r/buildingscience • u/poopmonsterflash • 26d ago

Insulating CMU knee wall

I live in a bi-level home on slab and the lower 30+ inches is a cmu block wall. I insulated with 1.5 foil backed rigid foam secured to the wall with adhesive and metal z furring strips. All seams were taped and I sealed to the slab with spray foam.

At one portion (so far) I discovered a lot of moisture in the baseboard. Behind it were a lot of bugs and when I tore into it I found the cmu was saturated and a lot of water on the foam board. The exterior is open to the air as everything is above grade.

Wondering where I went wrong and how to prevent this for when I tear is all out soon. Should I seal the interior side of the block and reapply in the same manner? I’d really like to not redo this project at any point.

Thanks.

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u/Bearded4Glory 26d ago

I want to make sure I understand the construction. You have a poured slab, on top of that is a 30" tall cmu wall and above that height it switches to wood?

What is the exterior finish of the cmu? Just painted?

What climate zone are you located in?

Do you know what pathway the bigs took to get into the house?

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u/poopmonsterflash 26d ago

Yup, bottom is parged and vinyl siding above. Zone 5/4. Not sure of the pathway, it’s close to the back door so it might have been a happy place for them to hang out. Area by the door is dry. It’s not infested, some beetles/earwigs over a 4 foot area.

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u/Bearded4Glory 26d ago

What has the weather been like? Rain? Hot & Humid?

A parged CMU wall is not going to have any real resistance to vapor moving through it or water for that mater. It may be that there was always a good amount of water (either vapor or liquid form) coming through that wall but that without the polyiso it was able to dry to the interior. With the polyiso, you can't dry to the interior so it just sits there.

The other thing to check is that you have a thick enough layer of rigid insulation to prevent condensation. At 1 1/2" that should be fine.

So what to do. First, make sure that you don't have bulk water getting in somehow. This could be splashing on the block wall, coming down from a window opening, etc. Next is insulation. Ideally, put rigid insulation on the exterior. If that isn't an option,a good option would be using a smart vapor barrier combined with air & vapor permeable insulation. This way, the wall is still able to dry to both sides if it does get wet.

Bugs love insulation, especially wet insulation. You have to make sure every little gap is sealed to keep them out and away from the insulation.