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Thread: basement joists embedded in concrete

  1. #1

    Default basement joists embedded in concrete

    I have an old house. In the basement I want to take down exising plywood walls and replace with proper insulation and wall board. It seems it is recommended to insulate the rim joist area as well as the walls but I've noticed that the floor joists are embedded within the concrete(house built in late 1920s). Is it wise to insulate the rim joist area if the joists are embedded in the conrete?

    Thanks for any advice.

  2. #2

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    I would insulate the joists above the concrete with 2 inch foam and seal the sides, top and bottom (to the concrete) with spray foam to seal it. Then insulate the basement walls up to the floor joists with unfaced fiberglass leaving 2" between the back of the studs and foundation. Look at this thread for some tips. The rim joist loses a lot of heat and has a lot of air leaks and you would be wise to seal it.

    http://www.selfhelpforums.com/showthread.php?t=16408
    ________
    Vaporite Vaporizers
    Last edited by Fischer; September 15th, 2011 at 11:19 AM.

  3. #3

    Default

    Quote Originally Posted by Fischer View Post
    I would insulate the joists above the concrete with 2 inch foam and seal the sides, top and bottom (to the concrete) with spray foam to seal it. Then insulate the basement walls up to the floor joists with unfaced fiberglass leaving 2" between the back of the studs and foundation. Look at this thread for some tips. The rim joist loses a lot of heat and has a lot of air leaks and you would be wise to seal it.

    http://www.selfhelpforums.com/showthread.php?t=16408
    I read the link. Interesting. Why is it that you read everywhere to attach foam board directly to the wall if so many people have noticed mold growth when it is done this way? I'm pretty sure the outside of the foundation wall is not insulated, would that make a difference?? Also, if my framed inside wall should be 2" away from the concrete why is it okay to attach foamboard directly to the concrete in the rim joist space? Would I be risking mold in the rim joist space? I should also note that about 2 feet of the upper wall is above grade, the rest below if that makes a differnce?

    Appreciate the response. Thankyou.

  4. #4

    Default Insulation

    Quote Originally Posted by garybsp View Post
    I read the link. Interesting. Why is it that you read everywhere to attach foam board directly to the wall if so many people have noticed mold growth when it is done this way?

    Me,
    I've had experience with mold forming between foam (which does not pass mositure) against block or concrete foundations. This has occurred in all types of soils, even soils that perk very well (sand). I can't speak for the people or the manufacturers that recommend this, but in my experience they're lucky if they don't have a mold problem. Personally I think they're dead wrong. Foam should only be placed on a concrete based wall on the exterior.

    I'm pretty sure the outside of the foundation wall is not insulated, would that make a difference??

    Me,
    No it would not make a difference.

    Also, if my framed inside wall should be 2" away from the concrete why is it okay to attach foamboard directly to the concrete in the rim joist space?

    Me.

    Because it is wood, and you would have a vapor barrier only on one side, and the rim joist will not wick water to the interior if it is properly sealed.


    Would I be risking mold in the rim joist space? I should also note that about 2 feet of the upper wall is above grade, the rest below if that makes a differnce?

    Me,

    No, see above answer., and no as far as how far the foundation is above ground or below. Foam should only be on the exterior.

    A further note, is tha if you have the option you can lay 2" foam about 1 foot below the surface next to the foundatiion and sloping away from the foundation. That will keep the soil next to the foundatiion warmer. If you've ever noticed that a piece of insulation or a bunch of hay that is left on the ground serves as a terrific insulater. You would keep the frost above the foam in the dead of the winter.

    Appreciate the response. Thankyou.
    Hope this answers some questions.
    ________
    Nexium Sickness
    Last edited by Fischer; September 15th, 2011 at 11:20 AM.

  5. #5

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    Thanks again Fisher for the advice.

    The rim joist space is strange in my house. Maybe I'm describing it wrong but the only thing that is wood in this space is the actual floor joists. The rim joist itself is not visible. I'm guessing the rim joist itself is embedded in the concrete wall or the concrete is somehow acting as a rim joist. Haha. I hope that makes sense. This is why I'm wondering how to (if at all) insulate that space because it looks like no other "rim joist space" that I've ever seen.

    Thanks.

  6. #6

    Default

    I'm guessing that your joists were laid on top of the foundation and then the concrete was placed between them. Is the concrete kind of sloped back towards the exterior? Common practice.

    I guess I would then run unfaced insulation up your stud walls and then up between the floor joists. Stud walls should be about 2" away from the foundation. 2x4 or 2x6 on exterior walls your choice.
    ________
    Nexium lawsuites
    Last edited by Fischer; September 15th, 2011 at 11:20 AM.

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