What insulation should I put on basement block walls that is resistant to mold?

Ken Gee Published: July 31, 2019, 3:25 p.m.Last updated: July 31, 2019, 4:03 p.m.

Concrete block basement wall. Moisture issue in spring ( and I suspect fall).

Responses (3)

Mike Reynolds
Mike Reynolds July 31, 2019, 4 p.m. Reply


Hi Ken,

Before we can really answer we’d have a couple of questions first - what climate zone are you in, and what is the condition of your wall? Cinder block foundations aren’t as solid as poured concrete, so you need to be a bit careful about insulating them from the inside, because if the wall has never been insulated, then you will be changing the thermal conditions it is used to. Meaning, if the walls have always been kept warm due to heat loss, you will be subjecting them to the freeze / thaw cycle if you are in a cold climate and you insulate the wall from the interior. That makes them a bit more susceptible to cracking at the joints.

Our first recommendation if you can do it, would be to insulate it from the outside. From the sounds of it you could use some moisture and water management; it would end up being quite a bit more expensive, but it would be a much better job and alleviate the concerns of you doing something now that will come back to bite you later on.

That would be a case of excavating around the perimeter and installing rigid insulation, a drainage matt and weeping tile. You can see a detail of that here on our page about how to insulate basements properly.

Having a professional (an engineer or at least an experienced foundation contractor) inspect it first is a good idea to make sure you do it properly. That said, here are a few other pages to read that may be of value, but feel free to write us back with a few more details and maybe we can help a bit more.

Ken Gee Aug. 5, 2019, 2:07 p.m. Reply

Thank you so much Mike. I am in Vermont. Sometimes it goes below 0F for weeks. Gave insulating exterior walls, but can only excavate about 50% of foundation due to concrete stairs and platforms around the exterior. Not really sure what to do at this point. Read many conflicting recommendations and opinions on-line while researching this topic. I was reluctant to insulate from the interior for the reasons you mentioned. House is an all block house including foundation and interior of basement is about 820SqFt. I appreciate your input but am still not sure where to go with this. ANy other advice is welcomed.

Mike Reynolds
Mike Reynolds Aug. 8, 2019, 10:58 a.m. Reply

Given there are a few issues it may be best to address them one at a time. On moisture - does it flood or is it just high humidity? If you get bulk water then you would be wise to install a drain.  If it's not a finished floor then you could smash the concrete with a sledge hammer around the edge , dig it out and intall a drain. If its about high humidity then I'd run a dehumidifier. 

Forgetting about insulation for the moment, you could at least stick a 6 mil poly vapor barrier on the walls if that's where the moisture issue is coming from. If it's a poured concrete floor and it has no vapor barrier beneath then that will for sure be a source of moisture, but a dehumidifier will help with that and we can cover that after. I  hesistate to recommend interior insulation without physically seeing the wall, but the poly won't do you any harm. It will help prevent mold, and it will help with heating costs a bit since it will keep it drier heating moist air consumes a lot more energy than heating dry air. 

Give us more details though -  Ceiling height, finished walls or not (is it living space), poured concrete floor or not, flooding, moisture levels, mold or no mold, etc, whatever you can tell us will help us help you pick a solution.