Tips on Assemblies – Above Green

Air Barrier

Mid-wall is the best place for the air barrier, exterior or interior placement is more vulnerable.

Two Wall Types

Keep in mind the “perfect wall” concept - structure to the inside, insulation to the outside. Diffusion open or exterior rigid foam, both versions can work moisture-wise and superinsulated.

Slab

Moisture barrier and air barrier are typically placed between foam layer and slab.
Polyisocyanurate insulation is typically not to be used below grade. Thoroughly follow the application guidance of the manufacturer’s warranty and any ICC Evaluation Service report that pertains.

Things to Avoid

Exterior Load Bearing Double-Stud Wall: This wall type is discouraged in climate zones 3 and higher. These will generally incur additional certification cost for hygrothermal analysis (WUFI), need a spray foam global warming potential (GWP) impact calculation, or both. They tend to be too vapor-closed on the outside for those climates and also put the air barrier on the exterior where it will be less durable than a mid-wall air barrier.

In heating dominated climates, double stud walls should be of the interior load-bearing type, either Larsen truss (standoff truss) or the kind described in.

When insulation with a high global warming potential is used (some closed cell sprayfoam or XPS), PHIUS may ask the project team to complete a global warming potential calculation to assess the overall impact of the insulation.

“Vapor-sandwiched” or “double vapor barrier” assemblies: These should generally be avoided and can make a project uncertifiable. There are some subtleties, e.g., it may be acceptable in compact roof assemblies. See these articles for a more detailed discussion [BSI-073] [BSI-092]. That discussion starts with "For the record, it makes no sense to have a vapor barrier on both sides of an assembly. If the assembly gets wet or starts out wet we are pretty much doomed…except if we are perfect and we start out dry.”