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Community and Q&A

Air-Sealing Concrete Blocks

singingdriver | Posted in Energy Efficiency and Durability on

Existing basement has cmu walls and has been dry for 2 years of my ownership. Wanting to finish basement for living space. There is no coating on the cmu and they are clean. I have read that blocks are porous for air and therefore water vapor. The sheet foam I am planning to use will not seal the wall like a liquid applied material would. I am considering applying a sealer to the blocks before the foam sheets….. any thoughts from you who have been there, done that before?

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  1. Expert Member
    NICK KEENAN | | #2

    There are several different sources of moisture you have to guard against in basements.

    Concrete is somewhat porous. In most places the soil is damp. Moisture from the soil can wick through the concrete and into the basement air.

    Rainwater can hit the foundation wall and pass through small cracks as liquid water. Or it can wet the concrete and wick through.

    In many parts of the US the air is humid enough in the summer to cause condensation if it hits a cool basement wall.

    It sounds like your basement doesn't let in liquid water. You want it to be tight, so that moisture in the concrete can't dissipate into the air, and so that humid outside air can't enter. Foam board is actually enough sealing, if you tape all the seams and seal all the edges. If that won't work, then a sheet of 6-mil poly stapled to the mudsill should be what need. Tape the seams on the inside. Make sure you seal your rim joists against air as well.

    A waterproofer like Charlie mentioned may also be a good idea, for a number of reasons it's a good idea to keep water out of your foundation. But it is no substitute for an interior air and vapor barrier.

    I don't like painted on coatings on the inside of basement walls. If liquid water gets into the wall they can make things worse.

  2. Expert Member
    BILL WICHERS | | #3

    If your primary concern is an air barrier, any rigid foam should provide that if you tape the seams and seal the edges. If your wall has always been dry, you don't need to worry about bulk water problems which is usually the biggest concern when insulating the interior side of a basement wall with rigid foam.

    If you really want to "seal" the block on the interior side first, you could use block fill paint. This is the type of paint typically used on block walls commercially, which will seal the little voids and provide a good finish. It's not typically used as a "sealer", but it will work for this purpose in a low-demand application like this. If you want something a little up from that in terms of robustness, try Drylok. You're unlikely to need anything more heavy duty than Drylok if your wall is already dry.


  3. singingdriver | | #4

    Interesting, thank you both. I was so concerned with sealing the blocks because i knew water moved through a block so rapidly. Maybe the blocks are not as porous to air movement as I thought, and perhaps the rapid transmitance of water through a block has more of a basis in capillary draw. House was built around 1960 and probably has oil base tar on outside of the wall and a sump pit in the floor at one corner, brick veneer above grade. Foam panels seem to shrink as they age so the plastic film stapled to the sill would be a backup if the joints opened up. Do you think I would be safe placing unfaced fiberglass batts in the stud walls inboard of the foam to compliment the r value of the foam board, and then covering with sheetrock. Location is southern Indiana and the basement temperature fell to 46 degrees last week In the cold snap!

  4. singingdriver | | #5

    Thanks Bill, just got your post! I just noticed a product called Radon Seal. It looks easy to apply with a pump sprayer. I don't know if I have a radon problem but it claims to seal the blocks. Maybe a two for one benefit with that product!?

    1. Expert Member
      BILL WICHERS | | #7

      I'm not familiar with that product, but I'd expect that it does create an air barrier. Usually you don't have a lot of air coming through the blocks themselves, the air leaks tend to be where differerent structural parts come together in the rim joist area on top of the block wall.

      If this were my home, I'd put up polyiso on the walls with seams taped, and not worry about sealing the block first. I'd put my air sealing efforts into the rim joist area, making sure to tie into the top of the polyiso there for a continuous barrier.


    2. charlie_sullivan | | #9

      I've used it. It is easy to apply and does seem to work to fill pores.

    1. singingdriver | | #8

      Thanks Tom, I didn't know about this company, they have a lot of good products.

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