Insulating Painted Concrete-Block Foundation Walls
I live in Michigan, Zone 5A.
I’ve read the posts here regarding basement insulation which involves foam board glued to the walls and rim joists, air sealing as much as possible, then a stud wall and drywall. Batts between studs are optional. But I have a few questions…
The latest science says it doesn’t matter if you use an EPS which allows vapor to move through, or XPS which does not. Concrete walls can be humid for hundreds of years they say. And I like the idea of XPS lowering my room humidity so I can save money on running that dehumidifier. But…my block walls have a couple layers of regular latex paint on them that seems to have been applied decades ago. The paint is well adhered and not flaking at all, fortunately. If the relative humidity between the XPS and block is constantly high, isn’t the likelihood of mold growing on that paint very high? If so, I think I have two options here: put on a fresh coat of mold resistant paint/Drylok, and use XPS; or use EPS and run a dehumidifier as needed.
Also, my joists sit directly onto my block foundation. There is no sill plate. I worry a little bit about the humidity leaking out of the top of wall in a high concentration and accelerating the demise of my joist ends.
Finally, is there such a thing as over-insulating? For example if I do the foam board and batts am I encouraging the wall to be too cold which increases the chance of condensation anywhere where I can’t get a perfect foam board seal like around my circuit breaker box? Am I potentially going to have a deeper freeze around the perimeter of my house which could put more pressure on my walls?
Also, for the time being I do not plan on insulating the floor yet. There’s currently modern peel and stick vinyl tile that is well adhered. The basement seems to be free of any signs of liquid water ingress, but I still want to give it a few years and be on the watch for water before I put anything else on the floor. This way if I do see a trickle I’ll know where to double check my grading outside.
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Replies
I'd use polyiso over either EPS or XPS in a basement, for more R per inch. Polyiso is great here, and the "slightly less R in cold temperatures" doesn't matter, since the basement walls won't get as cold as above grade walls would.
Even if you were to get mold between the wall and the insulation, the insulation would seal it in, so you shouldn't have problems. If you're concerned, do a quick coat of mold killing paint/primer prior to insulating for some extra insurance against mold.
You don't have to put up a studwall at all. You can use polyiso that is rated to be left exposed (Dow Thermax, etc.), or you can drywall right over the insulation -- without studs. If you want to run electricals in the wall, I'd use 2x3 studs on the flat instead of a full studwall to save space. Remember that any wall you build here is non-structural, so it doesn't need to be framed out as a regular studwall.
You can't really overinsulate here. You do increase the risk of freeze/thaw issues with the block SLIGHTLY by insulating, but that's usually not a problem in practice. I'd be most concerned with "rising damp" getting to those joists. If you can install a capillary break, that would be the best way to go. If you can't, you might consider leaving a small band uninsulated on the interior side to allow for some drying, and insulate the rimjoist with foamed in place EPS panels which also allows for a small amount of drying. The block itself can be wet forever, just like concrete, there is NO need to worry about drying for the masonry part of the wall.
Bill
Echoing Bill as I had many of the same concerns.
Current building science (as far I can tell) is that any mold that may grow in this location (between wet concrete, paint, vapor barrier or mold resistant insulation (foam)) is not an issue as long as your vapor barrier is properly sealed/terminated. With seams taped, edges sealed, and no air leaks, there is no way for the mold to cause issues.
Certainly, if desired, a coat of mold primer or drylok will not cause issues, though surface preparation and adhering to the wall may be difficult.
I've also seen recommendations for polyiso for walls. Usually with a caveat to leave it off the floor an inch or two as it can wick more moisture than EPS, which degrades its capability.