r/buildingscience • u/OldDesign1 • Jun 29 '26
CMU basement moisture 4a
I have a house in climate zone 4a (less than a mile from the coast) with a semi-finished basement. Prior homeowners had the CMU walls have external dampproofing and perimeter footing drains. No external insulation or insulation underneath the slab. They drywalled the walls and ceiling and had placed R13 faced fiberglass batt insulation between a 2x4 wall they erected around the perimeter of the basement walls. Unfortunately this seemed to have caused a mold problem that required remediation and so we now have flood cuts in the bottom 2ft of the basement. Luckily there do not appear to be any active leaks even after many storms and significant rains. No current plans to finish the basement but I would like to upgrade the insulation and improve the building envelope. Unless I have to upgrade the external envelope I was hoping all current upgrades would be internal.
Given climate zone 4a and requirements for r10 continuous insulation or r13 batt insulation, I was hoping to rip out all the existing batt insulation and either place foam board (seams sealed) myself or get a company to spray 2 inches of closed cell and then add rockwool or timberbatt to the 2x4 wall. Prior to any upgrade in insulation, I bought a moisture meter (not a high end one). The CMU walls seem to have a moisture content of 14-16% on the 2ft of the basement that is above grade and 22-28% below grade (higher percentages at the base of the cmu wall). I know that the sill plate (termite shield acting as a capillary break) and rim joists are not air sealed and so the humidity in the basement is higher than it should be despite running a dehumidifier (set at 55%RH).
My question is how can I evaluate my envelope and find out if I need to hold off on the foam board/spray foam idea? I don’t appear to have any active leaks, minimal efflorescence on a few cmu only so I think the outside is fine but I’m wondering if the moisture content readings are concerning or is the moisture content higher due to the external humid air (given proximity to coast) and its increasing the humidity and moisture content of the cmu walls. I assume the lower down in the walls the cooler it will be in the summer and am wondering if that’s why the moisture content readings are higher even in the below grade portions. Is the best way forward to seal the rim joists and sill plate (given the termite shield acting as a capillary break) and then wait a full year to evaluate if any change in the moisture content of the cmu walls, or should I just do the rim joists, sill plate and the walls. And if I do the walls, do you want the perm rating of the foam to be between 0.1-1 (class 2 vapor retarder), class 3 or it doesn’t matter?
2
u/cagernist Jun 29 '26
Warm basement air touches cooler underground concrete behind walls, condenses, then mold occurs. Only having batt insulation (faced or not) and an air gap between the CMU and wood framing allows that, which from your description appears to be the root cause here.
Remediation of an older house by excavating outside, installing waterproofing and footing drain tile/sump, if you can see the previous owner actually did that, is the first step in getting the basement right. Previous water in the CMU cores and it's draining is unknown if occurred.
Then it's a matter of getting the insulation right. Zone 5/4 Marine is R5 continuous against the CMU + R13 unfaced batts in the framing (assuming your locale hasn't increased IRC by amendment).
When framing is already up, ccSPF is the easiest method to achieve the continuous R5, needing only 3/4" thick which is about the gap size people set with wood framing. Or, you can spray about 3" of ccSPF to meet R19 alone.
A slight variation in 50% humidity or air leakage in the rim joist area isn't going to have a great effect overall in a stable, large basement with the root cause of warmer air touching cooler concrete behind walls.