Quote:
Originally Posted by Shin Pad
I've had this discussion with Mass_Nerder before, but polyethylene is not a vapour barrier - it's a vapour re tar der. The thing is, the sheet material might meet the requirements of a vapour barrier, but when you have it installed on the wall, with all the penetrations, etc, it ends up with a perm rating above 1 US perm - thus it is not considered a vapour re tar der not a vapour barrier. Anyway, you and Topfiverecords are both sort of right. The vapour re tar der always goes on the warm side of the insulation. The air barrier can be anywhere in the wall system - it doesn't have to be at the exterior side or interior side (unless you are doing an insulated stud cavity wall like in residential construction where the air barrier (building wrap) goes on the outside of the wall sheathing.
In commercial construction where you have a backup wall - whether it is stud/sheathing or concrete block or concrete or whatever, and the insulation goes on the outside of that backup wall, then you would normally be using a material that is both an air barrier and a vapour barrier (not vapour re tar der) - such as a self adhered membrane like blue skin or similar, which would be installed on the outside face of the backup wall prior to installation of the insulation which would normally be some sort of board insulation and would normally be mechanically fastened through to the studs in the backup wall. Self adhered membranes are considered air/vapour barriers because when installed they seal around screw penetrations, and around all other penetrations through the wall thus giving a perm rating of 1 US perm or less over then entire installation) The cladding would be installed on the outside of the insulation, with an air space between the outside face of the insulation and back side of the cladding - so you would have a "rain screen" type wall (I'm simplifying this but there is a lot more to this than I care to type here).
Anyway, I have worked as an Architectural Technologist (specializing in Architectural Specifications), for 40 years before I retired 2 years ago. Building science is my thing.  |
I believe the correct term now is barrier challenged.
It is a barrier when it leaves the factory on the roll so you spec for a barrier to be installed, not to turn it into a barrier challenged material prior to installation. When it becomes barrier challenged due to the semantics penetrating it, that’s after my specifications.