Sanity check on install of exterior foam on foundation walls
Builder put up foamboard on my foundation walls while I was out of town, and I have some concerns, but I don’t know if they’re worth complaining about so I’m asking here first.
– The basement has a high ceiling, so while the first run of boards was vertically oriented, the run above it was oriented horizontally to save labor.
I’m pretty sure this is not an issue.
– Zip tape used (poorly) instead of the adhesive that was on site that he bought for this purpose; not sure why his subs didn’t use it.
Also don’t think this is a huge issue; it all gets covered in backfill and the boards are wet all the time anyhow
– One instance where the board isn’t flush to wall
This is one I’m more concerned with – I imagine there’s no real insulating value when the board’s not on the wall. Mitigated because it only happened in one place
– Board coming up to uneven heights on wall, and not wrapping around walls entirely in some places. We will have a slope down on the side of the house, so the foundation is stepped, and I know that requires more work to cut everything to the right height, but I don’t want things exposed above grade
And this is the one I’m most concerned about.
I hope the pictures help illustrate my concerns; hoping someone can let me know if it’s stuff I should actually be concerned about or if it’s no big deal. Thank you so much!
Links to images here:
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Replies
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David,
Can you tell us a bit more about what the whole insulating strategy is for the walls. What happens above this foam? How does it tie into the framed walls above? Is there going to be any insulation on the interior of the basement walls?
EDIT: Didn't read closely enough.
On the outside of the house, there will be exposed concrete above grade for a few inches, followed by Zip R-9 covered in Slicker and composite siding. The inside of the above-grade walls will have dense-packed cellulose.
Below grade there will be foamboard against the inside of the walls, spray foam in the joists, and foam under the slab pour.
David,
I'm mostly interested in what happens at the top of the exterior foam. Is the concrete just exposed above grade?
Yes, above grade will be exposed concrete, sorry.
David,
Unfortunately if you go with exterior insulation it needs to be continuous. Concrete has a very low R-value, and the exposed area will be a thermal bypass that makes the exterior insulation below almost completely ineffective.
Malcolm, continuous exterior insulation is definitely the best way to do exterior insulation. My builder didn't have a ton of experience with exterior foam, and we were concerned about insects. I've seen lots of discussions on here about meeting code with a combination of exterior plus interior insulation on above-grade walls - does everyone have insulation all the way down to grade and stucco/stone covering?
I'd hoped that by having spray foam in the joists we mitigated the thermal issues somewhat...I've never lived in a home that didn't have exposed concrete for a foot or so. Hoping this wasn't a big mistake; I even hired a third-party performance consultant referred to me by someone well-regarded in the field who signed off on this approach...
David,
The walls will work fine with adequate interior insulation, but the exterior foam won't
be contributing much if anything to that. An 8" concrete wall is about R-0.7. The cold will simply bypass the exterior insulation.
To answer your original questions:
1. Orientation is not an issue.
2. Tape of any type make no real difference to the performance of exterior foam.
3. As long as air can not get behind the board it is fine.
4. Any lack of continuity largely negates the effectiveness of the exterior foam - this is especially true above grade, and near the top of the stem-walls.
Thanks, Malcolm - that's disappointing to hear. I'll make sure we use plenty of CC spray foam at the joists, then. When we moved in to our last home we put up foam against the walls in the basement and spray foamed the joists, and it helped some - hopefully that's enough here as well.