Polyisocyanurate on basement walls
BuildingNewb
| Posted in General Questions on
I was considering putting 1 layer of rigid, dual foul faced R13 Polyiso on my basement walls. Is this alright in climate zone 5? I have read conflicting information about requiring either R10 or R15 in my zone. If it is in fact R15, will putting R13 really pit me at a risk for condensation? I could fill the rest of the cavity with cellulose or fiberglass but read that it’s not advisable. Thanks.
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Replies
Building Newb,
The 2012 IRC (and subsequent codes) calls for a minimum of R-15 basement wall insulation. Local codes vary, however, so the only people who can definitively answer your question work at your local building department. Call them up and ask.
If you skimp on wall insulation and install less than the code minimum, you won't have condensation. You'll just have higher energy bills that a house that is code-compliant.
If you end up building a stud wall on the interior side of R-13 rigid foam, you can safely insulate between the studs with mineral wool if you want.
Thanks so much! Also, in your opinion, do you think it's better to use xps rather than polyiso so water vapor can still diffuse to the interior? Or is it better to use faced /unfaced polyiso?
Building Newb,
This is the third time you've posted a comment in which you have implied that moving moisture through a building assembly is a good thing. It almost always isn't a good thing. It's a bad thing.
On the exterior side of your concrete basement wall is damp soil. You don't want to encourage moisture to move from your damp soil to the interior of your house. So you want your insulation to have a low vapor permeance.
Either polyiso, XPS, or EPS will work in this location, but green builders try to avoid using XPS because XPS is manufactured with a blowing agent that has a high global warming potential. For more information, see Choosing Rigid Foam.
I just didn't know if there was potential to trap moisture behind the foam if it wasn't absolutely perfectly glued to the wall.
Furthermore, 95% of these prefabricated, insulated subfloor products I see use xps on the underside with ventilation channels (for "moisture control"). Therefore I want to be consistent in the type of insulation I use as I'm not sure it makes sense to use an impermeable insulation for walls and a semi-permeable product for the floors.
I'm not asking these questions to be difficult, I'm obviously new at this and while I see the overall moisture control concepts remain constant, I'm not sure how the subtleties of each situation (such as the aforementioned subfloor product) would have an overall impact on the project.
Building Newb,
Yes, there is moisture in your concrete wall. You want it to stay there. You don't want to invite it indoors. It isn't "trapped." It just is. The world is full of moisture.
The important thing is to make sure that your indoor environment is comfortable. Don't worry about your concrete -- concrete can stay damp for 100 years without being damaged.
For more information on this issue, see Joe Lstiburek Discusses Basement Insulation and Vapor Retarders.