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Community and Q&A

Insulating a painted basement wall

ranson | Posted in General Questions on

I’m going to be moving to a house near Worcester. It has a poured concrete walkout basement. The rim joist has been spray foamed. The walls have been painted. In some spots on the high grade side there’s a bit of mold at the base of the wall.

I would like to insulate the basement with foam board, but I’ve got a few concerns:

1. Is something like PL300 going to be adequate to hold foam board to the painted wall? (Does it need to last longer than the time it takes to frame the wall?)

2. Since I’ve already got a bit of mold, what do I need to do before installing rigid foam to make sure that I don’t get a mold problem. Is the paint going to feed the mold?

3. Should I have a spray foam company spray the top edge to seal it to the rim joist insulation and make sure the wall is fully covered?

4. Should I leave a gap at the floor and fill it with canned foam?

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  1. DC_Contrarian_ | | #1

    In that climate the mold is likely caused by condensation of warm humid air in the summer. On a painted wall your probably can just wipe it off and I wouldn't worry about it. The insulation will keep it from coming back.

    You want the foam to form an airtight barrier, to keep humid air from getting behind it and causing condensation.

  2. Expert Member
    Dana Dorsett | | #2

    If there is any blistering or spalling of the paint on the wall it's risky to rely on adhesive alone, unless the finish studwall wall is tight to the foam, trapping it in place. DO tape the seams of the foam for better air & radon tightness, and DO seal the gap between foam & foundation or sill plate at the top of the foam, but DO NOT seal the bottom edge, in order to allow drainage of any wall leakage down to the slab.

    Assuming there is no history of flooding in that basement, leave a 0.5" - 1.5" gap between the bottom of the foam & slab as a drain space for any seepage entry. If there IS a history of flooding, stop the foam an inch or three above the high-tide mark.

    Green Insulation Group in Worcester, MA is a good very-local resource for used & reclaimed foam board at a fraction of the new-perfect distributor pricing. See:

    Only slightly further afield is Nationwide Foam in Framingham:

    These two generally have a decent stock of various foam type, thicknesses, and conditions, and are fairly competitive. There are other, smaller operations that can often beat their pricing, but the availability is a bit hit & miss.

    I insulated my own Worcester basement using 3" of 2lb reclaimed fiber-faced polyisocyanurate roofing foam (labeled R17, derated to ~R15 due to age.) To conserve interior space it is strapped in place with 1x furring TapConned through to the foundation, with the code-required half-inch wallboard thermal (fire) barrier screwed to the furring. In my case the foam extends only to 4-8" above the (none too level rat) slab due to a history of the water table rising above slab level during spring thaws. Sump pumps are able keep it dry, but (like the power grid) can and will fail at inopportune moments, which is why in my case it will never be used as living space.

  3. Expert Member
    BILL WICHERS | | #3

    1- PL300 is fine BUT if you glue to a painted wall, you're now relying on the adhesion of the paint to the wall -- if the paint peels, so does your glue. If you're planning to frame over the rigid foam, and the framing will be tight to the foam, then the adhesive is effectively a temperary support until your framing goes up, so it's much less critical. You can think of the framing as the long-term support that will keep the foam tight to the wall. Note that that "tight to the wall" part is important for mold and moisture control.

    2- You can clean the mold with a mold killing cleaner. You can also get mold killing primers that you can paint right over the mold. I usually use both, cleaning the mold first and then using the mold killing primer as a preventative to avoid future problems. If you only have a small amount of mold, you should be OK skipping the mold killing primer step though. Once you have the rigid foam up, you are essentially "encapsulating" the wall, so the mold issue isn't as big a deal.

    3- This is not a particularly big deal. I wouldn't have a company come in to do this, but I might use a bead of sealant or some canned foam to seal the top edge. The most important thing here is to air seal the rim joist area to avoid air leaks to the outdoors, insulating is a secondary concern for such a small area.

    4- Gaps at the bottom are usually done when there is a potential for bulk water (minor flooding) in the basement, and you're using insulating that will wick water (like polyiso). If you're using XPS or EPS, water isn't really a problem, so you can run the foam board tight to the floor. If you don't expect to ever have any liquid water on the floor, you can run polyiso tight to the floor too. The gap is only needed if you think there might be issues in the future, so it's not a requirement in all situations. I would not fill the gap with canned foam if you are in a situation where the gap is needed, since filling the gap will cancel out some of the reason for having the gap in the first place.


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