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Community and Q&A

Half wall basement ?

KelliSean | Posted in GBA Pro Help on

I priced spray foam and it was 3k so we want to use xps on the block walls and in the rim joist area. Since the 2×4 walls are only 1/2″ from the block wall I can’t fit any foam back there can I cut the foam to fit between the studs and fill the area behind the stud with canned foam?
Do I caulk the sill area or foam it?
The area above the block wall is the above grade wall and it has fiberglass batts and is 2″x6″ framing then the 2″x4″ frame is in front of that to make a flat wall to the ceiling how should this upper wall be insulate? XPS? Fiberglass a combo and how much R-value?
I live in Alaska near anchorage I think cold zone 8.
I really want to start this project but I don’t want to do it wrong.

Thank you

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  1. jklingel | | #1

    EPS is a "greener" foam. The 1/2" gap will result in convective loops, killing your wall's R value somewhat. I would be inclined to fit foam between the studs and shove it against the concrete, spray foaming behind each stud (in between the foam blocks). Martin suggested to a person earlier to cut the foam a tad small, as it is easier to fill a larger gap than a real tiny one (1/4" or so, imo). Above grade, I would try to get some Roxul, if you can. Uresco can order it, if you have a Uresco down there. Be sure to air seal everywhere you can, either with foam, an appropriate tape, or non-hardening caulk, like Tremco Acoustical sealant. How much insulation? You'll have to figure your ROI on that, but R19 (really more like R15 overall) and another (approx) R10-15 or so inside that is not excessive for Anchorage. I ain't a pro, and maybe others will have better suggestions.

  2. GBA Editor
    Martin Holladay | | #2

    You live in Alaska, so you really need to insulate carefully. You've already made your first mistake -- you framed your 2x4 wall much too close to the concrete block wall. Ideally, you would have left at least a 4-inch gap -- for 4 inches of rigid foam -- and now your gap is too small.

    If you want to do a really good job, you can dismantle the 2x4 wall and move it out. If you want to leave the wall in place and go forward, I guess I would fill the 1/2-inch gap behind each stud with spray foam, one stud bay at a time, after installing rectangles of rigid foam in the previous stud bay.

    To insulate the above-grade wall, spray foam would be the best option (from the perspective of air sealing and insulation), but I assume the cost is too high. You can use blown-in cellulose or blown-in fiberglass -- either product is better than fiberglass batss.

  3. KelliSean | | #3

    Unfortunately the basement was done by the builder and we are trying to fix it. We bought the house a year ago and it was built in 2005. We would like to redo the whole basement at some point because it is finished with fiberglass batts and vapor barrier. We started this process because our energy bills were very high even though this house was 5 star energy rated when it was built. We are the typical family that show up on Holmes on Holmes with a poorly finished basement that the inspector didn't notice and the previous owner lied about. We had an energy audit done and are sealing air leaks and trying to make things more comfortable.
    Thank you

  4. GBA Editor
    Martin Holladay | | #4

    Sorry about the reference to the mistaken placement of the wall. I understand it was not your mistake and I'm sorry if my tone sounded accusatory.

    So, it's your choice: work with the existing 2x4 wall or try to move it a few inches inward to allow for a better insulation job.

    Here is an article with more information: How to Insulate a Basement Wall.

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