Fiber-faced rigid foam as basement insulation?
whitenack
| Posted in Green Building Techniques on
I have some extra fiber faced rigid foam sheets that I have left over from our build. Would this be OK to use as wall/floor insulation for a small crawlspace? The fiberglass facing has me wondering if it will be a problem.
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Replies
Clay,
What type of rigid foam are you talking about? I'm guessing that you are talking about polyiso, but I may be guessing wrong.
Oops, sorry. Yes, polyiso.
Fiberlglass faced polyiso works fine on the interior side of foundation walls. Keep the cut edge bottom edge off the slab (or dirt) to keep it from wicking ground moisture.
FWIW: Most of my poured concrete foundation has had 3" reclaimed fiberglass faced polyiso on it for about a decade, covered with 1/2" gypsum board. In the few places I've had to cut into it for things like vents & plumbing there hasn't been even a hint of mold or moisture issues in evidence. Mine stops about 3-5" from the slab due to seasonal high water table issues. (An extended power failure & sump pump failures while I was away had resulted in 2-3" of standing water on the slab a few of times in the 2 decades of my occupancy there. )
Thanks guys. Dana, how much clearance do you recommend (assuming no water table issues)? Just enough to keep it from making contact with the ground or is it best to leave an extra inch or two?
While we are in the same area.. It is my understanding that since I have 3" of exterior rigid foam that covers the rim joist, which is more than enough insulation (in zone 4A) to avoid moisture issues on the inside of that rim joist, an air-permeable insulation is safe to use. Am I correct on that? I know I don't want to use rigid foam on both sides of the rim joist, so I am planning on using the polyiso up and over the concrete wall, then some sort of fluffy stuff in the rim joist cavity. Is fiberglass or roxul acceptable in this instance or should I use something else like cellulose? This is a job I am going to try to attempt myself (I have the best builder...so patient with my efforts to "help") so I want to keep it simple.
Clay,
Q. "How much clearance do you recommend [between the bottom of the polyiso installed on the wall and the slab] (assuming no water table issues)? Just enough to keep it from making contact with the ground or is it best to leave an extra inch or two?"
A. A half inch or 1 inch should be fine.
Q. "It is my understanding that since I have 3 inches of exterior rigid foam that covers the rim joist, which is more than enough insulation (in zone 4A) to avoid moisture issues on the inside of that rim joist, an air-permeable insulation is safe to use. Am I correct on that?"
A. Yes.
Q. "I am planning on using the polyiso up and over the concrete wall, then some sort of fluffy stuff in the rim joist cavity. Is fiberglass or Roxul acceptable in this instance?"
A. Yes.
Thanks Martin (dang! You were up early (or late?))
Last question (I think...) For air sealing that bottom edge of the polyiso....do I seal up that entire 0.5-1" gap or just seal at the bottom edge of the polyiso and leave the rest of the gap exposed?
Clay,
I would put a bead of caulk between the bottom of the polyiso and the wall. I would leave the 1/2 inch to 1-inch gap between the bottom of the polyiso and the slab unsealed.
I used expanding can-foam to seal the foam edges to the wall due to some unevenness in the concrete creating gaps, in some places as much as 1/2". But if it's pretty smooth with gaps no larger than 1/4" caulk would be fine.
What is the best way to secure fiber faced Polyiso to an uneven concrete foundation basement wall?
Steve,
I suggest that you read this article: How to Insulate a Basement Wall.
In that article, I advise: "Rigid foam can be adhered to concrete with foam-compatible adhesive or can be attached with special fasteners like Hilti IDP fasteners or Rodenhouse Plasti-Grip PMF fasteners. (For more information on using Hilti IDP fasteners to attach rigid foam to a basement wall, see Marc Rosenbaum’s article, Basement Insulation — Part 2. For more information on Rodenhouse Plasti-Grip PMF fasteners, see New Green Building Products — June 2013.) To prevent interior air from reaching the cold concrete, make sure to seal the perimeter of each piece of rigid foam with adhesive, caulk, a high-quality European tape, or canned spray foam."
Note that this advice refers to relatively smooth walls of poured concrete. I'm not sure what you mean by an "uneven basement wall" -- but if it isn't relatively smooth and co-planar, you may need to insulate it with closed-cell spray polyurethane foam rather than polyiso.