FR2930623A1 - PRODUCT FOR THERMAL INSULATION - Google Patents

Abstract

The invention relates to a thermal insulation product, formed of a layer structure (s) containing at least one fibrous layer provided with at least one metal coating deposited directly on the surface of at least a portion of the fibers of said fibrous layer, as well as its manufacturing method.

Description

The present invention relates to a product (insulation) for thermal insulation, in particular of buildings and containers (such as tanks, pipes, etc.), as well as its method of obtaining. Insulation used for the thermal insulation of buildings (especially walls or roofs of buildings) is traditionally in the form of wound felts or panels made of mineral wool (glass or rock) up to 300 mm thick or more. For reasons, in particular, of size and flexibility of use, it has more recently appeared desirable to develop insulating products of smaller thickness without adversely affecting the desired properties, in particular thermal insulation. Thus appeared in recent years so-called thin insulators formed of various layers that curb the heat transfer, generally including at least one layer reflecting infrared radiation in the form of a metal sheet (in particular an aluminum foil) usually associated with a polymer film (in particular based on polyolefin (s) or polyester) serving as a support or allowing its assembly to the other layer (s) of the insulation. Thin insulators with thermal performance equivalent to the aforementioned traditional products are however more expensive than the latter. In addition, the presence of metal reflective sheets does not generally allow the breathing of the substrates coated with the insulation (in particular does not allow the passage of water vapor, etc.). The present invention has sought to develop improved insulating products employing reflective materials but which are both efficient and economically reasonable, particularly allowing the breathing of the coated substrates or structures of said products while presenting good thermal performance for a preferentially limited space requirement. This object is achieved thanks to the insulating product according to the invention, formed of a layer structure (s) containing at least one fibrous layer provided (at its surface) with at least one metal coating deposited directly on the surface of at least a portion of the fibers of said fibrous layer. The insulation according to the invention may be in the form of a layer structure (or multilayer or stack of layers or sandwich structure) identical or different selected from different types of layers. It may comprise one or more fibrous layers provided with at least one metal coating according to the present invention, said layers being able to be distributed in one or more layers (or beds) situated at different levels (or different thicknesses) of the insulation.

The fibrous layer according to the invention may be in a form used where appropriate in the insulators, for example in the form of a mat or a felt, or in a form generally not valued as an insulating material, such as a sail (the sails are used if necessary in the insulators on the surface to obtain a particular aspect or a particular quality of touch). These forms of fibrous layer have the advantage of having a large porosity open to the outside, permeable to the diffusion of water vapor. In particular, this layer is in the form of a sheet formed of fibers (particularly son and / or filaments) entangled (forming a porous or discontinuous structure), these fibers being cut or continuous (in general and advantageously it is is cut fiber). Although an ordered structure such as a fabric may optionally be conceivable, it is preferentially and generally a nonwoven type structure such as a veil, or optionally a mat, a felt etc. This nonwoven structure can nevertheless be associated with regularly arranged organized reinforcing elements, such as continuous yarns extending in one or more directions in the surface of the layer, especially in the form of parallel lines or a wire mesh arranged perpendicularly or at a given angle between them constituted by a grid which can be incorporated or associated with the nonwoven layer. The fibrous layer may be formed in a known manner, for example by dry process with deposition of fibers from a spinneret or other fiberizing device on a carpet and optionally junction of the fibers between them by mechanical fastening, in particular by needling, or chemical by means of a binder applied to the fibers (in particular to obtain mats or thick webs) or wetly from a suspension of fibers in a liquid filtered on a carpet (in particular to obtain sails). The fibrous layer may be formed of different types of fibers (mineral fibers such mineral wool such glass wool or rock, etc., and / or optionally organic fibers such as polyester fibers, polyolefin, bi-component organic fibers etc.), fibers of the layer which may be of one type or which may be a mixture of fibers (for example mixture of mineral fibers and organic fibers, and / or composite fibers). Preferably, at least a portion of the fibers of the layer, or, advantageously, all the fibers of the layer, are inorganic / mineral fibers, in particular glass fibers, for example and advantageously fibers of a glass having good reflection and / or absorption properties of infrared radiation (glass E, glass C, high alumina glass, in particular greater than 18% by weight, etc.). Generally, the fibrous layer also comprises at least one binder, which makes it possible, if necessary, to bind the fibers together, this binder generally being (but not necessarily) aqueous-based and generally comprising various organic compounds (resin (s), additive (s) , etc.), this binder being chosen so as to withstand, if necessary, the subsequent processing temperatures. The binder content may be of the order of a few% or tens of% by weight relative to the layer, generally less than or equal to 30% (in the case of the web for example of the order of 15 to 30% by weight). weight of dry matter in relation to the veil).

Advantageously, the fibrous layer mentioned according to the invention has a porosity (expressed by the volume fraction of air present in the layer, namely the proportion in% of the volume of air contained in the layer relative to the overall apparent volume of the layer) greater than 10% (and possibly up to 99% or more), preferably greater than 30%, especially greater than 50% and particularly preferably greater than 80% (in particular between 90 and 99, 8%) and / or has a basis weight of between 30 and 5000 g / m 2, in particular of the order of 30 to 1500 g / m 2, in particular of 30 to 500 g / m 2, for example of the order of 30 to 150 g / m 2. g / m 2, in particular from 50 to 120 g / m 2, for a glass veil. This fibrous layer also advantageously has a permeance to water vapor (measured according to ISO 12572) of greater than 100 perm at 75% relative humidity and greater than 200 perm at 25% relative humidity.

Surprisingly, as it appears in the following description, the combination of such a porous / permeable fibrous layer with water vapor and its metal coating is a sufficiently reflective structure (although discontinuous) while making it possible the respiration of supports coated with said structure, which is not the case of known reflective products incorporating one or more metallized polymer sheets. Advantageously, the fibrous layer coated with at least one metal layer as mentioned in the invention (or the product incorporating said fibrous layer) retains good permeance to water vapor (little or no difference, in particular a reduction not exceeding 20%, generally not exceeding 10 or 15%) with respect to the uncoated fibrous layer (or respectively with respect to the product incorporating the uncoated fibrous layer), said metallized fibrous layer advantageously having a higher permeance at 90 perm at 75% relative humidity and above 190 perm at 25% relative humidity. According to an advantageous embodiment, the insulating product according to the invention comprises, as fiber layer mentioned in the definition of the invention, at least one fiberglass web provided with at least one metal coating. This haze can be obtained by any technique known per se, in particular the so-called wet technique of preparing an aqueous suspension of chopped glass fibers, depositing this suspension in a film on a filter mat subjected to a suction to eliminate a part water from the deposited film, apply a binder composition (to hold the fibers together) on the wet film, dry the veil and crosslink the binder in an oven, then condition the veil in the desired manner, the product final being in the form of a fairly thin sheet (for example with a thickness of the order of 0.2 to 0.8 mm) generally packaged in rolls. This web or thin-film material is traditionally of essentially isotropic structure (no preferential orientation of the fibers) and is optionally qualified by means of the isotropic ratio (ratio of the tensile strength of the web in the machine direction on the web). the tensile strength of the web in the transverse direction) generally of the order of 1 to 1.5, sometimes up to 2. The glass fibers used for the production of webs are preferably in the form of cut son, for example having a length of about ten millimeters, especially 6 to 30 mm, in particular about 10 to 20 mm, but may also be (at least partly) cut to a greater length or continuous.

According to another embodiment, the insulating product according to the invention comprises, as fiber layer mentioned in the definition of the invention, at least one mat of glass fibers or a thick glass mat which can be obtained preferentially by the dry route, and whose thickness can be up to a few millimeters, for example of the order of 0.8 to 5 mm, in particular of 0.8 to 3 mm. According to another embodiment, the insulating product according to the invention comprises as fibrous layer mentioned in the definition of the invention at least one glass wool or rock wool, which can be obtained by conventional fiber drawing techniques for the production of mineral wool, and whose thickness may be of the order of 5 to 200 mm, in particular of 10 to 200 mm (such as one of those traditionally used in insulators), with a density which may be the order of 7 to 50 kg / m3, especially 7 to 25 kg / m3, the layer thus being sufficiently flexible so that the insulating product is able to be packaged in roll. Thus, in general, a product according to the invention may comprise a coated fibrous layer with a thickness chosen between 0.2 and 200 mm. The metal coating deposited directly on the surface of at least a portion of the fibers of the fibrous layer is advantageously a thin surface layer (or film) (in particular of nanometric thickness), this layer being directly deposited on the surface of the layer fibrous, in particular by a technique of deposition by evaporation under vacuum as explained later. This metal layer continuously covers the surface of the exposed fibers on the outside of the layer. In this respect, it may appear continuous at a certain distance from the fibrous layer (respectively of the product), but the close or microscopic examination reveals a discontinuous surface conforming to the surface of the fiber network, the porosity of the coated fibrous layer. the metal layer being approximately preserved. The metal coating is generally provided on at least one face of the fibrous layer (at least one of the faces of larger dimensions), or even at least two faces (in particular can cover the two opposite faces of larger dimensions, which allows in particular to arrange the layer in the product without taking care of a particular orientation). The metal coating may be for example based on aluminum, copper, zinc, silver, titanium, chromium, nickel, and / or other metals or metal compounds (for example conductive metal oxides such as tin-doped indium oxide, aluminum-doped zinc oxide, antimony-doped tin oxide, etc.), especially good conductors of electricity, and / or alloys between these metals (for example a nickel / chromium or silver / copper alloy, etc.), etc., pure (metal used) or almost pure or possibly doped or alloyed (for example to protect the layer of the effects of corrosion or for reasons of ease of deposition or adhesion, etc.). Although the coating of the fibrous layer by a single metal layer is generally sufficient, the fibrous layer may also optionally be coated with several layers, these layers may be of the same material (metal) or of different materials (metal or not, at least one of the layers being metallic according to the invention). The additional layer or layers are preferably of similar structure to the metal layer previously described, namely, continuously coating the surface of the fibers exposed on the outside of the layer. They may nevertheless be of generally discontinuous structure, that is to say that they are deposited in islets on the surface of the fibrous layer. For example, if appropriate, a first metal layer may be surmounted by another layer, for example protection against corrosion (for example a layer of silica), or biocide (for example a layer of copper or silver) etc. The insulating product according to the invention generally comprises one or more (for example up to 10 or more) fibrous layers provided with at least one metal coating as mentioned in the definition of the invention, said fibrous layers possibly being internal (For example in the middle of the product) and / or in at least one of the faces of the product (or on each of its two opposite faces of larger dimensions), the performance generally increasing with the number of fibrous layers coated according to the incorporated / present invention. The insulating product according to the invention may optionally comprise other layers of a nature to provide in particular a thermal insulation function, and / or to confer on the product of the mechanical strength, and / or to allow the connection between the layers of the product , and / or to improve the quality to the touch or to facilitate the handling of the product. By way of nonlimiting examples, it will be possible to choose at least one of: an insulating material based on mineral fibers, or organic fibers of natural origin (vegetable or animal) or synthetic (polymer), such as mineral wool for example, glass wool or rockwool, or hemp, sheep, feather, or polymer fiber felt, in particular based on recycled or non-recycled polyester; preferably in a flexible or semi-rigid rollable form, these insulating materials may be chosen with a density of the order of 7 to 50 kg / m 3, - foam, or other cellular material, organic or mineral, reducing the exchanges thermals, for example a cell structure, of the bubble film or blister type for example (the cells containing in particular air and / or at least one material making it possible to increase the insulating power of the product, such as a rare gas , an airgel, fumed silica, etc.), - another reflective layer not deposited on a fibrous layer (for example in the form of a sheet or film or layer, if necessary initially independent or free-standing or free, that is to say not needing another layer in support, or possibly in the form of a layer deposited on another support layer such as one or more plastic sheets, etc.), - possibly a film to base of material (s) gold synthetic ganic (s) (especially polymers), glue, a hooking grid to facilitate the laying or holding of the product, a veil of mineral and / or organic fibers in particular to constitute at least one external face of the product. In addition, one or more layers of the insulator may be additive, especially when one or more properties and / or functions are sought (presence for example of an infra-red opacifier), and / or comprise one or more binders and and / or oils and / or silicones, etc. Where appropriate (in particular on one and / or the other face of the insulator), one or more layers (in particular the fibrous layer, for example in the form of a sail) may be surfaced or reinforced and / or reinforced ( especially for not tearing) for example by the addition of a grid of organic or inorganic fibers (glass fibers). Also preferably, the product according to the invention comprises only breathable layers, that is to say layers which are permeable to the diffusion of water vapor, in particular porous or perforated (for example based on fibers, and / or open-cell foam, and / or in the form of a perforated film, etc.) or having a permeability to the diffusion of water vapor in at least appropriate ambient conditions (for example an adaptive hygroregulatory membrane, typically based on polyamide or other polymer, such as the product sold under the name VARIO by the company SAINT-GOBAIN ISOVER). In a particular embodiment, the product according to the invention mainly comprises, or even only, essentially mineral-based layers (with the possible exception of binder (s) and / or additive (s) and / or layers or films layer coating) which achieves high levels of fire resistance. Organic layers may, however, be tolerated if they contain fire retardant additives or are otherwise fire resistant, the total amount of organic material being preferably limited depending on the desired level of fire resistance.

The layers forming the product are laid one on top of the other and can be joined together in different ways, in particular the different sheets, films or other (initially) independent layers can be assembled by gluing, stitching, sewing, needling, possibly by welding (thermal, ultrasound) in the presence of suitable materials (eg polymers) in one or more layers or by the provision of a fusible interlayer (such as a hot-melt layer or a polyethylene or polyethylene terephthalate layer) ). The superposed layers may optionally be held together inside an envelope (so-called encapsulation technique) with or without joining the layers together.

The envelope is made of a packaging material, such as a plastic or other film, preferably permeable to air, which may be in tubular form closed or not at the ends. Preferably, the independent layers are made integral by localized or pointwise bonds (by sewing, needling, etc.), for example by lines or points, in various specific locations of the product (for example at the edges or at regular intervals along the length product) especially to maintain a certain flexibility of the product (allowing for example its winding) and avoid thermal bridges. The insulating product according to the invention is preferably of small thickness, its thickness not exceeding in this case 100 mm, and being advantageously less than 50 mm, although advantages are always observed for thicknesses up to 200 mm. . The thickness of each fibrous layer provided with at least one metal coating as defined according to the invention is generally less than about 40 mm and preferably less than 8 mm, in particular less than 5 mm (especially for a layer in the form of a web), the thickness of each metal layer deposited on a fibrous layer is generally less than 2 μm and in particular less than 1 μm, for example of the order of 500 nm or more. The insulating product according to the invention may be in the form of a sheet (generally multilayer) or a sheet or panel (generally multilayer), preferably flexible, and may optionally be packaged wound. The particular structure of the insulating (reflective) product according to the invention, due in particular to the presence of fibrous layers coated with a metal layer as previously mentioned, makes it possible to obtain good thermal insulation properties at the same time as a breathable structure (permeable to the diffusion of water vapor) if appropriate (if it is devoid of other non-breathable layers), having a good mechanical resistance at the same time as thermal and if necessary incombustible (especially when the fibrous structure is formed of mineral fibers), the insulation also remaining economically advantageous. The improved thermal performance of the insulation according to the invention results in values of thermal conductivity λ advantageously less than about 35 mW / mK, and preferably less than about 32 mW / mK (for comparison, the wool mattresses conventional minerals have a thermal conductivity to commonly greater than 35, in particular of the order of 40 mW / mK). Thermal conductivity A (in W / m.K) represents the amount of heat passing through the insulation (one meter thick, per m2 and when the temperature difference between the two faces is 1 ° K). The values of thermal conductivity Δ (compared with identical pressure and temperature, in particular at atmospheric pressure (1 bar) and ambient temperature (between 10 and 25 ° C) are measured on the model of ISO 8301). The performance gain in terms of thermal conductivity related to the presence of at least one fibrous layer coated with at least one metal layer according to the invention can range in particular up to 20% (less in thermal conductivity) compared to same product does not have such a fibrous layer, this gain is generally at least 1% (case of the addition of the fibrous layer to a product already high performance), the insertion into an existing insulation product of a layer fibrous coated with at least one metal layer as mentioned according to the invention to reduce the thermal conductivity of said product is also referred to in the present invention.

In parallel, the insulating product according to the invention generally reflects on the order of at least 30%, or even at least 40%, of the incident infrared radiation (wavelengths of between 5 and 22 μm, the measurements being made according to the technical regulation ACERMI RT A). The invention also relates to a method for improving the thermal performance (including reduction of thermal conductivity) of an insulating product in layer (s), without reducing the ability of the product to breathe, this method of inserting (or comprising at least one step of incorporating, in and / or on the surface of said product, at least one fibrous layer provided with at least one metal coating deposited directly on the surface of at least a portion of the fibers of said fibrous layer . As indicated above, the gain obtained can be significant, for example several% and up to at least 20%. The invention also relates to such a fibrous layer provided with at least one metal coating deposited directly on the surface of at least a portion of the fibers of said fibrous layer, as described above, capable of be used for the manufacture of a thermal insulation product or in other applications. The subject of the invention is also a process for obtaining an insulating product according to the invention, said process comprising at least one step of depositing a metal layer on a fibrous layer (intended to form part of the insulating product, said layer is then assembled where appropriate to one or other layers to form the insulation). The metal layer is preferably deposited on the fibrous layer by a vacuum evaporation technique (of PVD, CVD, optionally assisted by plasma, etc.), activated (for example by cathodic arc, sputtering, beam sputtering). ion or electron, by laser) or not, this technique of evaporating the material to be deposited, from 'targets' arranged around the load, by a thermal effect (temperatures above the melting temperature of the metal and less than its boiling point, assisted if necessary by an electric arc, an electron beam, etc.), the evaporated particles (or if appropriate, especially when the evaporation is activated, the ionized metal vapor) condensing in a continuous thin homogeneous layer (depending on the support to be coated) strongly adhesive on the substrates (here the fibrous layer) to be coated placed in a vacuum chamber. The resulting metallic deposit, of nanometric or micrometric type, for example based on aluminum, copper, etc. (as detailed in more detail above), produces a metallization at the surface of the fibrous layer serving as a support (in the case of a fiber layer in the form of haze, the assembly formed by the support layer and the reflective deposit is thus in the form of a metallized, for example aluminized, foil). Surprisingly one can thus obtain the metallized fibrous layer mentioned in the present invention, in particular despite the porosity of the support (fibrous layer) and the pre-existing fear that the binder or other residues related to the support disturb the deposition of the layer metal (for example by desorption of water) and adversely affect its quality (the binder in particular does not disturb the final deposition of the metal layer). The deposition of each metal layer may optionally be carried out online during manufacture or after manufacture of the fibrous layer, for example by unrolling the fibrous layer, coating it by vacuum evaporation and then rewinding it on a support, these three operations taking place continuously in the same vacuum chamber, rewinding the fibrous layer coated, unexpectedly, not altering it. As indicated above, the method may comprise a step of assembling the different layers of the insulation by means of various operations such as gluing, seaming, seaming, needling, seaming ), located (s) or continuous (s), wrapping (in an envelope permeable to air in particular), etc. The method may finally comprise a finishing and conditioning step, for example comprising a cutting step when the layers are assembled in line in the form of an endless band. Preferably, the insulation is flexible enough to be packaged as a roll which can be contained in a suitable packaging material. The following comparative examples illustrate the present invention without limiting it. In each of these examples, an insulating product consisting of 6 layers of glass wool with a density of 14 kg / m 3 and a thermal conductivity of 33.4 mW / m 2 K of 8 mm thickness each is considered. In the first example used as a reference (product not in accordance with the invention), the insulating product is only formed of the 6 aforementioned layers. In the second example, illustrating the present invention, is intercalated between each previous layer an intermediate layer (5 interlayers in all, the insulation then being formed of two types of layers arranged alternately), each intermediate layer being formed of a metallized glass fiber fleece 0.8 mm thick. This veil is manufactured according to the invention from a veil marketed under the reference APH75 by the company Saint-Gobain Technical Fabrics (and having a porosity of about 95%, a basis weight of 75 g / m 2 and a permeance of 150 perm 75% relative humidity and 210 perm 25% relative humidity, this veil having intrinsically no significant infra-red reflection property, the latter being barely 5%), that 2 μm thick aluminum metal layer is deposited on one of its faces by the technique of vacuum evaporation. In a Balzers evaporator where an aluminum atmosphere is produced at a pressure of 2.10-5 mbar and a temperature lower than 30 ° C, the glass veil is scrolled with one side exposed to the aluminum atmosphere at a speed adapted to obtain a deposition rate (growth rate of the aluminum layer on the fibers) of the order of 2 nm / s, the temperature of the web being less than 50 ° C. The coated veil is characterized by the ability to reflect infra-red radiation at 40% of the incident infrared radiation (metallized side, for wavelengths between 5 and 22 pm, the measurements being made according to the technical regulation ACERMI RT A), the permeability of the coated web being approximately conserved, said coated web having a permeance of 100 perm at 75% relative humidity and 205 perm at 25% relative humidity. The insulating product of the present example illustrating the invention is manufactured by superposition of a layer of mineral wool, then of an aluminized veil with its face coated on the top, then this stacking sequence is repeated 4 times, and the we finish with a layer of mineral wool. The product obtained has a final thickness of 52 mm. The thermal conductivity, measured according to the ISO 8301 standard, on 300mmX300mm test pieces of these products, is 33.4 mW / mK for the reference example, and 31.7 mW / mK for the example according to the invention is a gain in thermal performance of 5%. It is also observed that the reflection and transmission properties of the infra-red radiation of the metallized veil used are preserved over time. In particular, after aging in a climate oven at 54 ° C. and 95% humidity for 15 days, the transmission and the infrared reflection measured on the metallized film remain unchanged (in particular the infrared reflection on the metallized side remains high, of the order 40% for wavelengths in particular between 5 and 22 pm, the measurements being made according to technical regulation ACERMI RT A). The insulation according to the invention is particularly suitable for use inside the building envelopes, especially in the attic, on the walls or roofs inside the houses, for the insulation of containers (heater water, tub, hose, etc.), etc.

Claims (19)

REVENDICATIONS1. Thermal insulation product, formed of a layer structure (s) containing at least one fibrous layer provided with at least one metal coating deposited directly on the surface of at least a portion of the fibers of said fibrous layer. 2. Insulating product according to claim 1, characterized in that said fibrous layer is a nonwoven, in particular is a veil or a mat or felt. 3. Insulating product according to one of claims 1 to 2, characterized in that at least a portion of the fibers of said fibrous layer are mineral fibers, in particular glass fibers. 4. Insulating product according to one of claims 1 to 3, characterized in that said fibrous layer has a porosity greater than 10%, preferably greater than 30%, and particularly preferably greater than 80% and / or present a basis weight of between 30 and 5000 g / m 2, for example of the order of 30 to 500 g / m 2 for a glass veil. 5. Insulating product according to one of claims 1 to 4, characterized in that said fibrous layer has a water vapor permeance greater than 100 perm at 75% relative humidity and greater than 200 perm 25% d 'relative humidity. 6. Insulating product according to one of claims 1 to 5, characterized in that the metal coating of said fibrous layer consists of a single metal layer. 7. Insulating product according to one of claims 1 to 6, characterized in that said fibrous layer is provided with at least two metal coatings deposited each directly on a surface of the fibrous layer. 8. Insulating product according to one of claims 1 to 7, characterized in that the metal coating is based on aluminum, copper, zinc, silver, titanium, chromium, nickel, or alloys of these metals together with and / or other metals or metal compounds. 9. Insulating product according to one of claims 1 to 8, characterized in that it comprises at least one fibrous layer provided with at least one metal coating associated with at least one layer selected from an insulating material based on mineral fibers or organic of natural or synthetic origin, or based on foam, or other cellular material, organic or mineral, reducing heat exchange. 10. Insulating product according to one of claims 1 to 9, characterized in that it comprises only mineral-based layers. 11. Insulating product according to one of claims 1 to 10, characterized in that its thickness does not exceed 100 mm. 12. Insulating product according to one of claims 1 to 11, characterized in that it is flexible enough to be packaged in roll form. 13. Process for improving the thermal performance of an insulating product in a layer (s), said method consisting in inserting, in and / or at the surface of said product, at least one fibrous layer provided with at least one metal coating deposited directly. on the surface of at least a portion of the fibers of said fibrous layer. 14. Process for obtaining an insulating product according to one of claims 1 to 12, said method comprising at least one step of depositing a metal layer on a fibrous layer. 15. The method of claim 14, characterized in that the metal layer is deposited on the fibrous layer by a vacuum evaporation technique. 16. Method according to one of claims 14 or 15, characterized in that the deposition of each metal layer is performed in line during manufacture or after manufacture of the fibrous layer, for example by unrolling the fibrous layer, the coating by evaporation under vacuum and then rewinding on a support, these three operations taking place continuously in the same vacuum chamber. 30 17. A fibrous layer having at least one metal coating deposited directly on the surface of at least a portion of the fibers of said fibrous layer. 18. The fibrous layer according to claim 17, comprising a nonwoven of mineral or organic fibers, such as a veil, a mat, or a felt, of which at least one surface has been exposed to a metallic vapor deposition. 19. Fibrous layer according to claim 17 or 18, characterized in that it reflects infra-red radiation at a rate of at least 30% of the incident radiation.