It is common in heat-insulating roof structures to place an underroofing length on the rafters and across the insulation to prevent water from snow or rain as well as airborne pollution from penetrating. When such an underroofing length is to be laid out, care must be taken to ensure that an air space is provided between the latter and the insulation for the attainment of ventilation of the roof and to prevent condensation on the underroofing length, which condensation might drip down as water therefrom onto the insulation and harm its functioning. If said airspace provides a correct ventilation, an underroofing length made from a water and steam impermeable and water and steam resistant, optionally plane stabilized plast material foil will function substantially without problems.

Due to the increasing demands on heat insulation, thicker layers of insulation than before are used within the building industry, for which reason there will in case of a roof insulation be less space at disposal for the above-mentioned ventilation-securing airspace between the underroofing length and the insulation, and furthermore a greater temperature difference will occur across the layers of insulation.

Due to short times of construction and quick occupancy of new buildings, building humidity often occurs which has to be driven off by strong heating. All this results in a need for building foils, like e. g. under- roofing lengths, which in addition to being water and steam proof and water impermeable now further to an increasing extent have to be reversibly steam per- meable.

Suitable plast material foils for such under- roofing lengths have, as already said, to be water and steam resistant, which first and foremost means that the plast material in such foils must not be chemically degradable by water or steam, not even after use for a long time with attendant risk of aging of this material. Furthermore, these foils have to be mechanically strong in order to substantially resist the influence of tearing and impacts which are inevi- table in a building site. They have, to the same extent as steam impermeable plast material foils, to be able to resist a high water pressure, even during contact with another material ("tent effet") or after local contamination with e. g. detergents, and finally they have, in order to yield the desired effect, to possess a high permeability for steam.

Plast material foils with the above-mentioned properties will not only be suited for underroofing lengths, but may also in a broader sense be usable as building foils for insufficiently ventilated construc- tions, i. e. in addition to said underroofing lengths also as moisture barriers, vapour inhibitors, wind barriers, steam traps, and the like.

Moreover, they might be usable in quite different fields, their use being only limited by a demand for a simultaneous impermeability for water and a good steam permeability. Not limiting examples of such uses might be as insulation material for pipes, as protective covering of damp materials, as tent canvas for accom- modation tents, service tents or storage tents, etc.

Steam permeable and water impermeable plast material foils may be divided into foils made from microporous plast materials and monolithic ones. In respect of the first-mentioned the transport of steam takes place through micropores transversely to the foil, these pores being produced mechanically or by choice of a suitable combination of materials in the manufacture of foil, e. g. by stretching of a mixture of incompatible particles for the attainment of micropores at the particle boundaries. In films of monolithic materials there are, as the name implies, no pores present. The foils are made from special, in particular hydrophilic polymers, and steam is transported through the foil material by activated diffusion, the steam being dissolved in the material at the surface, above which the partial pressure for gaseous water is highest, diffuses through the film and is desorbed from the surface being exposed to a lower steam partial pressure.

The steam permeability for microporous materials depends on the thickness of the material and on the pore width, the risk, however-, of water penetration also increasing with the pore width. In respect of monolithic materials the chemical composition is in addition to the material thickness of particular importance for the steam permeability, and such materials can to a high extent be made safe against water penetration.

From the document EP-B-169 308 an underroofing length for the above purpose is known, said length consisting of an only slightly swellable polyurethane foil covered on one of the sides by a layer of non- woven fabric firmly connected with the polyurethane foil. The length is to protect subjacent construction members and spaces against penetrating thawing drift snow, rain, dust and soot, for which reason it is designed to be waterproof, but it is on the other hand designed to be water permeable to avoid condensation of steam thereon. For such an underroofing length a steam permeability has in practice been obtained of approx.

150-170 g/m2 for 24 hours, which cannot be considered sufficient in regards to the thicker heat insulation layers used nowadays and to a possible driving off of building humidity by means of strong heating units.

DE-C-34 25 795 discloses an underroofing length in shape of a water collecting foil of polyester block amides (PEBA) of hydrophilic type. In this manner the steam permeability may true enough be increased to approx. 1000 g/m2 per 24 hours, but the foil not protected by non-woven fabric is in a building site very much exposed to mechanical damage, which may render it permeable to water.

An attempt to combine the advantages of the two above-mentioned, known underroofing lengths by placing a hydrophilic PEBA-foil on a protective layer of non- woven fabric and by establishing a firm connection between the two layers led to an underroofing length not fit for use, as the PEBA-foil swelled by influence from water and after a short time period detached from the layer of non-woven fabric.

The document EP-A-708 212 describes in an attempt to overcome the above-mentioned problem an underroofing length constituted of three layers, viz. two water and steam permeable and water and steam resistant layers of non-woven fabric and a film-like barrier layer placed between these layers of non-woven fabric, which barrier layer in addition to being able to withstand water and steam should further be impermeable to water, but during water or steam uptake be permeable to steam. The presence of the two layers of non-woven fabric positioned in this manner is said to stabilize the adhesion between these layers and the barrier layer, such that an undesirable loosening of the connection between the layers of non-woven fabric and barrier layers does not occur. It is stated in the document that the barrier layers are preferably made from a plast material having the desired properties in respect of water impermeability and steam permeability, another possible solution being the use of a material which is normally water and steam proof, but which by microper- foration has been made adequately permeable to steam.

The examples state layers of non-woven fabric of polyethylene, polypropylene and polyester fibres and barrier layers made from a water-absorbent, thermoplas- tic polyurethane, a hydrophilic polyether polyamide block copolymer and a water-absorbent polyether ester, and the three-layer composition optionally being reinforced by a netting inserted in the barrier layer, e.g. embedded therein. A steam permeability of 700-1000 g/m2 per 24 hours was obtained hereby.

It has, however, in connection with layers of non- woven fabric, in particular two layers of non-woven fabric surrounding the steam permeable plast foil, appeared to be difficult to obtain liberation of a residual water content therefrom, and this may moreover in case of a local contamination of an underroofing length, which reduces the surface tension for water, lead to permeability for water, if the barrier layer is made from a microporous material. If on the building site the product undergoes mechanical damage, an error/leakage taking place will,. on account of the layers of non-woven fabric, not be immediately visible and might subsequently lead to water penetrating into the construction.

In the light of the above the object of the invention is to provide a reversibly steam transmitting plast material foil for building purposes of the above- mentioned kind, which, apart from retaining mechanical strength, preventing water from penetrating and providing a better steam permeability than is the case in the known lengths described, is of a simple construction.

It has now unexpectedly been found that certain plast material foils, which are known from their use as cover for textile materials, are most suitable for attaining the above-mentioned object.

Accordingly, the reversibly steam transmitting plast material foil for building purposes according to the invention is characterized in that the barrier layer therein is made from a monolithic, hydrophilic and while swelling steam absorbing and steam transmit- ting plastic material with a steam permeability measured according to DIN 53122 at 38°C and 90% rela- tive humidity at a foil thickness of 0.15 mm of at least 1000 g/ (m2 x 24 hours).

According to a preferred embodiment the mono- lithic, hydrophilic and while swelling steam absorbing and steam transmitting plast material with no additives added possesses a tensile strength according to DIN 53 504 of at least 38 MPa, an elongation at break accord- ing to DIN 53 504 of at least 500% and a tear resis- tance according to DIN 53 515 of at least 60 kN/m.

Said plast material is preferably a thermoplastic polyurethane material.

Such materials have a high tensile strength and elongation at break and they are moreover hard-wearing, resistant to chemicals and water as well as to UV degradation. By introducing a greater or lesser amount of suitable hydrophilic groups in their polymeric basal skeleton, the permeability for steam may be adjusted.

Concerning a further control of the mechanical properties, including the W resistance and the colour, as well as steam penetration, one or more additional thermoplastic polyurethanes may be added to said thermoplastic polyurethane material, the first ther- moplastic polyurethane material, in a smaller amount, preferably 0 to 10% by weight and especially 0 to 5% by weight as a master batch containing additives common to the field, such as e. g. finishing aids and UV stabilizers and further extenders and white/coloured pigments. The additives used may, however, also be mixed directly with the utilized first thermoplastic polyurethane material. All additives used are well- known to the skilled man in the field of plast material foils and thus need no detailed comments here.

In an especially preferred embodiment of the invention the plast material used for the plast material foil of the invention comprises 90-100% by weight, preferably 95-100% by weight, of a first thermoplastic polyurethane material, and 0-10% by weight, preferably 0-5% by weight of one or more thermoplastic polyurethane materials in combination with additives common to the field and, optionally, extenders, pigments and/or coloured pigments.

Certain types of thermoplastic polyurethane materials with 5 to 6 times higher steam permeability than standard thermoplastic polyurethanes are now available on the market. These improved polyurethanes are sold under the trade names Estane 58237 and Estanes 58245 (B. F. Goodrich Company, Cleveland, Ohio, USA); the 58245"has according to the manufacturer approx. 50% higher steam permeability than the"58237", but is made of a somewhat softer material. Both the Estane materials are most resistant to water penetration, as, with a thickness of 25y, they are able to resist a water pressure from a water column beyond 10 meters. The materials are according to the sales leaflets of said firm especially suited for use within the textile industry, e. g. as coatings on textiles for sportswear. Nothing has, however, been mentioned or suggested about their suitability for building pur- poses.

According to a preferred embodiment the first thermoplastic polyurethane material is Estane 58237 and/or Estanes 58245.

Reversibly steam transmitting plast material foils for building purposes, in which the barrier layer is made from other monolithic, hydrophilic and while swelling steam absorbing and steam transmitting plast materials other than the ones mentioned specifically above, are also to be considered as belonging within the field of the invention provided their steam per- meability meets the requirements of claim 1.

The present steam transmitting plast material foil has contrary to the hydrophilic PEBA foil according to DE-C-34 25 795 proved most suitable as building foil for direct positioning on or in immediate vicinity of an insulation layer.

With a view to a broader field of application the said plast material foil may, however, also be plane stabilized.

It is known to reinforce non-swelling (water vapour impermeable) barrier foils mechanically by embedding a netting therein of a suitable material which is compatible with the foil material, and the above described already stabilized three layer system above may as mentioned also as an additional reinforce- ment comprise a netting.

It is, however, as far as Applicant knows, not known to reinforce a steam permeable (swelling) barrier layer for water, in particular a monolithic such layer, by embedding therein a reinforcing/plane stabilizing structure without a simultaneous double faced protec- tion and/or reinforcement of the barrier layer with layers of non-woven fabric.

A plane stabilizing by means of a film, a web or a netting laminated with the barrier layer does in view of the above-mentioned discouraging experiences with lamination of a single layer of non-woven fabric on a hydrophilic PEBA foil not appear to be attractive to the skilled man.

It has, however, unexpectedly turned out that not only is a reinforcement possible by means of a web or netting embedded in the barrier layer of the plast material foil according to the invention, in that the embedded structure does not detach from the barrier layer even after a high absorption of water in the barrier layer, but tests with films, webs or nettings laminated on such barrier layers also appear to show a surprising coherence stability for such form stabilizing structures.

In accordance with the above-mentioned, the plast material foil according to the invention intended for building purposes is characterized in that the barrier layer is stabilized by means of a plane stabilizing structure of a water and steam proof and water and steam impermeable material.

In an embodiment thereof the plane stabilizing structure is a film, web or netting laminated with the barrier layer.

In another embodiment the plane stabilizing structure is a web or open netting of mono or multi fi- laments or split fibres embedded in the barrier layer and having a mesh of 0.1 to 5 cm.

The film, web or netting laminated with the barrier layer may be a known material of non-woven fabric, e. g. polyethylene, polypropylene or polyester fibres, or it may be a netting as described hereafter.

The embedded structure is preferably a web or an open netting with a mesh of 0.1 to 5 cm. The structure may be made from natural materials (cellulose, cotton, flax) and is in this case often multifilamentary, i. e. consisting of yarns of thin filaments; it may, however, also be made from synthetic materials and may in this connection be mono-or multifilamentary or consist of split fibres. The choice of material is not especially critical to the invention, as long as the requirements of claim 5 of a water and steam resistant and steam impermeable material, which can be plane stabilizing, are met; among preferred synthetic materials for the reinforcing structure may be mentioned high density polyethylene, polypropylene, nylon and polyester materials. At present a preferred plane stabilizing structure is an open netting of polypropylene split fibres with a mesh of 0.5 to 1.5 cm.

The thickness of the reversibly steam transmitting plast material foil according to the invention may vary within rather wide limits, balancing requirements of mechanical strength, steam permeability and the price of the material against each other. The thickness is normally within the range of 0.02 to 0.5 mm and is preferably 0.1 to 0.2 mm.

The steam permeability of the reversibly steam transmitting plast material foil according to the invention, measured according to DIN 53122 at 38°C and 90% relative humidity at a foil thickness of 0.15 mm, is as mentioned above at least 1000 g/(m2 x 24 hours).

It is preferably more than 1200 g/(m2 x 24 hours) and especially at least 1400 g/ (m2 x 24 hours).

The reversibly steam transmitting plast material foil according to the invention is made by a technique known per se within the field of manufacture of previ- ously known water and steam impermeable plast material foils, and details of the manufacutring are not of importance to the invention and need no detailed discussion here.

In the manufacture of the plast material foil according to the invention, which foil has been plane stabilized by laminating as described above, the plast material foil itself is made, as mentioned above, by a technique known within the field, and a lamination thereof by a film, web or netting is also a commonly known method.

When manufacturing a plast material foil according to the invention, which foil having been plane stabilized by embedding of a web or netting in the barrier layer, the reinforcing structure may be placed on a film of the material used for the barrier layer and the structure either coated with a layer of said material or hot-laminated with an additional film thereon.

The reversibly steam transmitting plast material foil according to the invention is of a simple con- struction, is mechanically strong and has a superior steam permeability compared to known steam transmitting and water retaining underroofing plast material foils.

Moreover, it is transparent, except when plane stabilized by an opaque web or film laminated with the barrier layer or when being very darkly pigmented to resist UV ageing, in contrast to underroofing films reinforced by non-woven fabric on both sides, which makes it easy to inspect a subjacent insulation, and it is on account of its far smaller surface than foils reinforced on both sides by non-woven fabric far less exposed to ageing due to UV action than the films reinforced by non-woven fabric.

Accordingly, the inventive reversibly steam transmitting plast material foil may according to the invention be used as a building foil, either as a foil immediately adjacent to the surface of insulation material facing the deck of the roof, or in a plane stabilized condition as an underroofing length, mois- ture barrier, vapour inhibitor, wind barrier, steam trap and similar building foils. According to the invention the inventive plane stabilized plast material foil may also be used as a tent canvas for accom- modation tents, service tents or storage tents.

The following Example elucidates the invention.

EXAMPLE Using known technique a film with a thickness of 0.075 mm was manufactured from 97% by weight of Estane 58237 and 3% by weight of a master batch from a thermoplastic polyurethane"T50019" (delivered by the firm Goodrich) containing known, but unspecified additives, like finishing aids and UV stabilizers.

Hereupon an open netting of split fibres of poly- propylene was placed as a plane stabilizing structure having a mesh of 1 cm and then, a coating by a thin layer of a melt of the same Estane material was applied.

On the foil obtained, the thickness of which was 0.15 mm, a steam permeability of 1400 g/(m2 x 24 hours) was measured at 38°C and 90% relative humidity.