Basement wall insulation: batts vs. EPS vs.?
Zone 4.5. Just had foundation walls poured. On about 23% grade front to back. Front wall is concrete 12 ft high, 12 in thick. Back wall is all stick walk-out on top of a 6 ft deep buried wall for stability. Side walls step down & transition from concrete to wood framing. Walls are about 50-50 concrete-wood frame.
I’ve read everywhere to not put fiberglass batts in the finished basement due to mold issues. My builder has always put fiberglass in finished basements and thinks my concern is unwarranted. I was thinking about 2 inch EPS on the concrete walls and floors I saw a local house being built that way.
Given the 50-50 nature of the basement, would the fiberglass batts be OK on the concrete walls, or should I stay focused on the EPS or is there another good option?
The way I saw the EPS installed was on the footers before the slab was poured meaning the slab is 2 inches from the walls. While this makes sense from an insulation perspective, seems concerning from a structural one. On top of that, the floor EPS also sat on the footer, meeting the wall EPS. So the slab doesn’t touch the footer at all. Any thoughts on this? As far putting the EPS outside, sounded like detailing the exposed part is too much trouble.
Lastly, the water-proofer offers the option of adding 2 inches of EPS to the inside top 2 ft of the buried back wall, I guess with the theory of the top 2 ft being above the frost line. Thanks for any input.
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Replies
Bruce,
1. Fiberglass is OK (although not great) on the above-grade wood-framed walls.
2. The concrete walls need to be insulated like basement walls -- which means only rigid foam or closed-cell spray foam against the concrete.
3. All of your questions should be answered in this article: "How to Insulate a Basement Wall."
Bruce,
Q. "The way I saw the EPS installed was on the footers before the slab was poured meaning the slab is 2 inches from the walls. While this makes sense from an insulation perspective, seems concerning from a structural one. On top of that, the floor EPS also sat on the footer, meeting the wall EPS. So the slab doesn't touch the footer at all. Any thoughts on this?"
A. It sounds like you are describing the standard way to install insulation under a basement slab. See the 4 details below. The first is from GBA; the second and third are from Building Science Corp.; and the fourth is from a U.S. government lab (Oak Ridge National Laboratory).
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