Do I need house wrap under exterior wall rigid foam ?
steveworks
| Posted in Green Building Techniques on
Hi,
Do I need to put house wrap ( Tyvek etc) between exterior 3/4 “iso rigid foam (taped)? Can I go naked or just 15 lb felt?
I’m looking at tech drawings on the site and it’s not shown.
My guess is not, but why not ask before I go through all this an d do something dumb.
Thanks
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As you probably know, every wall needs a water-resistive barrier (WRB) that is integrated with wall and door flashing. You can choose from three possible WRBs: asphalt felt works fine; so does plastic housewrap; and it's even possible to use rigid foam as a WRB. Whichever product you choose for a WRB, be sure that it's integrated properly with other flashings on the wall.
If you use felt or housewrap for a WRB, it can go under or over the foam. Either way works. However, since most builders install windows on top of the foam, it's easier to integrate the WRB with window flanges if the felt or housewrap goes over rather than under the foam.
Some builders use the foam itself as a WRB. In that case, it's acceptable to omit the felt or housewrap. But you will be depending on the integrity of tape rather than overlaps to be sure that water doesn't get through the seams of your WRB. Whether or not this system is acceptable depends upon how you feel about the longevity of the adhesive in your chosen tape.
Do you mean between the ridgid foam board and the sheathing?
In addition to Martin's caveat about the longevity of tape, I would also consider any gables that are above the thermal envelope (unconditioned attic?). The WRB must be continuous along all outside wall surfaces, including gables (where it's often ignored).
Given the importance of the WRB to the long-term durabiliy of the structure, why compromise? I would recommend a WRB, whether felt or polymeric, outside the insulation - since it's function is to prevent weather from intruding into the thermal envelope and the exterior foam is part of the thermal protection and could be undermined by water penetration and freeze/thaw cycles.
Also, given that the exterior foam is an exterior vapor barrier, I would take extra precautions to prevent moisture accumulation in the sheathing or stud cavities, both from outside and from inside - since drying to the exterior (dominant wintertime direction) would be prevented. Alternatively, use a more vapor permeable exterior foam board, such as 1" or thinner XPS.