Location of drainable housewrap in wall
chris814
| Posted in General Questions on
I’m looking at the wall design cutaway from Martin Holladay’s “Five Rules for Wall Design” of February 21, 2020 and have a question about the location of the drainable house wrap. It’s shown as being between the sheathing and the exterior rigid insulation. Won’t the water drainage paths on the housewrap thereby provide an air path up/down the wall at this point reducing the effectiveness of the exterior insulation? Why isn’t it better to put the house wrap outboard of the rigid insulation?
I’m in climate zone 5b.
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Replies
Small gaps (less than 1/8") between insulation layers don't allow for enough air movement to effect the R value of the wall. You won't loose any insulation value by putting your WRB between foam and the structure.
It is best to have the house wrap in the same plane as your window flange. So if you have your windows against the sheathing, housewrap should be behind the foam.
If you have outie windows on bucks, best to have the WRB above the foam layer.
This makes detailing the drainage layer much easier and less chance of the installer making a mistake.
Joe Lstiburek reinforcing what Akos said:
"But won’t the tiny gap cause a loss of thermal performance of the foam sheathing? Yes. How much? About 5 percent of the thermal performance of the foam sheathing (not the entire wall assembly) with the 1/8-inch gap, less with a smaller gap. With “crinkly” stuff you loose next to nothing. Is it worth it? Yes, in my opinion, the loss in thermal performance is trivial compared to the reduced risk and improved durability."
Hi user-7672941 (It would be great to get your real name).
You may find this article helpful: Where does the Housewrap Go?
Thanks everyone - food for thought. Sorry about the no-name, didn't realize my profile wasn't up to date. chris814