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Community and Q&A

Insulating basement cinder block wall

MikeSG | Posted in GBA Pro Help on

Hi All,
My name is Mike and I am currently planning to finish my basement in Michigan. My basement walls are hollow cinder block and I plan to paint the with drylock or a Ames block and wall liquid rubber paint (not sure which yet) to prevent moisture from seeping to the interior. Following this, I plan on putting 2in of rigid foam insulation (eps) on the basement wall followed by the studs. I plan on keeping the space between the studs as air space in front of the rigid foam for electrical and plumbing. I was told by my contractor that it’s best not to insulate the basement wall to allow the cinder block to dry to the inside. He said that if I put rigid foam insulation it won’t allow the cinderblock to dry and may result in a mold issue. Most of what I have read said to use rigid foam insulation. 

I have read most of the articles regarding insulating basements on this forum but would like some additional advice regarding this specific issue.. Can you please shed some light on if using a drylock type sealer of the cinder block followed by 2in of foam would cause a mold issue or if even rigid foam on exposed cinder block would cause a mold issue?

In addition to that, I have read that adding rigid foam insulation to cinderblock wall could cause the cinderblock to freeze in the winter cause cracks in foundation. Is this true?

Thanks in advance.

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  1. Expert Member
    NICK KEENAN | | #1

    The mold is a non-issue, if mold is going to form it's going to be on the other side of the moisture barrier and not connected to your living space. The dirt on the other side of the cinder block is full of mold, but you don't worry about it because there's not way for it to get into the house.

    The issue of masonry getting damaged when it stays wet and freezes is real. It is true that insulating the interior makes the above-ground portion of the wall colder and wetter. I've read stories about historic brick buildings being damaged when the basement was insulated. I don't have any advice on how to prevent it or determine whether it's a risk in your particular case.

    1. Expert Member

      I'll add an addendum, in that it's only when block or brick gets wet, and then freezes, that damage happens. Maybe it's a small detail, but if those are kept dry, the the cold matters little.

      1. MikeSG | | #3

        Thanks for the info. I am fairly certain our cinder block gets wet here in michigan, it rains/snows quite a bit.

        Would excavating the exterior and water proofing then using rigid board insulation prevent any freezing issues ? In this case there would be insulation on both the interior side and exterior side of the cinder block.

        If so, what thickness of rigid foam board would be necessary?


        1. Expert Member
          PETER G ENGLE PE | | #4

          Excavating and cleaning the foundation and then adding waterproofing and rigid insulation is the gold standard for foundation treatment and it would certainly minimize any chances of frost damage. It is not cheap though. The most important thing is to manage water on the exterior of the foundation by using good grading for runoff, a drainage layer in the backfill (dimple mat if not using fibrous or other self-draining insulation), and footing drains routed to a sump pump or daylight. If you go this route, a 50-50 mix of insulation inside and outside would be more than adequate. 2" applied both in and out would give you about R20, which is very good for foundation insulation in your climate.

          That said, IMVHO the risk of freeze-thaw cycling damage with CMU is low, even on very wet lots. I have seen this damage with very old CMU that used coal cinders for aggregate, but not with any block manufactured since about 1970.

          With good exterior waterproofing, you don't need Drylock or equivalent on the inside. And no, rigid foam on the interior is not going to cause mold growth. In fact, if you use foil-faced polyiso insulation and tape the seams, you dramatically reduce the risk of mold growth. Make sure to air seal and insulate the band joists and tie this treatment to the interior foam for best results.

          1. MikeSG | | #5

            My house was built in 1951 so I assume it is coal cinder as you said. In that case would you say there is a risk if I insulate only on the inside?

            Also a few questions:
            1. If I water proof exterior foundation, would I use tar or some kind of waterproofing paint the place a dimple barrier followed by rigid board?
            2. Could the drain pipe connect to city pipes?
            3. In the event of excavating exterior is prohibitively expensive, what would the alternative be to protect cinder blocks from cracking if I were to put insulation on the interior only?


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