Foil-faced polyiso — basement insulation, moisture, and radon
You all have given me great feedback in the past.
I have a unique home which has 3 distinct basement sections — about 20% of the basement is old fieldstone with a poured concrete floor, 35% is dirt crawlspace covered in 6mil poly sheeting sprayfoamed as best I could to walls/rubble for airtightness, and 45% is poured concrete walls and concrete floor. The exterior foundation around the 45% “new basement”/1990 addition appears to have 2″ XPS — i see it starting about 2” below the dirt and dont know how far down it goes. Previous owner installed fiberglass bats between first floor and basement.
I have an interior french drain which was retrofitted. There are two sump pits — 1 in the fieldstone portion of the basement, 1 in the new portion of the basement. I recently sealed both sump covers to reduce evaporation/humidity and radon levels.
Radon levels: 7.4 pCi/L in basement, 0.4 pci/L first floor. Tested two weeks ago just prior to sump sealing.
The basement is not damp in that there is no standing water. and its not super super humid or muggy, no water on walls etc. In the winter its pretty dry. Forced air ducts run through this area which likely helps. However in the summer it can get musty. If I leave clothing out in boxes for instance it will get mildew/mold. To combat this, I open windows to circulate air which seems to work.
I want to now insulate the 7′ poured concrete walls, to start. They face north, east and west, and about 2.5′ is above grade. Sill/rim-joist is insulated with 2″ XPS and spray foam + batts or rock wool. I have a 12×12″ piece of plastic taped to the concrete walls for 24 hours to see if moisture collects.
I’m leaning toward 2″ or 2.5″ Dow Thermax because I don’t want to add drywall. My question is: can I use the thermax from top of the walls to 1″ or so off the floor, with all seams taped? My contractor friend said he would be worried about moisture to the interior having nowhere to go with the foil faced polyiso. He suggested maybe I only go 2′ below grade with the insulation (top half of walls only). Is that a myth, and if so is top of foundation walls to just off the floor okay? I have a foam capillary break between the concrete and the sill.
Any insights/advice would be great. Also, 2.5″ is preferable to 2″ I presume. If anyone thinks the fact that I cannot encapsulate my entire basement perimeter makes this a wasted effort, please let me know that too.
Also, pertaining to humidity. I will work to air seal the crawlspace better. It’s pretty good, but where the poly rests on the rubble it’s hard to get a 100% seal. I will use more spray foam. While my in-house radon level is not concerning, I read that radon mitigation would reduce basement humidity. I am weighing whether a system pulling from the interior drain tile and under the crawlspace poly would lower my humidity levels? Conversely, would it pull out house heat if I don’t have a 100% crawlspace seal?
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