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Air sealing basement beam pocket?

(Zone 4c)

I have a concrete-wall basement from 1950. There are two beams in the basement that are triple 2x8 or 2x10. They rest in beam pockets in the foundation walls. There is a piece of sheet metal below them where they rest on the concrete, as a capillary break. I'm in the process of air sealing my sill plate and rim joist with expanding foam -- should I seal these beam pockets? If I don't, they will definitely leak some air, as a small length of the sill plate-to-foundation joint will remain unsealed. If I do, they supposedly may rot more readily over time, as it seems to be common practice to leave an air gap around the beam end to facilitate drying.

3 out of 4 of the beam ends are on foundation walls that are about 18" exposed above grade. The other one has more like 6" exposed above grade, so part of the beam is likely a little bit "underground". There is no waterproofing or dampproofing on the foundation exterior that I'm aware of. I'm planning to add a few inches of rigid foam on the interior foundation walls.

Should I air seal these beam pockets, or not?

Asked by Nick Welch
Posted Mon, 07/14/2014 - 14:16

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Nick,
Beam pockets are tricky. First of all, there is no guarantee that air leakage near the beam pockets will necessarily help keep the beam ends dry; during the summer, such air leaks could easily help keep the beam ends wet.

As you have probably figured out, adding interior insulation to the foundation wall will make the concrete colder than it used to be, raising the chance of condensation in the beam pocket. Sometimes, insulation makes things worse.

The best way to insulate this type of wall would be on the exterior. Exterior insulation is more likely to keep the beam pockets warm and dry. If you can't insulate on the exterior, I would probably go ahead and insulate as planned. You can air seal near the beam pockets, and then keep a close eye on the beam ends to see whether they appear to be staying moist. It's a risk worth taking, in my opinion. The advantages of insulation outweigh the (small) risk of beam end rot.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 07/14/2014 - 15:12

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