If you plan to install a layer of continuous rigid foam on the exterior side of your wall sheathing, where do you put the housewrap? There are many opinions on this issue, but the usual answer is that the housewrap can be installed either between the sheathing and the rigid foam, or on the exterior side of the rigid foam. Either approach can work, as long as the builder knows which layer will function as the home’s water-resistive barrier (WRB), and as long as the WRB is integrated with the window flashing, door flashing, and penetration flashing. (For more information on this issue, see “Where Does the Housewrap Go?”)
The first time that a builder sandwiches housewrap between his wall sheathing and a layer of rigid foam, he or she may feel uncomfortable. Among the questions we’ve received on this issue are the following: How can the housewrap work as a WRB when there is no clear drainage path for liquid water? Shouldn’t I be installing one of the wrinkled or bumpy housewraps in this location to facilitate drainage? (For more information on wrinkled housewraps, see the Fine Homebuilding article, “Are Drainable Housewraps Enough?”, or the GBA article, “All About Rainscreens.”)
Here’s the short answer: It’s perfectly OK to install a wrinkled housewrap in this location if you want to. But wrinkled housewrap may not be necessary.
To address the question of when wrinkled housewrap is nessesary, it’s helpful to step back and broaden the discussion somewhat—specifically, to establish when walls need a rainscreen gap. Before delving into rainscreens, though, I’d like to tip my hat to building scientist Joseph Lstiburek, on whose analysis I rely heavily. (For those who want to read what Lstiburek has to say on rainscreens and wrinkled housewrap, I recommend…