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With 2 1/2 inches of exterior foam should you spray foam the box sill or regular insulation

Wiscoguy | Posted in General Questions on

I have finally gotten to the point of insulation in the next week so I need to figure out one last thing. My insulator normally spray foams the box sil around the outside of the house. However I have been under the impression that this is not necessary when using enough exterior insulation . I have 2 1/2 inches which is r12 plus a foam backed siding brings me up to about r14 for exterior foam. So do I just use batts on the box sil or would it be safe to spray foam this area and not trap moisture. I do have a heated basement however the ceiling is not Drywalled at this time. 

appreciate any thoughts here everything else on the build seems to be going well. So just want to make sure I get the insulation correct. 

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  1. Wiscoguy | | #1

    Bumping this one just to get some ideas need some help on this subject. Appreciate any thoughts

  2. Walter Ahlgrim | | #2

    In general fluffy insulation around the sill and rim joists is a risky game.

    In that you can’t be sure what the dew point will be in your basement every hour of every day. Will the surface temp of the rim joist ever be cooler than the dew point of the basement air? Fluffy insulation will allow the moist air to come in contact with that surface.

    Will the wood get wet enough for long enough to grow mold and rot seems unknowable.

    This is the one spot that spray foam makes sense to me in new construction. If acts as a vapor barrier and tends to seal a lot of aid leaks when used in this location.

    You may find this article interesting


    1. Wiscoguy | | #3

      I have always thought this was the correct way as well but Ive also read on this same forum and throughout the site that you shouldn’t sprayfoam behind thick exterior insulation.

      So I have 2 1/2 inches of thick exterior insulation that goes completely over my outside rim joist and box sil area. This transitions from the below grade 2” foam and is taped all the way up the wall.

      My concern is by spray foaming this area will a cause a moisture trap in between spray foam and exterior foam?

      Or it the 16” space so small that it won’t be a problem so much as the upper area. Eventually the basement will be finished and this area will be under a drop ceiling or drywall so somewhat segregated from the living space.

      For the upstairs it’s already batts and blown in insulation in the ceiling and I’m using a smart vapor barrier certainteed membrane I know it’s not necessary but it helps with the inspector and accomplishes what I need.

  3. Expert Member
    Zephyr7 | | #4

    I would use cut blocks of EPS here and seal them in place with canned foam. EPS is the most vapor open of the rigid foams, so it’s the least risky here. I insulated the rim joist in my own home this way for similar reasons, since I have foil faced polyiso on the exterior which is a vapor barrier.

    With ~R14 of exterior insulation, you’re actually pretty safe with batts as long as you don’t go overboard. What matters here is that the first condensing surface, which is the inner face of the rim joist here, doesn’t drop below the dew point temperature of the air in the basement. Any insulation you add on the interior will reduce the temperature of that surface according to the temperature differential between indoor and outdoor air and the ratio of the R values between the inside and outside insulation. Less insulation on the inside means a warmer rim joist, so less chance it will drop below the dew point of the inside air.

    If your basement air is reasonably dry, you’re probably safe up to maybe an R11-R15 batt, but EPS would be safer. I would use 2” thick EPS here, which would be around R8.4 or so.


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