Housewrap left exposed for a year before siding: what UV does and when to replace it

DuPont's product page gives HomeWrap, StuccoWrap and DrainWrap a 120-day UV exposure limit. Past that, DuPont says, the builder can strip the old wrap and replace it or install a new layer of Tyvek over it. Either way the new layer has to be tied into the window and door flashing so the drainage plane stays continuous. DuPont adds that two layers together pass less water vapor than one.

The limit carries weight at inspection. The IRC, in Section R703.2, requires exterior walls to have a water-resistive barrier that runs without gaps behind the siding and says approved materials other than No. 15 felt go on according to the manufacturer's instructions. That pulls the exposure limit into what an inspector can enforce. DuPont's warranty also assumes installation by those instructions, GreenBuildingAdvisor editor Martin Holladay noted.

How fast wrap breaks down depends on how much sun it gets. In a 2016 lab study by Cosella-Dörken, a WRB maker, reported by building scientist Allison Bailes, some products lost half or more of their water resistance after 500 hours under UV lamps. By the study's conversion, that equals roughly three months in south Florida or 11 months in Toronto. Holladay pointed out that a competitor paid for the work, and the test results were not tied to brand names in Bailes's write-up.

Field reports vary. One GBA reader saw only faded print on the south side of his Typar after about 10 months. A former siding installer wrote that he had pulled siding off old Tyvek with no tear strength left. A wall that has been bare for a year in a sunny climate is three times past DuPont's limit, and the maker's fix is a new layer or a replacement, lapped and flashed, before cladding.