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Community and Q&A

Spray Foam for Heavy-Timber Rim Joist

kevinkeegan | Posted in GBA Pro Help on

Climate zone 4 very old house  4 x 8 heavy timber joist header  sitting directly on top of the rubble stone foundation wall and the joist timbers 8 x 3 are tenoned into the 4 x 8 joist header. full depth foundation/ basement is where all the mechanical equipment is located , no duct work in the basement. In relationship to grade, one side of the foundation is completely below grade, two sides are all above grade and on the fourth side of the foundation the joist header is above grade, and the basement floor is poured concrete. The basement is not heated.  I have closed most of the spaces between the subfloor floor boards and joist header and I will use sealant and can foam where there remains any openings so cold air and critters don’t get up into the first floor. My question is with this type of construction does it make sense to closed cell spray foam the heavy timber joist header around the perimeter of the basement or will cause more harm than good? I’m mostly concerned about the completely buried side of timber joist header, could I be trapping any moisture in the timber header by spray foaming it?

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  1. Expert Member
    Michael Maines | | #1

    For the above-grade portions, in a heating-dominated climate, as long as the sill can dry to the exterior--i.e., not covered in ice and water shield or other impervious materials--spray foaming the interior is good.

    For the section of your house where the sill is below grade, you should change the detail so the sill is at least 6-8" above grade. With or without foam, that's a recipe for failure.

    1. kevinkeegan | | #2

      Thank you Michael, Yes that is my concern, however the sill beam is at the front of the house and there is a concrete patio running the length of the sill beam projecting out 10 feet. The good thing is there is a roof over most of the patio, but the patio is still exposed to the weather on the ends and the outbound side farthest away from the front house wall. It would be a killer to have to remove that concrete patio. I have to image the concrete patio slab is wicking in water and that moisture will find its way to the sill beam. That is why I am hesitating to spray foam that length of sill beam. Hoping that if it is getting wet that it can dry to the inside. If you have any thoughts I am listening with great interest. Thank you, Kevin

      1. Expert Member
        Michael Maines | | #3

        Kevin, I promote energy-saving (and planet-positive) actions whenever possible, but only if they don't negatively affect occupant health or building durability. In your situation I would not foam the interior of that sill, and I would keep a close eye on it to make sure it is drying, preferably with a moisture meter with a long probe. Depending on site conditions it may or may not dry readily toward the interior.

        1. kevinkeegan | | #4

          Hi Michael, I really appreciate your help. I was thinking I would just insulate under the sub floor, and leave the sill beam uninsulated. Do you think there would be any benefit to putting a small fan on top of the foundation wall and let it blow along the length of the sill beam?
          Thank you, Kevin

          1. Expert Member
            Michael Maines | | #5

            Kevin, while a fan would help clear humid air hanging out next to the sill, allowing it to dry more easily to the interior, my guess is that it would not make enough difference either way to justify the expense of running it. I recommend fixing the cause of the problem (or potential problem) and not relying on a bandaid solution. Easier said than done, I know.

  2. kevinkeegan | | #6

    Hi Michael, Yes I agree. It's always better to solve the cause than treat the symptoms. Yesterday I stuck my head up in that framing bay and looked along the length of the sill beam and it was dry.
    We have had rainy days, and some more rain expected. So I will be checking it. I also noticed some gaps between foundation top and bottom of sill that need non shrink grout. The mortar they used back then seems to be mostly sand with very little cement. I'm wondering since it is a rubble foundation and the mortar is so permeable if the rain water is finding it's way down the foundation wall instead of collecting against the sill beam. Could I be that lucky? I can't thank you enough for your time and attention.
    Peace & Good Health, Kevin

    1. Expert Member
      Michael Maines | | #7

      Kevin, the moisture would likely accumulate on the outside face of the sill, and would likely not be visible. That's why I suggested using a moisture meter with a long probe. It's not liquid water leaking that is a concern; it's the fact that the sill can't efficiently dry toward the exterior if damp, cool soil and concrete is directly on the exterior.

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