315 400 nm wherein the membrane is held at a distance of one meter from the uv source at a membrane temperature of 140 u00b0 f the membrane does not lose strength and shows no visible signs of damage an example of a suitable breathable membrane is a two layer composite sheet with tyvek u00ae hdpe available from e i du pont de nemours and company as the inner layer and a durable spunbond polypropylene sheet as the outer layer the composite sheet can be made by joining the two layers with an adhesive and subjecting them to a thermal calendering process the temperature of the calendering process should be sufficient to melt the adhesive and the nip pressure should be sufficient to force the molten adhesive around the fibers of the two layers to lock the two layers together mechanically and ensure high delamination strength of the composite sheet other examples of materials suitable for use as the breathable membrane in the invention are spunbond polyolefin nonwoven sheets including for instance a three layer spunbonded polypropylene fabric such as the roofing underlayment sold under the trade name roofshield u00ae available from the a proctor group ltd uk other materials suitable for use as the breathable membrane are a nonwoven sheet comprising sheath core bicomponent melt spun fibers such as described in u s pat no 5 885 909 herein incorporated by reference and a composite sheet comprising multiple layers of sheath core bicomponent melt spun fibers and side by side bicomponent meltblown fibers such.