Insulating existing plastered basement wall
Mkendall456
| Posted in Green Building Techniques on
Is there a way that will not create a moisture problem to insulate over an existing furred out plaster wall in a basement and drywall over? I have been reading about polyiso, xps, and closed cell foam from https://www.greenbuildingadvisor.com/blogs/dept/musings/how-insulate-basement-wall. What product would you recommend and why? There is no electrical on these walls. Or plumbing. There are two plastered walls over furring with plaster ceiling in a corner of basement. I might want to add a few outlets in this process. I’m trying to insulate the room to make it more comfortable. Location is southeast Iowa. Thanks.
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Replies
Oh. I forgot to mention. No water or moisture issues are present.
Michael,
You didn't explain whether your basement wall is built of concrete blocks, poured concrete, or stone & mortar.
If I were going to insulate this wall, I would remove the plaster and furring strips, exposing the structural wall underneath. It's hard to choose the best insulation method without knowing more about the construction of the wall.
Michael,
If, instead of taking my advice, you install insulation on the interior side of the plaster, the insulation will lower the temperature of the concrete or stone wall. By lowering the temperature of the wall, you increase the chance of condensation -- and the air channels between the furring strips provide a pathway for the delivery of warm interior air to the cold wall.
Sorry about that. The basement wall is poured concrete. I believe it is 8" thick. It supports the brick veneer around the whole house. Does that change anything?
Michael,
If the structural wall was stone-and-mortar, you would be restricted to using closed-cell spray foam. But if the wall is poured concrete, you can insulate with rigid foam. You don't want any air channels between the rigid foam and the concrete, so you still need to demo the plaster and furring strips.
With no electrical there, what would be the best way to demo? Sledgehammer? In my area is polyiso, xps or something else? Is closed cell foam better to achieve the R value needed? What would the recommended R value be for this area? Since the ceiling is directly attached, would this need to come down too? Or does it matter? Insulating rim joists? XPS and foam around edges?
XPS is about the least-green foam available, and it's performance drops over time to that of EPS(~R4.2/inch instead of it's labeled R5/inch. SE Iowa is US zone 5A, where R15 continuous insulation is IRC code min. At your subsoil and average winter temps 3" of polyiso would have higher performance than 3" of XPS, and it's easier to reliably air-seal than XPS. Just be sure to leave an air gap at the cut edge at the bottom (or install an inch of EPS there), to keep the polyiso from wicking ground moisture up through the slab.
On the rim joists 2" of unfaced EPS foamed in place for a tight seal, covered with carefully sculpted R15 rock wool batts works, and can't create a moisture trap. In zone 5 with more than 1/3 of the total R being the foam it doesn't really matter if there is no interior side air-barrier for the air-retardent rock wool, but if you wanted go higher than R15 over the ~R8-8.4 foam you'd have to do something. Air sealing the interior side of cut up batts is pretty tough to do reliably. At only 2" the vapor permeance of 1.5lb density "Type-II" EPS is well under 2 perms, but still above 1 perm, which is about the right compromise between wintertime moisture migration into the band joist and overall drying capacity toward the interior. If you go any tighter than that it would be better to use 2" of 2lb closed cell spray polyurethane applied directly to the wood, at about 0.5-0.7 perms. Between the higher polymer weight and the only marginally less damaging blowing agent, 2lb polyurethane isn't a whole lot greener than XPS, but makes for an easier more reliable air seal than cut'n'cobble sealed with can-foam.
Without knowing the full construction of the ceiling it's hard to say if or how much of the ceiling has to come down to make it work.
Michael,
Q. "With no electrical there, what would be the best way to demo? Sledgehammer?"
A. Demolition requires many of the same tools as construction. While a sledgehammer and several wrecking bars (in different sizes) are of course useful, so is a circular saw and a Sawzall. Experiment.