Basement Insulation – Remodel
I have a question I haven’t been able to find a good answer to. I own a home in Minneapolis-St. Paul that has an unfinished walkout basement. Home was built in 1988. Basement was never finished. Basement is drain tiled with sump pit but no pump was ever installed. Nonetheless, the basement appears completely dry.
The walls in the basement are concrete block for the full height on three walls. On those walls, the previous owner nailed 2x2s, installed fiberglass batts between them, and installed poly over that. The fiberglass actually doesn’t have any mold or other moisture damage that I can see. I intended to finish the basement and will tear this out.
I’d ideally install rigid foam on the block walls and frame inside of that. But those 2x2s present a problem…
My question is, should I tear out the 2x2s that are nailed to the block? I’d have to hope for the best in pulling out the nails and then fill the nail holes somehow, maybe with hydraulic cement. Or could I do something different? Your suggestions are appreciated!
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Replies
John,
It's always nice to have a continuous layer of insulation, uninterrupted by framing.
If this were my house, I would install two layers of rigid foam. The first layer would be 1.5 inch thick, and would be cut into strips to fit between the 2x2s. The second layer of rigid foam would be a continuous layer -- the thicker, the better.
Minnesota code diverges pretty dramatically from the IRC on basement insulation issues, and expressly disallows more than R10 continuous insulation on the interior of foundations unless there is at least R10 of continuous insulation on the interior. Local inspectors have a great deal of discretion on varying from that in retrofit situations, but be sure to run it by them before going forward.
As long as you have at least 2" of foam (any type) on the above grade section you'll be fine to use unfaced batts or kraft faced batts on a 2x4 studwall without interior vapor retarders, as long as the gypsum is reasonably air tight and painted with standard interior latex paint, mirroring Table 702.7.1 for climate zone 6 in the IRC:
http://codes.iccsafe.org/app/book/content/2015-I-Codes/2015%20IRC%20HTML/Chapter%207.html
At 2" all types of rigid foam is more vapor retardent than latex paint, and will be a sufficient vapor retarder against ground moisture diffusion into the wall cavities, and sufficient R-value for dew point control on wintertime moisture drives on the above grade portion for up to R15 cavity fill.
Be sure to put an inch of EPS under the bottom plate of the studwall as a thermal & capillary break against the slab coolth & ground moisture.
Of the rigid foams, EPS is the cheapest and has the most stable long term R. Even low-density (Type-1) EPS would run R7,8 @ 2" @ 75F, but R8.3+ when the average temp through the foam is 40F , and even higher when it's colder than that (which it will be on the above grade section during the wintertime weather where condensation is a concern.) Polyiso will have higher shoulder season performance, but comparable to worse mid-winter performance. XPS would have initially higher performance, but loses performance over time, eventually dropping to the performance of EPS of similar density.
D Dorsett, what are your thoughts about removing the 2x2s that are currently nailed to the block wall? Can I leave them or must I remove?
Martin, would you have any concerns about trapping the existing 2x2s behind the foam against the concrete block? I assume they would rot, but maybe that's not worth worrying about if it's trapped behind a good foam vapor barrier?
John,
If your basement wall seems dry, I wouldn't worry about the 2x2s rotting.
And if your basement wall seems damp, you need to address the moisture problem before insulating.
I think it's a question of which is easier--cutting the foam to fit between, or ripping the 2x2s off. Home depot sells EPS sheets pre-cut to fit between 16" on center 2x framing, so that might be a nice solution. If for some reason that doesn't work for you, I might try taking a few 2x2s off to see how that goes before deciding which is easier. I might try grinding the nail heads off and then pulling the wood off leaving the nails behind,and the simply sticking the foam over the nails.