Will adding more insulation in basement create a problem?
Hello,
I’ve got a large basement space which I’m renovating into a living area. It’s in zone 5, Ontario, Canada.
Floor, and Lower 2/3 of walls are bare concrete, upper 1/3 is fiberglass insulated stud wall with drywall.
Walls had been insulated with 1-1/2″ of paper faced fiberglass bats between furring strips on which hung wood paneling. Paneling and fiberglass have been removed after 20-30 years, without any signs of mold.
I’d like to increase the insulation on the concrete by adding more fiberglass, but am concerned about vapor condensation. The old system may have worked due to how little insulation was being used.
Will adding an extra 3″ of insulation over the previous 1-1/2″ create a problem?
Foam is last resort for me.
Thanks for your advice,
-Richard
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Replies
Richard,
It's always risky to insulate below-grade concrete walls on the interior with fiberglass batts. I recommend that you insulate the concrete with rigid foam or closed-cell spray foam.
For more information, see How to Insulate a Basement Wall.
When going higher-R on the foundation wall, it's safer to use at least some foam between the fiber and foundation for wintertime dew-point control, since an interior side vapor barrier would trap ground moisture in the fiber & furring/stud layers. But in a climate as warm as yours it doesn't take much foam.
In a southern Ontario climate comparable to US climate zone 5, and inch of foam against the concrete with seams & edges sealed with an interior side 2x4 wall insulated with R13 batts does not need an interior side vapor retarder, and is pretty much code-min under IRC 2012. With 1/2" wall board that takes up 5" of space between the concrete and interior. If that's too much, 3" of foam board held in place with 1x4 furring through-screwed to the foundation with masonry screws gets you comparable performance at only 4.25".
If the stud edges of the pony wall on top of the foundation is flush with the concrete, it's fine to go from the slab to the joist bottoms with 1-3" of EPS, and no interior side vapor retarder, leaving the existing kraft faced batts in place. If there's a bit of ledge, adding enough EPS to the studwall to make it co-planar with the concrete THEN adding a continuous layer of EPS up to the joist bottoms still works.
Unfaced EPS is still semi-permeable to water vapor at 3", and does not form a serious moisture trap. Other foams, or foams with facers are much lower permeance, and present more issues, but can be used if chosen judicioiusly for location withing the stackup.
Thanks for the detailed responses to my question.
I believe the choice has been made to install 1" of closed cell spray foam, with a mineral wool insulated stud wall in front to accommodate plumbing and electrical.
The approach should work- just don't let a less-informed inspector force you to install polysheeting or some other extreme vapor barrier on the interior side of the studwall, since that would increase the mold/rot potential of the studs themselves.
If the slab is not insulated below, put an inch of EPS between the bottom plate and the slab as a capillary & thermal break, to keep ground moisture from wicking into the framing, and to keep the bottom plate above the summertime dew point temperature, both of which would otherwise contribute to mold & rot issues.