GB2536702A

GB2536702A - Determining the U-value of a wall or other construction element

Info

Publication number: GB2536702A
Application number: GB1505226.9A
Authority: GB
Inventors: Atherton Eric
Original assignee: SENICO Ltd
Current assignee: SENICO Ltd
Priority date: 2015-03-26
Filing date: 2015-03-26
Publication date: 2016-09-28
Also published as: EP3274679A1; WO2016150856A1; GB201505226D0

Abstract

A method and an apparatus are disclosed for determining the U-Value of a wall, or of any other construction element having two opposed substantially fiat surfaces. The method comprises: measuring the initial temperature of both wall surfaces at two positions which are substantially opposite each other on either side of the wall, and either, if the two measured temperatures are different modifying the temperature of a first closed volume defining a first predetermined area of wall surface surrounding at least one of said positions by applying energy to neat or to cool the first volume and the first area of wall so that the temperature of the area of wall is substantially uniform and substantially the same as the initial measured temperature on that side of the wall or, if the two measured temperatures are substantially the same, modifying the temperature of a first dosed volume defining a first predetermined area of wall surface surrounding at least one of said positions by applying energy to heat or to cool the first volume and the area of wail so that the temperature of the area of wall is substantially uniform and differs from the measured temperature and using the measured temperatures, the first predetermined area and the heating or cooling energy to determine the. instantaneous U-Value. Also disclosed in a method comprising the use of first and second closed volumes, one on each side of the wall.

Description

Intellectual Property Office Application No. GII1505226.9 RTM Date:27 October 2015 The following terms are registered trade marks and should be read as such wherever they occur in this document: Bluetooth Intellectual Property Office is an operating name of the Patent Office www.gov.uk/ipo Determining the Value of a waU or other construction element flEW OF 'ME INVENTION The present invention relates to the methods and apparatus for the de-ithe "U value" of a wail of a building or the like.

BACKGROUND ART

the ield of building physics, 0-value (or 1J4actor) is the term used to describe the heat transfer coefficient of a building element (e.g. a wall); it is a measure of the rate of heat loss or gain through the material of the building e:ement. The unit of 1.1-Value is W1rn2K. The lower the U-factor, the greater the material's resistance to heat flow and the better is the insulation quality of the buildino element. The U-Values of the elements forming the exterior of a building determine the building's fuel consumption, CO2 emission, cooling load or running cost.

Figure 1 shows in cross-section a part of a typical building having wails:;) and a roof 5; there is a temperature difference (dt) between the interior 7 of the building and ambient or atmospheric temperature outside 9 the building, in the drawing, the internal temperature is greater than that outside the building, so there is a heat loss, or flux, (Hi) in the direction of the arrow (if the ambient temperatures were reversed, the heat flux would be in the Other direction). The heat transmission through the building wall (or any similar construction element) can be expressed as: where ti-value"(Wfm2K) A =wall area (m2) dl = temperature difference Techniques are known to extend the above expression so as to allow for construction elements farmed of more than one layers, such as that formed by brick and Mortar in many wails. The typical U-Value for a wal4 150 mm, thick of poured concrete having a density. of 2.2x10b.kgith', it about 3,9 Winlzk; fOr a wail 250 rnM thick made of brick the U-Value is about 2,0. \filmic Measurement of U-Value is frequently carried out during building surveys, for *example so as to predict heating costs, or to determine the before and after's effects of adding insuiation, etc and it is important that the Li-Value be determined quickly and accurately. Known ways of determining.UValue involve using temperature sensors on the internal and/o' external surfaces of the wall (or other construction element), together with sensors for measuring the ambient temperature on either side of the wall: on at least one surface of the wail there are usually several, spaced temperature sensors so as to enable measurements of heat flux to be taken over an area of the wall, rather than simply a point reading. Usually the readings are taken over a period of time, often days or even weeks in order to obtain accurate values; although this allows accuracy, it is too slow to be accomplished in the normal building survey, which is usually conducted in no more than an hour or two. One factor which has a significant bearing on the speed of determining U-Value is the thickness of the wall or other construction element; combined with the normally low thermal conductivity or conventional wail material(s), the equilibration of heat flux through and temperatures inlon the wall following a change in the ambient temperature on one or both sides of the well takes a significant time, and any U-Value readings taken before equilibration is completed will be inaccurate (which is why U-Values are normally only measured accurately over a long period of time, so that equilibration errors are allowed for).

The accuracy of U-Value measurements is affected by such factors as the in homogeneity of or variation in many building elements (they may contain voids or regions of greater or lesser density, or heating pipes or other building utilities, all of yvhich can give, 30 a U-Value reading which is not representative of the wall over its entire surface) and variations in ambient temperature. Fluctuations in temperature over time have already been discussed, however there are very often variations in temperature over the surface area of the wall as well as through its thickness, for example because the air inside a room is usually warmer towards the ceiling than towards the floor, or because part of an external wail is in sunlight and part in shade.; in these circumstances the heat flux is not directed squarely across the wail, as shown in Figure 1, but instead is angled other than at 90' to the surface of the wall. This can mean that when a U-Value reading is taken over a relatively small surface area the ti-Value determined may not be representative of the wail as a whole (because the thermal difference across the wall at one point differs from that at other points).

There is a need to be able to determine the U-Value of a vvall (in what follows any reference to a 'wall" or "vvallsr7shou d be construed as encompassing any form of construction element, such as a roof, floor, door, window, etc.) more quickly and/or accurately than conventional techniques and devices allow.

SUMMARY OF THE INVENTION

The present invention arises partially from the recognition that, where there is a temperature difference through a wall and also a temperature gradient along the wail surface theil the isotherms within the wall are not parallel to the surface of the wail, and therefore their intersection with the wall surface at or adjacent the area where the UiAtalue reading is being made introduces error into the U-Value measurement. It is also prompted by the realisation that the thickness of the wall is one of the main factors which prevent rapid and accurate U-Value measurements to be made, and that reducing the wail thickness would enable an improvement in speed and/or accuracy, Actively modifying the temperature of the wail surface(s) can effediveiy reduce the wail thickness, in temperature distribution terms, as will be explained.

The present invention therefore provides a method of determining the Li-Value of a wall (or of any other construction element having two opposed substantially fiat surfaces) comprising; measuring the initial temperature of both wall surfaces at two DOS tom are substantially opposite each other on either side of the wall, and either, if the two measured temperatures are different, modifying the temperature of a first closed voibme defining a first predetermined area of wall surface surrounding at least one of said positions by applying enemy to heat or to cool the first volume, and the first area of wail so that the temperature of the area of wail is substantially uniform ano substantially the same as the initial measured temperature on that side of the wall or, if the two measured temperatures are substantially the same, modifying the tempera:tare of a first closed volume defining a first predetermined area of wall surface surrounding at least applying' energy to heat or to not the first volume and the one of said positions by area of wall so that the temperature of the area of wail is substantially uniform and difTers from the measured temperature: and using the measured temperatures, the first predetermined area and the heating or cooling energy to determine the instantaneous U-Value.

'Such an arrangement is effective, where there is a temperature difference through the wall and also a temperature gradient along the wall surface and hence the isotherms are not parallel to the general plane of the wall, of changing the teraperature distributon within the wall so that the isotherms adjacent one side of the wall the side of the wail on wilier: the first area is located) are brought parallel, or closer to parallel, with that surface of the 15. wall and/or the general plane of the wail.

A succession of instantaneous Li-Values may be monitored and reccircied in a processor or data store until the difference between successive instantaneous U-Wkies reaches a threshold amount, and taking the last U-Value deterinined as the final U-Valtie of the val Where the structure reacts sufficientiy quickly, thS allows a speedy U-Value measurement to be made. Alternatively, a succession of instantaneous Li-Values may be recorded over time until sufficient information has been 6cquired to extrapolate a representative U-Value, and extrapolating the final U-Value of the wail from the recorded inhormation: this is where the structure is such that temperature stabilisation takes too long, in which case extrapolation can be carried out using known techniques.

Where the two initially measured temperatures are different, the method may -further comprisa modifying the temperature of a second closed volume defining a second predetermined area of tivall surface surrounding the other of said positions by applytno energy so as to heat or cool the second volume and the second area of wall so that the temperature of the second area of wall is substantially uniform and substantially the same as the initial measured temperature of the said other position, Where the two initially measured temperatures are substantially the same, the method may comprise modifying the temperature of a second closed volume defining a second predetermined area of wall surface surrounding the other of said positions by applying energy so as to heat or cool the volume and the second ai-ea of wall so that the temperature of the second area of wall is substantially uniform and differs from the measured temperature of the said other position.

Preferably the arrangement is such that on one side of the wall the temperature is lowered, whilst at the same time on the other side the temperature is raised, This brings the temperature distributiion inside the wall more quickly into equilibrium, certainly as compared to applying heat (or cold) to only one side of the wall as the re-distribution of the isotherms has to reach only from the surface of the wall to its centre from each side, and the quantum of heat energy to achieve a notable change is less than when applying a temperature difference on only one side of the wall, The method may fLirther comprise modiyng the temperature of at least one third closed volume defining a third predetermined area of wall surface surrounding the first and/or second predetermined area of wail and applying energy to heat or cool the third volume(s) and the third area(s) of wail so that the temperature of the third area(s) of wall is/re substantially uniform and/or substantially the same as the measured temperature of the surrounded first and/or second predetermined area of wail. In this way, concerto,.

regions or areas are defined on the surface of the wall. Control of the heating or cooling is carried out in two control loops; the first of these maintains the temperature of the inner part of the outer area the same as the temperature of the central or core area, at a higher or lower temperature than the initial measurement, whilst the second, on the other side of the wail, either maintains the temperature of the inner part of the outer area the same as the temperature of the central or core area at the initial measured temperature, or modifies it in the opposite direction (cooling or heating) compared to the modification on the other side of the wall.

The invention also provides an apparatus for measuring the Li-Value: of a wall comprising a first temperature sensor for mounting to and for measuring the temperature of 30 a surface of one side of the wall and two temperature sensors for mounting to and for measuring the temperature of two spaced points on a surface of an opposite side of the wall to the first temperature sensor and thereof, a first enclosure adapted to be held against the said opposite surface so as to enclose a volume of air against a predetermined area of the wall and so as to surround one of the two temperature sensors on said surface, a heater/cooier adapted to heat or to cool the volume of air and a processor operatively connected to the temperature sensors and the neater/cooler; the processor being adapted to control and monitor the energy supplied to the heater/cooler, to monitor the temperatures sensed by the sensors, and to determine the instantaneous U-Vaiue using the sensed temperatures, the first predetermined area and the heating or cooling energy,

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by ilyey of example and with reference to the accompanying figures, in which; Roure 1 is a schematic cross-sectional view of a part of a typical building illustrating heat flux through a wait Figure 2 shows a section of wail with temperature sensors attached; Figure 3 shows a section. of wail with a thermal management apparatus in accordance with the invention in place; Eioure 4 is a detailed view of the thermal management unit of Figure 3, and Figure 5 shows the placement of an apparatus in accordance with the invention in place on both sides of a wall.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Figure 2 shows a wall 1 in its initial state, prior to the measurement process. The internal temperature profile of the wall depends on the current and previous external r temperature and convection acting on the wall outer surface 9 and the current and previous internal temperature and convection acting on the wail inner surface 10, the thermal conductivity of the wail 1, and the thermal mass of the wall 1. Isotherms 2 are shown, each line being the locus of points at the same temperature ("contours') within the wa The exact pattern of the isotherms 2 shown indicative only, as this temperature profiie depends on many variables.

Surface temperature sensors 3 and 5 are attached to measurement module 7 on the wall outer surface 9; preferably this attachment is releasable, using adhesive tape or the like, so that the sensors can, be removed from the. wall without damaging. aby decoration, Surface temperature sensors 4 and 6 are attached' to measurement module 8 on the wail inner surface 10 and are located opposite the corresponding temperature sensors 3 and 5 and measurement module 7.

Air temperature sensors' 11 and 12 are attached via short posts to measurement modules 7 and 8 respectively., Portable IT device 60 (see Figure 5) receives temperature information from both measurement modules 7 and 8 via a wireless standard such as itluetoothil, Measurement modules 7 and 8 are identical apart from their serial numbers, so the operator of Portable IT device 60 can identify which measurement module is on the wall inner surface and which on the wail outer surface. Measurement modules 7 and 8 also contain physical location devices, preferably small coils designed to generate or detect a specific medium frequency magnetic field, $o that each can detect the location of the other. Which device is the transmitter, and which is the receiver is determined by the user through the portable IT device, so that the user is able to position the second measurement module installed to be exactly opposite the first measurement module to be installed by referring to the intensity of the received local signal displayed on the portable IT device 60. A gravitational sensor M each of the measurement modules 7 and 8 (the output of which is displayed portable IT *device 60) a lows the modules to be oriented so that the corresponding temperature sensors are directly below the measurement module, and hence also aligned.

The temperature sensors 3" 4, 5 and 6 are preferably physically small and of very low thermal mass so as to disturb the surface convention and heat flow in the wall as little as possible. Temperature measurements are logged and displayed as trend.s on. the portable IT device 60, The two thermal management units, 50 and 51 are both identical, but identifiabie through unique serial numbers. One thermal management unit is allocated for use on the wail inner surface 10 and the other IE allocated for use on the wail' outer surface The two thermal management units are then placed over their corresponding sensors as shown in Figure 3, where temperature sensors 4 and 6 are on the wail inner surface 10, are covered gy thermal management unit 51 and positioned So that the central axis cif thermal management unit 51 aligns with the temperature sensor 4; The thermal management unit 51 contains a core enclosure 26 that along with the wail inner surface 10, creates a core chamber 22. Similarly, guard enclosure 28 along with wall inner surface 10 creates a guard chamber 24. The positioning of thermai management unit 51 is such that it measures the temperature of the wall at the inner edge of the guard chamber 24. Temperature sensor 4 measures the temperature at the centre of the core chamber 22.

Referring to Figure 4 it can be seen that guard seal ring 43 acts to seal guard enclosure 28 to wall inner surface 10; Core seal ring 44 acts to seal core enclosure 26 to wail inner surface 10. Guard seal ring 43 and Core seal ring 44 are composed of soft rubber, and have a wedge shape profile so that they effectively prevent exchange of air, and also define a precise limit and hence surface area of each chamber.

Guard fans 40, 41 and core fan. 42 circulate air within the guard chamber 24 and 20 core cham.ber22, Peltier devices 34 and 35 can transfer heat between the guard chamber 24 and the surrounding air in ether direction, Peltier device 38 can transfer heat in either diredion between the guard chamber 24 and the core chamber 22.

Heat sinks 30, 32, 31, 33, 36, 37 are connected to Peltier devices 34, 35, 38 to aid heat transfer from the either side of the Peltier devices to.the air surrounding the devices, Heat flux sensor 46 is sandwiched between Peltier device 38 and heat sink 37. This precisely measures direction and quantity of the total heat flow intofout-cif the ore chamber 22.

Core enclosure 26 and guard enclosure 28 is preferably made. of a thin structurally robust, thermally insblating material with relatively low thermal mass such as a glass 'loaded composite naterial, Connecting rods 15 serve only to mechanically attach the core enclosure 26 to the gueit enciosure-28 to form the 'thermal management unit. 51. The connecting rods 15 are small diameter, and do not substantially Impede the air flow within the guard charnber 24, Figure 5 shows a preferred way of deploying the two thermal measurement modules, 7 and 8 and the two thermal management units, 50 and 51 to measure the thermal conductivity of the wall 1 of a house. Each thermal management unlit 50 and 51 is supported by an adjustable telescopic leg 53 and 52 respectively. The upper section of each telescopic leg holds control moduies 53 and 52 respectively that are wired to the sensors, Peltier devices and fans within the thermal management units. The control modules contain the necessary electronics to drive the Peltier devices and fans and also receive signals from temperatune sensors (not show) monitoring air temperature within the core chamber and the guard chamber. Other sensors, for example airflow speed sensors, can be incorporated into the core and guard chambers for enhanced information and control.

The control. module 53 is also wired to heat flux sensor 37. The control.module 53 contains rechargeable lithium batteries to enable. reasonable operating time. As the temperature differentials across the Peltier devices are small, they behave as efficient heat pumps and current co.nsumption is relatively small.

The upper ends of control modules 53, 52 are attached on hinged connectors joining them to thermal management units 50, 51. The lower end of each telescopic leg. 54, 55 terminates at a hinged connector Joining it to foot plates 56, 57 respectively, The out plates have a soft rubber lower surface designed for nigh coefficient of friction on a wide variety of surfaces, The hinged connectors and telescopic legs allow the height of the two ti-zermai management units to be adjusted to suit differing conditions, Each control module 53, 52 contain a radio link to the portable IT device, so that a control prooramme in the portable IT cievice 60 can receive, store and process information from and also control, both the measurement modules 31 and 32 and also the control 30 modules 52 and 5$.

Figure 3 shows the isotherms 2 when the control programme has been managing the heat flow in the wall 1 for a relatively short period. It can be seen that the isotherms in the region of the centre of the wall 20 are substantially parallel with the walls. Jr particular, they are pare Id in the coaxial cylinder of wail 19 that is coaxial with the same diameter as, the core chambers 45 and 46. Because the isotherms are parallel with the wall within and immediately surrounding the coaxial cylinder of wall 19, there is negligible heat is flowing either in or out of this region and the coaxial keylinder of wall 19 can be considered as isolated from the rest of the wall, with known cross sectional area, The heat flow through this cylinder is substantially the same as that measured by heat flux sensor 46 in the thermal management unit 5L As the effective cross section area, the heat flux, and the ten-iperature different al across the wall through temperature sensors 3 arid 4 alia. ali known, all the information necessary to calculate the thermal conductivity of the wall material is available. There is also confirmation and a quality indication from the thermal management unit on the other side of the wall that should be reading an equal and opposite heat flux.

To obtain the required isothermal profiles two control loops are active. In the first control loop, the temperature obtained from temperature sensor 6 is controlled so that it matches the temperature obtained from sensor 4. This control is effected by varying the heat flow into guard chamber 24 generated by Peltier devices 35 and 34. In the second control loop, the temperature sensed by temperature sensor 4 within the core diramber 22 is maintained at the original value from this sensor when it was in free air prior to attaching the thermal management unit Si. This control is effected by varying the heat flow into the core chamber 22 by varying the heat flow through Pether device 38. Two comparable control loops are also implemented using the corresponding sensors and Peltier devices on the other side of the wail within the thermal management unit 50.

nc If the initial temperature difference between temperature sensors 3 and 4 is sufficient to obtain reasonably accurate U-values, then the above control loops strictly applies, and this minimises the thermal disturbance of the wall, so that only enough energy is put in to make the isotherms parallel within the central region. If on the other hand the initial temperature difference between 3 and 4 is too low for accurate U-value measurement, then the control temperature set point for the highest temperature side is increased by a certain small amount (of the order of a few degrees C), and the other control set point temperature, beirig iower of the two on the other side of the wall, is reduced by the. same amount. The key point is that the total amount of thermal disturbance, particularly a the centre Inc of the wall, Is mini.mised by putting.substantially equal and opposite, heat inputs Into either side Of the wall to obtain an enhancedTheita-r.

Table of References

1 Wail. 25 * Core enclosure 49 _ 26 Core enclosure -p isotherm

-

3 Temperature sensor Thermal p management unit _ 51 Thermal management unit 27 Guard enclosure 4 Temperature sensor 28; Guard enclosure 52; Control Module. Control Module -Temperature sensor 29 53 6 Temperature sensor 30 I Heat sink 54: Telescopic leg 7 Measurement 31 I module, i i Heat sink 55 Telescopic leg 8 Measurement 32 i Heat sink; 56 Foot plate modtile I I 9 Wall outer surface -p---- .: 33 I Heat sink: 57 hot plate 1 10 Wall inner surface 1 34. * Peltier device I 58 i _: 11 Air teinperature 35 I Peltier device I 59 sensor i 12 Air temperature I. sensor 36 I Heat sink Portable IT device

I

13 37 Heat sink 14 38 Peltier device * * : Connecting rod 39 * I I, 16 40 Guard Fan I p 1 17 41 Guard Fan 42 Core Fan 19 Coaxial ylinder of 43 Guard seal ring I wall 44.

i Core seal ring I re m 45 21 core chamber I 22 Core chamber 46 I Heat flux sensor 23 Guard chamber 47 I 24 Guard chamber 48 It will of course be understood that many variations may be made to the above-described embodiment without departing from the scope of the present invention. For example, the various chambers are described above as being circular, however these could be square, rectangular, or any other shape. There may be a plate or thermal conductor within the volume to assist in transferring heat to or from the surface of the wail; however, this adds weight te the apparatus and presents difficulties in ensuring good contact and thermai conduction with the wait Where different variations or alternadve arrangements are described above, it should be understood that ernbodimen of the invention may incorporate such variations and/or alternatives in any suitable combination.

Claims

A method of determining the Li-Value of a wall comprising i, measuring the initial temperatures of both wait surfaces at two positions which are substantially opposite each other on either side of the wan; a, If the two measured temperatures are different, modifying the temperature of a first dosed volume defining a first predetermined area of w.all surface surrounding at least one of said positions by applying energy to heat or to cool air in the first volume and the first area of wag so that the temperature of the first area of wail is substantially uniform and substantially the same as the initially measuren temperature on that side. of the wall; or P. if the two measured temperatures are substantially the same, modifying the temperature of a first dosed volume defining a first predetermined area of wall surface surrounding at:east one of said positions by applying energy to heat or to cool air in the first volume and the first area of wall SG that the temperature of the first area of wall is substantaliy uniform and differs from the initially measured temperatures; and N. using the measured temperatures, the first predetermined area and the heating or cooling energy to determine the instantaneous. U-Value,
2. A method according to Claim 1 comprising monitoring a succession of instantaneous U-Values until the difference between successive instantaneous LI-Values reaches a *-e) threshold amount, and taking the last U-Value determined as the final U-Value of the wall.
3. A method according to Claim 1 or Claim 2 comprising recording a succession of instantaneou.s U-Values over time until sufficient information has been acquired to extrapolate a representative U-Vèlue, and exapolating the final U-Value of ke wait from the recorded information.A method according any one of Claims 1, 2 or 3 further comprising, where the initial measured temperatures are different, modifying the temperature of a sec dosed volume defining a second predetermined area of wall surface surrounding the other of said positions by apph4ng energy so as to heat or cool the second volume and the second area of wall so that the temperature of the second area of wall is substantially uniform and substantially the same as the initial measured temperature of the said other position.A method according to any one of Claims 1, 2 or 3 ftrthr comprising, ' e measured temperatures are substaritially the same, modifying the temperature of a second closed volume defining a second predetermined area of wail surface surroundino the Other of said positions by applying energy so as to heat or cool the volume and the second area of wall so that the temperature of the second area of wall is substantially uniform and differs from the measured temperature of the said other position,: A method according to any preceding claim further comprising mo temperature of at least one third dosed volume defining.a third predetermined il surface surrounding the. first. and/or second predethrmined: area of wall, and applying energy to heat or cool the third volumes) and the third.area(s) of well so that the temperature of the third area(s) of wall is/are substantially the same as the temperature of the surrounded first and/or seaOnd predetermined area of wall, A method. according to any preceding claim wherain the first and second volumes and/or the first and/or second areas are substantially the same, 8. A method according to any preceding claim c ircuiatino air with second and or third es so that the beating or cooling is assisted by air convection.method. according to any preceding claim comprisinci rieleasably attaching the temperature sensors to the wall surface.10, Apparatus for measuring tne U-Value of a wall comprising a first temperature sensor for mounting to and for measuring the temperature of a surface of one side of the wall and two temperature sensors for mounting to and for measuring the temperature of two spaced points on a surface of an opposite side of the wall to the first temperature sensor and thereof, a first enclosure adapted to be held against the said opposite surface so as to enclose a volume of air against a predetermined area of the vvali and so as to surround one of the two temperature sensors on said surface, a heater/cooler adapted to heat or to cool the volume of air and a processor operatively connected to the temperature sensors and the heaterlcooier, the processor being adapted to control and monitor the energy supplied to the heater/cooler, to monitor the temperatures sensed by the sensors, and to determine the insthntaneous U-Value using the sensed temperatures, the first predetermined area and the heating or cooling energy supplied, 11 Apparatus according to Claim 10 comprising a second enclosure adapted to be held against the said opposite surface so as to enclose a second volume of air and the first enclosure and to surround the other of the two sensors, the second enclosure defining a second predetermined area of the surface of the wan and having a further heaterlcooler adapted to heat or to cool the second volume of air, the processor being operatively connected to the further he,atericooler, and being adapted to control and monitor the energy supplied to the further heaterlcooler and to determine the instantaneous U-Value using in addition the energy supplied to the further heater/cooler.Apparatus according to Claim 10 or Claim 11 wherein the processor is adapted to record a succession of instantaneous U-Values over a period of time until the difference between successive instantaneous U-Values reaches a threshold amount, and to output the last U-Value determined as the final U-Value of the wall.Apparatus according to Claim 10 or 11 further comprising a. control circuit operatively connected to each of the first temperature sensor and to the two temperature sensors, and adapted to transmit the sensed temperatures to the broCesSor, Apparatus according to Cairn 12 further comprising a position locator associated with each control circuit, the position locators being adapted to communicate with each 12. 13. 14,other and to measure the distance between them and/or their relative locations, and an audible andlor visual alert to indicate to an operator when the contrai circuits are located substantially opposite each other on opposite sides of the wall, 15. Apparatus according to any one of Claims 10 to 13 comprising a resilient seal provided around the edge of the or each enclosure and adapted to engage against a surface of the wall so as substantially seal the air within the respective volume, 16. Apparatus according to any one of Cairns 10 to 14 comprising a fan within the or each volume to circulate air within the volume and to 17. Apparatus according to any-one of Claims 10: to 15. further comprising a selectively telescopic leg attached at one of its ends to the exterior of the enclosure outside the volume enclosed and at its other end to a foot adapted to rest on the ground so as to hold the enclosure against the wail, 18. A method of determining the 'Li-Value of a wall comprising: measuring the initial temperatures of both wall surfaces at two positions on either side of the wall, at least one position on one side of the wall being substantially opposite a position on the other side of the wail; modifying the temperature of a first closed volume defining a first predetermined area of wall surface surrounding the at least one position on one side of the wall by applying energy to heat or to cool air in the first volume and the first area of wall so that the temperature of the first area of wall is* substantiaily uniform and substantially the same as the initially measured temperature on that side of the wall; iii, modifying the temperature of a second closed volume defining a second predetermined area of wall surface surrounding the at least one position on another side of the wall by applying energy to heat or to coo; air in the second volume and the second area of wail so that the temperature of the first area of wall is substantially uniform and differs from the initially measured temperatures; and uSind the measured temperatures, the first predetermine° area aria the heating or cooling energy to determine the instantaneous tl-Vah 19. A method according to Cairn 18 further comprising modifying the temperature of at least one third dosed volume defining a third predetermined area of wail surface surrounding the first and/or second predetermined area of wall and applying energy to heat or cool the third volume(s) and the third area(s) of waii so that the temperature of the third area(s) of wall Isiare substandally the same as the temperature of the surrounded first and/or second predetermined area of wall.20. A method according to Cairn 6, Claim 18 o ciaim 19 wherein the nioGificatior temperatures comprises heatino or cooling the. first volume, and cooling or heating the second volume, 21, A method of determining the U-Value of 11 substantially as herelnbefore described and with reference to Figures 2 to 5 of the accoimpanying drawings, 22. ApparatuS for measuring the U-Value of a wall substantially as nereinbe: ore described and with reference to figures 2 to. 5 of the accompanying drawings