I recently had an energy audit/blower door test on my 1963 single-story ranch home in SE Michigan. They noted a lot of air infiltration along the baseboards, and recommended pro sprayfoaming the rim joist to about 3 inches total, over the existing (1.5" thermax+ canned great stuff) DIY job I did 5 years ago.

In doing some research, I realized that my house doesn't have a sill plate or capillary break between the joists and poured concrete foundation. The sprayfoam company I had out for an estimate told me not to worry about this, but I've read a lot like this comprehensive reddit post saying that closed cell foam could lead to moisture and rotten joists in this situation. FWIW I removed a few sections of the thermax I'd added to check things out, but I don't see any signs of moisture damage.

Note that top of foundation/bottom of joists is a few inches above ground level.

East rim joist, with diy thermax removed

Air leaks are BY FAR the worst along the North wall end joist (parallel to the other floor joists). I also tore off some of my DIY job there, and found lengths of 2x6 along that wall sistered inside the end joist (possibly just for floor nailers? again, no sign of moisture damage). I am guessing that the voids between the sistered pieces are the cause of those drafts.

North end joist, showing gap between corner and 2x6 sistered to end joist

My questions:

  • Am I right in thinking that having the whole rim joist pro sprayfoamed is a bad idea?
  • If so, is my DIY canned great stuff and thermax job OK? Again, not seeing any issues at this point.
  • How should I handle the gaps where the end joist has sections that are sistered in that drafty North wall? Would it make sense to just redo my DIY work here, maybe caulking all the edges first before putting in the foam board and great stuff, or is there something else that would work better?

Thank you for any advice!

album of all images

  • do you live in a seismic zone? Commented Nov 11, 2023 at 5:10
  • Spray foam is a pretty standard solution, actually
    – keshlam
    Commented Nov 11, 2023 at 6:27
  • @FreshCodemonger not at all, the whole area is very low risk.
    – sonoflee
    Commented Nov 11, 2023 at 12:47

1 Answer 1


I'd probably start with aerobarrier which is a great air sealing option and won't fill up these cavities like spray foaming will. aerobarrier is a latex caulk like sealant so it also won't eventually crack like spray foam will.

If you don't like aerobarrier then I'd be tempted to take a screw jack and lift each joist up just enough to slip some sill gasket onto the foundation wall under the joist. You'd want to put a piece of dimensional under the bottom of the screw jack to disperse the load to a larger area of your slab to avoid any slab cracking potential.

It shouldn't take too much. There would be some minimal risk to finishes above, drywall joints cracking, floortile/floor grout cracking, etc but the 1/8" you need should actually be very doable.

I'd probably do that for any single joist and skip any built up joists. When foaming the built up joists I'd leave an air gap to allow for any capillary moisture to escape that area.

  • Thanks for this! I haven't heard of aerobarrier, is this the right product? aeroseal.com/aerobarrier Our house is occupied and full of our stuff, so not sure if an aerosol solution would work well. I had considered jacking up the structure to add a capillary break, but I think that would be tough to do without affecting the brick veneer.
    – sonoflee
    Commented Nov 13, 2023 at 16:44
  • That is the right product. Hard to do occupied with furniture but it can be done - really you just need to poly your surfaces. I suspect your brick veneer would be fine if you are only moving up 1/8". Houses at least in my area are built in rainy weather and the framing all dries 1/8" overtime. The center of the houses ends up worse as those have posts / beams vs 2 floors sitting on concrete (which doesn't shrink). Often a depression in the middle of the house as you get to the top floor which is most impacted. All that to say you probably have an 1/8" of give in your framing. Commented Nov 14, 2023 at 6:33
  • passive house projects often use aerobarrier to get to the .6 ACH50 requirement. It isn't cheap but it is hard to beat for performance and doing air sealing right make the house way more comfortable without drafts. Commented Nov 14, 2023 at 6:34
  • The current air leaks may be preventing moisture accumulation in the wood in contact with the concrete. Spray foaming might lead to moisture accumulation. Do you really need to seal and insulate this? Can you estimate the possible savings in heating costs and the possible increase in comfort? Commented Apr 9 at 22:05

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