UNDERROOF

Abstract: An underroof for a sloping exterior roof and comprising a moisture absorbing layer (1) which on both sides comprise spaced, substantially horizontal highly diffusion resistant zones (2, 3), the diffusion resistant zones (2) on the top side of the moisture absorbing layer (1) partly overlapping the diffusion resistant zones (3) on the underside and being water tightly connected with the diffusion resistant zones on the underside at their upper edges (4). The underroof is impermeable to water flowing along the top surface thereof but has a sufficiently high resistance to diffusion to prevent summer condensation. Furthermore, it is capable of absorbing and discharging condensate formed on its underside.

Description: The present invention relates to an underroof for a sloping exterior roof and comprising a moisture absorbing layer. It is well known to provide underroofs below sloping exterior roofs, such as tiled roofs, slate roofs, and roofs made from asbestos cement slabs. The primary function of such underroofs is to prevent that moisture which may penetrate the exterior roof through the inevitable gaps therein in case of heavy rain fall or snow drift causes damage on the building. The present underroofs typically consist of reinforced plastic films which are both impermeable to water and have a high diffusion resistance. However, the use of such underroofs presents the problem that during the winter months water vapours tend to be condensed on the underside of the underroof in such amounts that severe damage may occur. In order to prevent such damage it is normally required that the loft be effectively ventilated. However, in practice this is not always possible. If the loft has been converted into living quarters or if the ceiling located underneath the underroof extends parallel to the roof surface, the space between the underroof and such rooms or ceiling has to be thermaly insulated. In order to prevent the formation of condensate on the underroof it is normally necessary to provide a vapour barrier on the underside of the thermal insulation and to provide between the underroof and the insulation a space of a width of e.g. 4 cm which space can be vented to the atmosphere. In practice it has been found difficult to avoid the formation of condensate on the underroof because the vapour barrier on the underside of the insulation normally cannot be made sufficiently air tight to prevent humid air from migrating from the underlying rooms to the roof structure and to form condensate on the underroof. Since the underroof normally hangs down between the rafters and tends to contact the insulation layer within small or large zones, the required ventilation space between the top side of the insulation and the underroof normally cannot be obtained. In order to reduce the risk of moisture damage due to the formation of condensate on the underroof, the latter is often made from a material having a certain moisture absorbing capability and a low resistance to diffusion so that the absorbed condensate may diffuse through the underroof to the space between the exterior roof and the underroof which space normally is effectively vented to the atmosphere. Even in cases where the resistance to diffusion of the underroof is low, the rate at which accumulated condensate diffuses to the underroof is relatively low. This is due to the fact that the thin films from which the underroof typically is made have an insignificant resistance to heat transfer compared to the total resistance to heat transfer exhibited by the roof construction. In the well ventilated space between the top side of the underroof and the underside of the exterior roof the relative humidity will be the same as in the free air in which it is typically close to 100% during the winter months. Since the temperature on the moist underside of the underroof will be equal to the temperature of the top side thereof, there will be no partial pressure difference to force the accumulated condensate through the underroof. Therefore, the condensate will remain in the underroof until e.g. sunshine on the roof will force it back through the insulation to the vapour barrier on the underside of the insulation at which it will condensate and cause dripping from the ceiling. A low resistance to diffusion may also cause so-called summer condensation. Summer condensation arises when the roof coating has been wetted by rain and has absorbed moisture and the moisture under the influence of sunlight is forced into the underroof and the underlying heat insulation and is condensed on the relatively cold vapour barrier where it may cause moisture damage and lead to dripping from the ceiling. It is also well known to prepare underroofs of fibrous materials, said underroofs having a top side coated with an essentially water impermeable asphalt layer. The use of such an underroof may also lead to the formation of condensate on its underside and the condensate thus formed cannot penetrate the asphalt layer on the top surface of the underroof. An ideal underroof should have the properties: it should be impermeable to water flowing along the top side of such underroof, it should have a sufficiently high resistance to diffusion so that summer condensation is prevented, and it should be capable of absorbing condensate on its underside and allowing said condensate to migrate to its top side at which it should be capable of being evaporated. If the underroof has these properties, it will not cause the above mentioned condensation problems and the space between the heat insulation and the underroof can be eliminated or it can be used for increasing the thickness of the roofing. The underroof according to the invention is characterized in that both sides of the moisture absorbing layer comprise spaced, substantially horizontal highly diffusion resistant zones, the highly diffusion resistant zones on the top side of the moisture absorbing layer partly overlapping the diffusion resistant zones on the underside and being water tightly connected with the diffusion resistant zones on the underside at their upper edges. The highly diffusion resistant zones preferably consist of strips of a plastic film which is attached to the moisture absorbing layer and the invention will be described with reference to a moisture absorbing layer coated with such strips of plastic film. However, it should be pointed out that the above mentioned zones can also be provided in a different manner, e.g. by applying dense stripes of paint to the moisture absorbing layer. The underroof according to the invention prevents water which is flowing down along the underroof and which is partially absorbed in the moisture absorbing layer from dripping through the underroof. Furthermore, it allows condensate which is formed on the underside of said underroof to be absorbed in the moisture absorbing layer from which it can migrate to the exterior free zones of the moisture absorbing layer and evaporate. Thus, the above mentioned moisture damage which may occur during the winter months is eliminated. The underroof according to the invention also prevents the above mentioned summer condensation because the partially overlapping strips and the intermediate moisture absorbing layer offer such a high total vapour diffusion resistance that moisture cannot be forced through the underroof under the influence of sunlight. The width of the strips is e.g. about 7 cm and the spacing between said strips is preferably about 4 cm. The strips on the top surface of the underroof can be located in such manner that they overlap the strips on the underside with 1-3 cm. Such an overlapping produces a suitable vapour diffusion resistance. When the strips are composed of a thin plastic film, the latter preferebly has a weight of 15-25 g/m² and preferably consist of pylyethylene, polypropylene, or the like. The moisture absorbing layer preferably consists of synthetic fibres such as polypropylene fibres or glass fibres and the layer preferably has a thickness corresponding to a weight of 100-200 g/m². The water tight connections between the strips on the opposite sides of the moisture absorbing layer are preferably provided by welding through the moisture absorbing layer. The invention will now be described in further detail with reference to the drawing which shows a vertical cross-sectional view through an underroof according to the invention on an enlarged scale. In the drawing 1 is a moisture absorbing layer consisting of synthetic plastic fibres. Both sides of the layer are coated with strips 2,3 of a plastic film. The film strips 2 on the top side of the underroof are at their upper edges connected with the film strips 3 on the underside by means of welding seams 4. Arrows 5 indicate how the condensate formed on the underside of the underroof moves towards the uncoated zones on the exterior side where it evaporates.

Claims: 1. An underroof for a sloping exterior roof and comprising a moisture absorbing layer, characterized in that both sides of said moisture absorbing layer comprise spaced, substantially horizontal, highly diffusion resistant zones, the diffusion resistant zones on the top side of the moisture absorbing layer partly overlapping the diffusion resistant zones on the underside and being water tightly connected with the diffusion resistant zones on the underside at their upper edges. 2. An underroof according to claim 1, characteriz­ ed in that both sides of the moisture absorbing layer are coated with spaced strips of a plastic film. 3. An underroof according to claim 2, characteriz­ ed in that the plastic film strips on the top side are connected with those on the underside through welding seams.

Inventors: KORSGAARD, VAGN
Applicants: V.I.K. CONSULT APS
Application year: 1987
Main classification: E04B1
IPC codes: E04D12, E04B7