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Community and Q&A

Rim board insulation with outside 2 1/2 “ of foam

wiscoguy | Posted in General Questions on

Been living in my house for about 10 months all the insulation has paid off bills are great house is very very comfortable. 

however around the rim board I currently just have bat insulation but I have been considering possibly doing an open cell spray foam since I already have 2 1/2 of exterior foam even though its probably semi vapor permeable I’m guessing around 1 perm or slightly less than that even based on the literature I can find.

id like to use open cell only because it could still breath some but also seal off any potential areas of air infiltration or would I be safe using closed cell then I worry it potentially not allow the rim board area to breath. The area I’m concerned about is roughly 16” wide going around the entire outside of the house. I used floor trusses so it’s just sheeting on the outside covered by a really good water barrier then the 2 1/2” of foam.

lastly One area on the outside rim board only has a 1/2” of foam due to an over site so the concrete would not sit proud of where the concrete stoop was poured so we now have cold concrete up against only 1/2” foam and then they good rain barrier behind that. In this area I’m wondering if it would be ok to use closed cell spray foam since the 1/2” foam is not a great vapor barrier like the 2 1/2” foam is it should still be able to breath somewhat from the outside.

appreciate any thoughts on solutions for these areas 

ive had great success sharing thoughts and ideas in this forum look forward to your responses.  

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  1. wiscoguy | | #1

    Any thoughts from anyone ?

  2. onslow | | #2


    Guessing the name is a reference to Wisconsin. Before doing anything that can't be undone easily, I would propose first that you check the actual current surface temperature of the rim board. If you have an IR thermometer great, if not, many multi-meters come with a thermocouple you can put in place of the red/black leads. With luck you will see about 50 or so.

    Lacking either, put your wrist on the surface. It will be pretty obvious if it is colder than you like. To test for moisture risks, stapling some tissue paper (like the Christmas gift kind) to one of your joist pockets and then set a batt of your choosing like you would if doing all around. Wait a week or more and then remove to see how the tissue paper is doing. If the batt doesn't make the rim temperature drop to a condensing surface point then batts might be safe.

    Cut and cobble or spray foams will create a sandwich that will slow any drying potential you might have by using vapor open batts. Think of your situation as similar to finding the balance of interior to exterior insulation of your walls up above. Too much insulation inside the wall and the sheathing surface will not stay warm enough to not become a condensation condition. Of course, if you get another winter like the last one and get -20 for a week, the rim board is going to get VERY cold. You might even get frost. Not having batts would ensure rapid recovery, having batts would at least allow for a slower recovery. Most of the year the rim board should not be at a dangerous temperature, so batts are safer.

    I designed around the issue by dropping my exterior sheathing and the 6+inches of exterior insulation down past the sill plate by 5". The joist pockets are completely open and average about 56F when 0F outside. This is largely due to the floor losses to basement keeping them warm. Your pockets represent almost 1.8 sf per pocket so while you might be losing more heat than you like when very cold, it might be a livable penalty for most of the year. So no batts, no problems.

    1. wiscoguy | | #3

      Thanks for the response. My 2” of foam goes all the way down from the footing to the gable I just have a few spots I’d like to seal better around pipes.

      My main problem area is where the concrete gets up against the house on my porch it’s the only are where I used a thinner foam for about 6” and it gets pretty cold in this spot and there’s no easy way to fix it now. I’ve actually thought of cutting out the concrete and putt in foam but it would be a huge pain in the ass. So was hoping to correct from the inside possibly. I suppose I could just seal better around the exterior pipes but that concrete against the house with not as thick of foam is really a problem.

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