smoothing concrete walls before insulating
I live in climate zone 6a (coastal Maine). I have an unvented crawlspace that does not have any insulation (interior or exterior). I am looking to install foam board insulation on the interior crawlspace walls and probably spray foam in the rim joist area.
The crawlspace walls are poured concrete and are about 2.5 feet high. The house was built by a previous owner about 17 years ago. The walls are not totally smooth. There are a variety of bumps, ridges, and other imperfections. The biggest ones protrude maybe a quarter inch. My intention is to attach the insulation to the walls using some combination of adhesive and plastic insulation fasteners.
Two questions:
1) How smooth do the walls need to be before installing the insulation? The concrete walls are currently rough enough that pressing the insulation against the walls would put dents in the boards Is that bad? Does target smoothness depend on how I fasten them to the walls?
2) If the walls, do need to be smoothed more, what are good tools for doing that?
Thanks!
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Replies
ben_riegel,
1) Not very smooth. The only problem with uneven walls is if moist air can get in behind the foam and condense on the cold concrete walls. As long as the perimeter and joints in the sheets are air-sealed, a small air-space behind is fine. Some dents in the boards are common and to be expected.
2) An angle grinder with either a diamond or carbide wheel makes sort work knocking down bumps, but from what you describe I wouldn't bother.
Thank you Malcolm!
I have old concrete walls that have spalling. I intend to fasten foam boards to them instead of gluing them. Should save a small fortune in CC SPF costs.
If you are using EPS or XPS and the imperfections in the wall are small (like small protrusions), you can probably just bress the foam against the wall and let the imperfections dent themselves into the foam board. You could make a "tool" to help with this by taking a piece of plywood or framing lumber around 8" or so square and mounting a drawer pull to it as a handle. Wherever the foam board presses out from the wall, lean on it with this "tool" so that the foam board flattens out with the imperfection poking into the foam. The "tool" will spread out YOUR force more evenly so that your finished front surface of the foamboard remains relatively flat and doesn't get fist and hand dents in it from your efforts pressing it against the wall.
If you have larger raised areas on the wall, then an angle grinder with a carbide wheel is the way to go as Malcolm suggested. Be aware that you'll make a big mess if you have to do a lot of that work though, and WEAR PROTECTIVE GEAR! A respirator is a MUST here, along with eye protection. A heavy old leather or denim jacket is a good idea too. You do NOT want to breathe silica dust when grinding concrete!
Bill
Thanks, Bill! Those are great tips. I'm planning on using polyiso. Would that be harder to press into bumpy walls?
Polyiso is a little more solid-feeling than lower density EPS, and probably a bit more solid than XPS too. That means polyiso will take a little more effort to "squish" against a rough surface, if your goal is to get those surface imperfections to press dents into the polyiso. It should still be doable though, it will just take a little more effort.
Bill