Exterior foam: what does house wrap attach to at the top and bottom?
airfix
| Posted in General Questions on
With exterior foam what does the house wrap attach to at the top and bottom of the wall?
I assume normally the house wrap is taped to the sheathing at the top and bottom but with the foam extending all the way up there is no exposed sheathing.
Steve
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Replies
Steve, it depends on your assembly. Most of us put the WRB directly on the sheathing (or rely on Zip sheathing to double as the WRB) with the insulation on the outside of that. Is your foam already in place? What is your air barrier at the ceiling or roof? What is your foundation and floor detail?
Yes the foam is already in place. We are doing outtie windows so per the information here we planned the wrap on the outside of the foam. However the guides here don't explain how to attach the WRB.
We have an unvented shed style roof. The air barrier at the roof to the wall is closed cell spray foam. Somehow we need to link the house wrap to that I guess?
Steve
At the top I staple it to the roof sheathing, just make sure it doesn't stick out past the siding+trim.
At the bottom I cut at about 1/2" longer than the foam and let it rest on the flashing at the bottom of the wall. If you are using perf flashing, make sure it is not covering all the drain holes.
Akos,
I'm not sure how I would attach the WRB to the roof sheathing? We have 3 foot eves. The wrap would have to go out over the eves and around the fascia to attach to the sheathing.
If I tape the WRB to the top of the foam then there is that gap between the top edge of the foam to the sheathing.
See the attached picture. How do I seal the WRB to the underside of the eves or the sheathing or the spray foam in my unvented roof?
Steve
Edit: Added picture
Steve,
The air barrier is best handled under the Tyvek. The rafter areas can be filled up with pieces of polyiso and the edges sealed with canned foam or completely sprayed with a small froth pack. Detailing house wrap as an air barrier is hard, trying to air seal that area with it is futile.
Acting as a WRB, there is little chance of water ingress directly under overhangs, you don't need to go overboard, just make sure it won't flop around. Stapling it to the underside of the rafters and taping it to the top of the foam should keep it in place. The rain screen strapping is what will secure it properly in place.
Akos,
Thanks for the reply. I think I gave the wrong idea above. We are not using the house wrap as an air barrier. Our air barrier is our sheathing that's tied into our roof air barrier (spray foam) with spray foam at the rafter blocking that can be seen in the picture. I only mentioned the possibility of tying the house wrap into that to prevent water getting over the top of the foam and between the foam and the sheathing.
I agree that if I staple the wrap to the underside of the rafters there is not much chance of water getting behind it. I was concerned because of our shed style roof with the high side eves being more exposed than the low side eves and the potential for water to trickle down the eve's and get behind the house wrap.
Is the way that you describe if stapling the house wrap to the rafters how it is typically done or do I have to explain this detail to my general contractor to make sure it's done correctly?
Steve
Typical install is running the house wrap up to the bottom of the rafters and stapling to the sheathing. Because of the foam, this can't be done, just need a way to hold it in place while the strapping goes up. I would leave it up to the contractor on how to keep it in place, this is not a critical detail. The way I've done it is slit the house wrap and staple it to the roof sheathing, that works well for 24OC, looks like you are 12" which would be pretty tight.
The important part is the details around your windows and doors, make sure they get those right.