A roof underlay is a layer under the exterior roof that has a primary function of working as the shield that protects the thermal insulation material and the building structure from the effects of the weather. The roof underlay acts as a condensation barrier, which leads condensate water away from the underlay, and thereby also from the insulation material.
The traditional diffusion tight (no vapour through the membrane), which in general is made up of a reinforcing grid of either polyester yarn or polypropylene, which is protected by two layers of low density polyethylene (LDPE). This build–up could also incorporate an additional layer of spun bond that are either hydrophilic (water absorbing) or hydrophobic (water repellent) dependent of the climate and building traditions.
The second type of breather membranes provided by Icopal is high quality coated products that are from a one layer substrate of PP spun bond add a top coating layer of special co-polymer, which gives the breathability.
Insulating roof underlay incorporates a reflective surface which minimises solar gain while also preventing heat loss.