Ronning's Ghost wrote: ↑Mon Oct 19, 2026 12:05 pm
BCExpat wrote: ↑Mon Oct 19, 2026 10:56 am
specify EPDM roofing material as the wall air/vapour barrier
And that wouldn't have worked because wall vapour barriers are supposed to breath, and EPDM water impermeable ?
Well, here are the definitions of each:
Vapour barrier - as per the term it stops water vapour from passing through from the warm side (interior) to the exterior. A material is considered a vapour barrier if it has a perm rating of 1.0 or less. Note: all vapour barriers and vapour retarders are installed on the warm side of the insulation. If water vapour gets through the wall into the insulation, it will condense when it hits the dew point in the wall. The vapour barrier and vapour retarder prevent that (for the most part)
Vapour retarder - this is a material that has a vapour transmission of more than 1 perm. Polyethylene on it's own does have a perm rating of 1, so it is considered a vapour barrier until it is installed. When you take into account all the penetrations and perimeters, the vapour transmittance exceeds 1 perm so when installed, it is considered a vapour retarder.
Air barrier: on it's own, an air barrier is a water barrier (for rain water getting through the exterior cladding). This is basically the rain screen principle. A typical air barrier would be something like Tyvek. This type of air barrier is breathable - which seems to contradict the name, but it does keep water and wind from passing through.
So on typical residential construction, the exterior sheathing membrane (air barrier) goes on the outside of the sheathing prior to installation of the cladding and provides a water barrier and a breathable membrane. The poly vapour retarder goes on the inside of the insulation to prevent moist warm interior air from passing through into the insulation cavity thus preventing condensation within the insulation space, leading to mould and wood and drywall rot.
Air/Vapour barrier: this is usually used more on commercial projects. it is both a vapour barrier and air barrier. In commercial construction, the insulation is installed outboard of the air/vapour barrier. the rain screen principle is usually incorporated (typically an air space between the face of the insulation and the back side of the exterior cladding, to allow water which penetrates through the cladding to drain through the air space and is expelled to the exterior through weep holes and flashing. Typically and air/vapour barrier is either a peel and stick material or a torched on material (although torch on is not used much anymore due to fire risks). Peel and stick materials might be something like Blue Skin or Perm-a-Barrier and they are rated as vapour barriers because they seal around all penetrations including fasteners (the material is self sealing in the case of fasteners through the material. They are also the air barrier as they stop wind from passing through the wall, but they are not breathable membranes.
As for EPDM, yes, it is a rubber material. It is normally used on large expanses of roof on projects such as ware houses and similar projects. The advantage is that it comes in 50' wide rolls so there aren't many joints. It would be my last choice for most projects as it is very easily penetrated and the leaks are hard to find. The point about it being used as a vapour barrier - attachment to the wall would be difficult, but more importantly, it is not rated as an air/vapour barrier, which was what the moron at DND didn't understand.
