“Housewrap is an air barrier.”
When housewrap was first marketed to builders in the 1980s, manufacturers touted its benefits as an air barrier. The marketing campaigns were so successful some builders still believe that “housewrap” and “air barrier” are synonyms.
In fact, the most important function of housewrap is as a water-resistive barrier (WRB). Installed between siding and sheathing, a WRB is designed to stop rain that sneaks past the siding.
Housewrap can reduce air leakage between sheathing panel edges somewhat, especially if the housewrap seams are taped. But the cracks between wall sheathing panels don’t account for much of the air leakage in a typical home; the big air leaks are elsewhere.
Air leaks occur in many locations, from the basement to the attic. For example, leaks are common between the top of a concrete foundation and the sill plates, between the subfloor and bottom plates, and around attic access hatches. Significant amounts of air can also leave a house through electrical boxes in partition walls, by traveling up the stud bays and into the attic through cracks between the drywall and the partition top plate. All of these leaks — and many others — need to be addressed before a builder can brag about the tightness of a home’s air barrier.
“Interior vapor retarders are a good way to prevent wet-wall problems.”
Northern builders tend to overestimate the importance of vapor retarders. Worries about vapor-retarder placement are often misguided, since wet-wall problems are usually caused by wind-driven rain or deficient air barriers, not vapor diffusion. Most of these baseless worries concern either the foam sheathing (sometimes vilified as a “wrong-side vapor retarder”) or the lack of an interior vapor retarder.
By keeping wall cavities warm, properly specified and installed foam sheathing actually reduces the chance of condensation inside a wall. And interior polyethylene can be safely omitted from walls — even in cold regions of the country — as long as kraft-faced insulation is used. Almost all walls are free of vapor diffusion problems, in part because even painted drywall provides a fair amount of resistance.
According to the 2007 Supplement to the International Energy Conservation Code, polyethylene vapor retarders are not required in any location in the U.S. In northern climates (Marine Zone 4, as well as Zones 5 through 8), the code requires that walls include an interior vapor retarder; either kraft facing or polyethylene is acceptable.