fourth embodiment of the invention.

It has been found that by setting up a pressure difference across panels 100 with filler material 112 (e.g. Tradical Hemcrete panels), it is possible to achieve much shorter drying times than by cooking or by natural air drying. In all cases a suitable fan 730 is used to pressurize one face of the Tradical Hemcrete (i.e. the filler material).

As shown in Fig. 7, a panel 100 has a polythene sheet 710 (or other airtight membrane) attached to one face. The sheet 710 is sealed in an airtight manner to * the panel 100 to create a volume 740 (see Fig. 8) of air between the panel 100 and the sheet 710. A conduit 720 is also attached to the sheet 710 in an airtight manner. The conduit 720 connects the volume of air 740 between the panel 100 and the sheet 710 to a pump 730 that pressurizes the volume 740. In this manner, the air on one face of the panel (the one to which the sheet 710 is attached) is at a higher pressure than the opposite face (which is open to atmospheric pressure).

-23 -The panel 100, sheet 710 and conduit 720 are shown in cross-section in Fig. 8.

As indicated by arrow 750, air flows into the volume 740 via conduit 720. The high pressure within volume 740 and the lower pressure outside the volume 740 causes a pressure gradient across the panel 100. This pressure gradient causes movement of air through the panel from inside the volume 740, through the panel and out the other side as indicated by arrows 760. The air flowing through the panel in this manner entrains moisture from within the wet filler material 112 and carries it towards the open face 770 of the panel 100. The moisture collects on open face 770 and evaporates therefrom.

As the moisture is carried through the panel 100, the panel begins to dry from the high pressure side towards the low pressure side, i.e. the high pressure face dries first and the low pressure face 770 dries last. During the drying process, a dry front 780 (indicated by dashed line in Fig. 8) moves through the panel from the high pressure side to the low pressure side. Thus when the low pressure side is fully dry (which can be determined by visual inspection as wet filler materIal 112 is darker in colour than dry filler material 112), it can be determined that the whole panel 100 is dry.

The pump/fan 730 may just blow ambient air into the volume 740, but it may also heat and/or dry the air so as to increase the potential of the air to carry water out of the panel 100. Alternatively other heating and/or drying equipment can be used along with the pump/fan 730.

This technique can also be run in reverse by depressurising on the fan side. The set up is essentially the same as that shown in Fig. 8 and described above, except that the pump/fan is run in the opposite direction so as to draw air out of the volume 740, thus lowering its pressure and causing air to move through the panel 100 from the higher pressure side (i.e. the unpressurised side which is typically at atmospheric pressure). It will be appreciated that a combination of these techniques may also be used, i.e. actively pressurising one side and actively depressurising the other, using pumps/fans 730 on both sides of the panel 1 00, each with its own sealed volume 740.

-24 -Fig. 9 shows an alternative embodiment for drying multiple panels 100 at once. A support frame 910 is mounted on the floor in an airtight manner. The support frame 910 is a rectangular frame. Together the support frame 910 and the floor form 5 sides of a cuboid box. The support frame 910 is sized so as to support panels 100, i.e. the width of the support frame 910 matches one of the dimensions of the panels 100, while the length of the support frame 910 matches (orexceeds)a multiple of the other dimension of the panels 100. In Fig. 9, the outer dimensions of the support frame 910 are shown slightly larger than the outer dimensions of the panels so as to allow a small tolerance in the placement of the panels, but this need not necessarily be the case. Any excess overlap is sealed off before pressurising the space.

Together, the floor, the support frame 910 and the panels 100 form a closed box surrounding a volume of air. The panels are engaged with the support frame 910 in an airtight manner. The volume of air inside this closed box is connected, via conduit 720 to pump 730 as in Figs. 7 and 8. The pump 730 is used to raise the pressure of the volume of air below the panels 100 and thus creates a pressure gradient across the panels 100 from the bottoms to the tops 920 (which are at atmospheric pressure). Thus the panels dry from the bottom up in the same fashion as described above.

Fig. 10 illustrates another drying arrangement employing the same principle as Figs. 7, 8 and 9. In this arrangement, several panels 100 are mounted together in a closed structure, in this case a building 1000. The windows 1020 and doors 1010 (and other orifices) of the building 1000 are sealed shut in an airtight manner so that the whole of the interior volume of the building can be raised in pressure via pump 730 and conduit 720 in the same manner as in Figs. 7, 8 and 9, thus creating a pressure gradient across all of the panels 100 simultaneously, with higher pressure on the inside and lower pressure (atmospheric pressure) on the outside.

The panels 100 dry from the inside out in the same way as indicated above.

It will be appreciated that panels may be dried in a factory in a similar manner, by forming the panels 100 into a closed structure (not necessarily a building) in a sealed manner. In this way, the panels can be dried much faster than in the conventional manner.

-25 -In the case of a building, the panels can be placed in situ while wet (or not fully dried). Alternatively, the building structure (e.g. a timber structure) can be constructed on site and filler material can be cast or sprayed on site onto that structure. The building can then be sealed and dried as illustrated in Fig. 10 (although no panels are involved in such method).

If only part of a building is site cast, battens/membranes may be used to isolate the area to be dried. Alternatively, battens may be used to isolate areas of wall to be dried so that the wall can be dried in stages.

It will be appreciated that in the above description, the terms "airtight" and "sealed" are used to mean sufficiently airtight and sufficiently sealed that the required pressure can be obtained within the volume in question. Better sealing will improve efficiency, but it will be understood that some leakage can be tolerated, so long as the required pressure gradient can be maintained across the structure which is to be dried.

As indicated above, conventional drying times for a building where Hemcrete is cast on site onto a timber frame can be up to 2 or 3 years to achieve full drying to an equilibrium moisture content. Drying times for a panel, air dried in a factory with simple circulating air at atmospheric pressure are of the order of 2 to 3 weeks. This can be compared with total curing/drying times of about a week (5-7 days curing time plus drying times typically 24 hours or less) which are achieved via the pressure drying method described above.

The following explains this drying method in more detail and provides specific examples to illustrate the efficacy of the process.

Air pressure and airflow are interrelated quantities. The same airflow over a larger area will result in a lower pressure. For the purposes of sizing, fans with a capacity of 500 m3h(1 per m2 of Hemcrete has been found to work well. Lower flow per has the effect of drying locally at the air input area, instead of drying uniformly across the whole panel surface. -26

To achieve the required flow across 120 mm of Tradical Hemcrete, a pressure difference of around 100 Pa is required. It will be appreciated that this figure may vary with the density of the filler material. It is currently believed that the required pressure generally increases linearly with the thickness of the filler material to be dried.

Towards the end of the drying process, a pressure drop has been observed at the pressurized side as the filler material (Tradical Hemcrete in the tests) shrinks and some airflow takes place around, rather than through it.

Pressurizing panels using higher temperature and lower relative humidity air has been shown to decrease the drying time.

During drying, temperature falls and WME (Wood Moisture Equivalent) rises as distance from the pressurized side increases. The low pressure face of the Tradical Hemcrete becomes very wet and the wall starts to dry from the high pressure side.

This moves through the Tradical Hemcrete as a Dry front" until it reaches the low pressure side. This provides an easy visual check on the low pressure side of the Tradical Hemcrete for when drying is finished, as no moisture will be visible.

Sensors may also be used to monitor the progress of the drying process. For example, a pressure sensor may be placed inside the volume to monitor the pressure that is being maintained therein. A pressure sensor may also be placed outside the volume so as to monitor the pressure gradient. In other words, pressure sensors may be applied on both the high pressure side and the low pressure side of the filler material. Additionally, or alternatively, one or more humidity sensors may be used to monitor the humidity of the air adjacent to the low pressure face. Temperature sensors may also be used to monitor the drying process as an increase in temperature can be detected when drying is complete and there is no longer any evaporation taking place from the low pressure face.

To illustrate the magnitude of the reduction in drying times, the following examples are provided for drying Tradical Hemcrete panels: -27 -In the following examples, "275 mix" refers to an aggregate/binder mix having a density of 275kg/rn3 and the mix contains 110kg hemp shiv for each 165kg of Tradical® HB. Similarly, "330 mix" refers to an aggregate/binder mix having a density of 330kg/m3 and the mix contains 110kg hemp shiv for each 220kg of Tradical® HB. A bale is a quantity of hemp shiv, about 200 litres in volume and weighing 18 to 22 kg.

Further, the moisture level of the panels is described in terms of Wood Moisture Equivalent (WME). Most moisture meters are based on measuring electrical conductance across a sample. This conductance will vary from one material to another, so the meter needs to be calibrated for each material. The meters are normally calibrated for wood and the standard reading is therefore based on the

Example 1

A 450 mm thick panel of Tradical Hemcrete (275 mix, 40L water per bale) was pressurized to 400 Pa, (dropping to 200 Pa after 24 hours). The input air was at 22 degrees C and 50% RH. The panel dried to below 10% WME in 36 hours.

Example 2

A 120mm thick panel of Tradical Hemcrete (330 mix, 45L water per bale) was pressurized to 180 Pa. The input air was at 22 degrees C and 50% RH. The panel dried to below 10% WME in less than 18 hours.

Example 3

A 120 mm thick panel of Tradical Hemcrete (330 mix, 45L water per bale) with perforated board on one side of the panel (the pressurized side) was pressurized to 300 Pa. The input air was at 22 degrees C and 50% RH. The panel dried to below 10% WME in less than 21 hours.

-28 -The perforated board had a square array of 7 mm diameter perforations, the perforations being spaced 25 mm apart. The non-pressurized side of the panel was left open.

Example 4

A 200 mm thick panel of Tradical Hemcrete (275 mix, 40L water per bale) with perforated board on one side of the panel (the presurized side) was pressurized to Pa. The input air was at 55 degrees C and 10% RH. The panel dried to below 10% WME in less than 15 hours.

The perforated board was the same as in Example 3.

It can be seen from the above that the drying times of the panels are significantly reduced using the depressurizing method. Even with an initial time period for allowing an initial cure of the binder after casting, the total curing and drying time for the panel can be about a week. This applies to both pressurized (blowing) or depressurized (sucking) methods.

Claims (26)

1. -29 -Claims 1. A method of forming a construction panel, comprising: forming a frame structure of the panel; filling the frame structure with a filling material; and drying the filling material by creating a gas pressure difference across the panel so as to cause gas to flow through the filling material.
2. 2. A method as claimed in claim 1 wherein said pressure difference is created by sealing a volume of gas against one face of the panel and forcing gas into said volume to increase the pressure therein.
3. 3. A method as claimed in claim 2, wherein the volume is formed by sealing a * flexible sheet around the perimeter of the panel.
4. 4. A method as claimed in claim 2, wherein the volume is formed as a box with one side of the box comprising the panel.
5. 5. A method of drying a plurality of panels, as claimed in claim 4, wherein more than one side of the box comprises a panel.

   *
6. 6. A method of drying a plurality of panels, s claimed in claim 4 or 5, wherein one side of the box comprises more than one panel.
7. 7. A method as claimed in any preOeding claim, wherein the gas driven through the panel has a temperature of greater than 20 degrees centigrade.
8. 8. A method as claimed in any preceding claim, wherein the gas driven through the panel has a relative humidity of less than 50%.
9. 9. A method as claimed in any preceding claim, wherein the pressure difference is at least 100 Pa.
10. 10. A method of drying a structure, wherein the structure comprises a framework and a filler material, the method comprising: -30 -drying the filler material by creating a gas pressure difference across the filler material so as to cause gas to flow through the tiller material.
11. 11. A method as claimed in claim 10, wherein the structure is a building and wherein the gas pressure difference is created by sealing all openings in the building structure and forcing gas into the interior of the building.
12. 12. A method of forming a construction panel comprising: forming a frame structure of the panel; casting slabs of a filler material; drying said slabs of filler material; and placing said slabs in said frame structure.
13. 13. A method as claimed in claim 12, wherein said frame structure has one or more parallel studs separating the panel into a plurality of elongate channels, and wherein said slabs are cuboid in shape, having a first dimension a second dimension and a third dimension; wherein said first dimension is equal to the thickness of the frame structure and wherein said second and third said dimensions are common inter-stud distances.
14. 14. A method as claimed in claim 13, wherein said slabs have a second dimension of about 400 mm and a third dimension of about 600 mm.
15. 15. A method of forming a construction panel comprising: forming a frame structure of the panel; attaching a perforated board to one face of the frame structure; filling the frame structure with filler material; and * drying said filler material.
16. 16. A method as claimed in claim 15, further comprising attaching a second perforated board to a second face of the frame structure before said drying step.
17. 17. A method as claimed in claim 15 or 16, wherein the or each perforated board contains perforations with a diameter of about 7mm.

    -31 -
18. 18. A method as claimed in claim 15, 16 or 17, wherein the or each perforated board contains perforations in an array with the distance between adjacent perforations being about 25 mm.
19. 19. A method as claimed in any of claims 15 to 18, wherein the drying step comprises blowing air around the panel.
20. 20. A method as claimed in any of claims 15 to 19, wherein the filler material includes Calcium Oxide.
21. 21. A method as claimed in any preceding claim, wherein the filler material is a mixture comprising a vegetable based aggregate, a mineral binder and water.
22. 22. A method as claimed in claim 21, wherein the vegetable based aggregate is a hemp based aggregate.
23. 23. A method as claimed in claim 21 or 22, wherein the mineral binder is Tradical HB or Calcium Oxide.
24. 24. A method of forming a filler material comprising: mixing a vegetable based aggregate with a mineral binder in a continuous action mixer so as to create particles of said vegetable based aggregate coated with said mineral binder.
25. 25. A method of forming a construction panel, comprising: forming a frame structure of the panel; attaching a board to one face of the frame structure; filling the frame structure with particles of vegetable based aggregate coated with mineral binder; and attaching a second board to a second face of the frame structure.
26. 26. A method as claimed in claim 25, wherein the filler material is formed according to the method of claim 24.