Flat Roof Insulation Vapour Barrier

A roof design that includes an adhered roof membrane with multiple layers of insulation with board joints offset and staggered over a vapor retarder air barrier helps lower the risk that air and the moisture it carries will infiltrate the roof system.

Flat roof insulation vapour barrier.

Classic compact flat roof the most famous double vapor barrier of them all. An airspace is left above the ceiling insulation and below the under side of the roof decking and the roof is vented either around its perimeter with soffit vents a flat roof or the roof may be include intake venting at its lower edge through a soffit and outlet venting through a half ridge vent or similar outlet vent along the roof s uppermost edge a low sloped roof. High density foam insulation partially fills roof rafter cavity and wall cavity. We are insulating a flat roof with tapered rigid insulation over a moisture barrier and then capped with modified bitumen.

Vapour retarders frequently are used in flat roof construction to prevent moist air from the inside of the building condensing onto the roof assembly and potentially causing damage to materials. However we lose r value where the taper is thinnest and need to insulate under the sheeting. That reduction of air and moisture infiltration can then help improve roof longevity. This barrier is meant to keep moisture from getting to the insulation in the walls and ceilings and it is required by building codes when insulating most houses.

Air impermeable spray foam insulation spray foam protected with thermal barrier in occupied attic space. If the roof is insulated with foil faced insulation blankets the aluminium side can serve as a vapour barrier at the same time. In any case the vapor barrier must point to the warm side. The bottom membrane s primary function is that of an air barrier but it s material characteristics are such that historically it has also been a vapor barrier.

Next to that you may also buy a moisture regulating vapour barrier membrane that can manage the damp even better. Vapor barriers are sheets of plastic or other material placed on one side of insulation sheets. No interior vapor retarder required in any climate zone with high density spray foam insulation. However note that one should seal the seams here as well.

Unvented flat roof assembly there is.