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Community and Q&A

Insulating Rim Joists / Air-Sealing Finished Basement

ryanmiday | Posted in General Questions on

Moved into 30 year-old house 2 years ago. (Suburb of Cleveland, Zone 5). Knew nothing about building science. Discovered rotted rim joists in basement when I pulled off fiberglass batt insulation. Joined GBA. I have followed Martin Holladay’s

How to Insulate Your Rim Joists (Thank you). Just finished — 85 rim joist cavities.  I have a mouse problem too, so sealing any possible entry was important. But I realized that I sealed the top of the finished side of my basement walls that are insulated with fiberglass batts between the 2×4 wood framing and cement wall. There were 1/2 gaps at the top of the wall. Did I create an environment for mold growth since there’s no longer a way for air to escape?  

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  1. Expert Member
    PETER Engle | | #1

    Depends on how the walls are constructed. But, if they were done with FG batts and a 1/2" gap to the foundation wall, that's already problematic. The prevailing wisdom is that there should be no fluffy insulation in basement walls unless there is an approximately equal amount of rigid foam or other inert insulation outside of the fluffy stuff, either between the FG and the wall, or on the exterior of the wall.

    FWIW, the air gap you observed was probably a big part of the rotten rim joists. Moisture from the foundation walls was climbing up through that gap, through the FG stuffed into the rims, and condensing on the rims. You've stopped that with foam, but you haven't eliminated the moisture source. Moisture in those walls comes either through a poorly waterproofed foundation wall, from moisture under the slab coming through the wall/slab joint, or from warm/damp interior air entering the space between the drywall and the foundation wall. The first two can only be corrected with major interventions. The last one can be controlled with a dehumidifier.

    1. ryanmiday | | #2


      Thank you for your response, and I thank GBA! The subscription is the best I’ve ever purchased.

      I now understand…several weeks ago, I discovered that my main sewer drain in basement (on the finished side) was leaking. It had cracked and had run under the carpet and tile in a 14x22 area. When I pulled the baseboards off the wall, I was surprised to feel a strong current of cold air. Your response led me to run a thermal meter around the rest of the finished basement. There’s an average 10 degree temp difference from bottom to top of drywall. On Sunday (25F outside), I marked the temp and moisture levels on the wall of drain pipe and cut open sections of the drywall at the bottom. I believe the white stuff is efflorescence, confirming what you indicated. (See pics). But no insulation. I guess that’s good but there is insulation that can be seen at the top of the walls. I will use a dehumidifier. Further evidence of my basement water issues was discovering rotted rim joists on the unfinished side, when I pulled out the FB batts. I attached pics. The cause was the sliding door to the deck. On the finished side, as I began to air seal the rim joists, I discovered snow in 2 of them, which are four feet in length and have a 2’ x 3’ foot hole inside them. (See pics).My front porch had a gap that led to damaging the rim joist area. It was 35 degrees in these cavities over the last several days; it was 25F outside. I am perplexed about the hole in them. I am finishing these long cavities: before insulating, I covered the 2’x’3 hole with plywood and treated lumber and then sealed 2’’ XPS with spray foam, added R15-mineral wool and then R-30 Unfaced. I used a lot of spray foam due challenge of air sealing the 4’ cavity, which concerns me as I’m unclear about external/interior ratio. I could have simply sealed the cavity at the entrance but I thought it was a bad idea to leave such a large air cavity plus it would not solve the problem with the temp of the small closets, adjacent to the front door, directly above; they registered 35F around 11pm last night with outside temp of 16F. Again, I very much appreciated your response.

  2. Expert Member
    BILL WICHERS | | #3

    The rot and issues around the sliding door were probably caused be insufficient, or incurrectly installed, flashing. That's a fairly common issue. You could potentially have drainage issues too -- you absolutely do NOT want water pooling up along the exterior of your foundation wall. Be sure to check both the flashing and the drainage.

    It's important to remember that insulating the rim joist area has the unfortunate side effect of making any water/moisture issues worse. It becomes a lot more important to get the drainage and flashing details right if you air seal and insulate, since the air sealing greatly reduces the ability of the assembly to dry, and the insulation makes things colder, which usually also means wetter.

    I think you're right about the white stuff. The makers of Drylok (the waterproofing paint) also make a special chemical cleaner that can remove that if it's a problem. If you're just sealing up the wall, I wouldn't worry about cleaning that stuff off since it won't be a problem as long as you get the water under control that was causing it to form in the first place.


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