Exterior foam has become a standard method, both in new construction and renovation, to improve the thermal envelope and reduce thermal bridging. Unfortunately, as with most technical "solutions", there are unintended consequences.

In the MEWS study performed by the National Research Council of Canada, for instance, when small amounts of water were deliberately allowed entry into the wall structure (simulating reasonably expected leakage), walls with exterior foam did not dry out for the entire two years of the simulation. And, because the "outsulation" was doing what it was supposed to - keeping sheathing and framing warm - it maintained perfect mold and rot conditions indefinitely.

"If plywood sheathing you may not need the tyvek, but you might tape all the joints for an air barrier."

Most construction tapes are not permanent enough to rely on as an air barrier, let alone a weather barrier. In another study, in which WRBs were subjected to water under pressure (simulating wind-driven rain), 50% failed at the taped diagonal cuts in the plastic housewraps over windows. These were tapes designed for compatibility with housewraps, and yet still failed. The same tapes are not designed for long-term adherence to plywood or osb.

- Robert Riversong

master housewright