Insulation – combining
Climate zone 7 Canadian prairies
cape cod style house with unconditioned unvented attic
I’m installing 3” EPS type II rigid foam on attic side of kneewall.
The rafters notch into the top plate of the kneewall. To cut the foam to fit the around the rafters I need to create an angled notch. This is proving to be quite difficult.
Because my angled notches in the foam are not precise there are gaps between the rafters and the foam. Are there any issues – condensation etc – that I need to be aware of if I use canned spray foam fto fill in the gaps? Roxul mineral wool will be used to fill the 2×4 stud bays and I am running smart vapor over the kneewall followed by drywall.
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Replies
I don't think so. Cutting rigid farm precisely, even into simple shapes is challenging. And even if you did get a nice tight fit, the chances of the foam being air tight are slim. So, consider it part of your air barrier and leave a gap to be filled with canned spray foam. You can seal the rest of the panels with the canned foam as well and that will help keep conditioned air out of the walls which is how most moisture gets into walls and collects in the first place.
Knowing I can use spray foam and not cause any issues with condensation is a huge relief.
I've been sticking the foam panels together using LePage's PL 330 foam board adhesive. I think I will tape the seams too so that they are airtight.
I’ve done this lots of times (with XPS, but that shouldn’t matter) without any problems. Canned foam is used all the time to seal up small leaks and to fill gaps around things. If your intent is to ensure a good airseal then you need to be sure you do a good job with the canned foam and don’t leave any gaps. If you are only trying to insulate the small space between your EPS board and the framing members, you just need to fill the void with the canned foam.
Bill