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  1. #1
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    Dec 2011
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    Default interior basement insulation

    We built a new house in 2007 and insulated the exterior basement walls with rigid styrofoam. We are finishing the basement and would like to insulate the interior walls also. We live in the upper Midwest, so our winters are very cold. We don't want any moisture issues that will cause mold or mildew. We've found information that advises against an interior vapor barrier of any kind, so the moisture can escape the poured concrete wall. What type of insulation should we use in the interior stud walls?

    Marg

  2. #2
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    nova scotia, canada
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    Default Re: interior basement insulation

    sounds like your talking about partitions.. if this is the case you dont need insulation for keeping the heat in. the only time partitions get insulated is for sound transmission and this is done between bedrooms and bathrooms along with theatre rooms. in this case rock wool insulation is used as opposed to fibreglass.

    rock wool has a better stc rating where as fibreglass is for r value
    fire up the saw and make some dust

  3. #3
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    Default Re: interior basement insulation

    Sorry, I wasn't clear. We insulated the exterior foundation walls and would like to also insulate the interior of the foundation walls. The foundation is poured concrete. The partition walls will not be insulated.

    Thanks. Marg

  4. #4
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    Default Re: interior basement insulation

    Quote Originally Posted by eagleton View Post
    We built a new house in 2007 and insulated the exterior basement walls with rigid styrofoam. We are finishing the basement and would like to insulate the interior walls also. We live in the upper Midwest, so our winters are very cold. We don't want any moisture issues that will cause mold or mildew. We've found information that advises against an interior vapor barrier of any kind, so the moisture can escape the poured concrete wall. What type of insulation should we use in the interior stud walls?

    Marg
    Marg --- welcome to the forum

    Having the rigid foam installed on the exterior side of the foundation wall below grade is a good thing. Not only for providing a continious thermal break for insulation but also helps with preventing moisture infiltration.
    If there was at least a continious damp proof coating applied first along with proper footing drains and if the rigid foam was extruded foam, there shouldn't be any concern for moisture issues.

    The exterior rigid foam should be only as far as the grade level with the rest of the exterior exposed. This is to allow any moisture in the concrete that may be present an avenue to exit.

    Perhaps some of the information you received was pertaining to vapour since you shouldn't have moisture ( from the exterior ) issues.
    Before finishing the walls I recommend cutting a number of clear plastic squares ( from sheet plastic such as used for a vapour barrier ) about a foot square.
    Tape these ( continiously around all the four edges ensuring a tight seal ) to the bare concrete walls in a variety of spots on all the walls.
    I like to place some near the corners and have them placed with one low near the floor --- midway --- and up at around grade level.

    After about one week inspect them to see if moisture has accumulated --- notate if any moisture is under the plastic or on top. If under then there is moisture migrating from the exterior inward and needs to be addressed before finsihing the walls. If any moisture is on top of the plastic this indicates an interior relative humidity ( RH ) condensation issue. This will likely be a non issue when insulation is applied and by keeping the humidity level under control.

    Personally I would apply a continious layer of one inch rigid foam panels to cover the inside walls --- from the floor to ceiling. Use a house wrap tape to seal all butt joint seams of the foam panels. In the corner butt joints and along the bottom ( where the panels meet the floor ) use foam in a can like a caulk to provide an air tight seal.
    Then frame the wall studs so they are in front of the foam but touching the foam. --- run the wires ---- apply the drywall and you will have dry warm walls.

    Hopefully this helps and makes sense.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  5. #5
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    Default Re: interior basement insulation

    Thanks for the clear and thorough answer. We have the patches of plastic on the walls and no signs of moisture or condensation yet.

    A follow-up question...On the exterior of the foundation, we insulated to the top of the foundation wall, above the ground level. We don't want to remove that insulation, but do want the moisture to have a way to escape the concrete. Our basement layout is such, that some sections of the walls will not be insulated. For example, one wall is 54 feet long, but only the middle 30 feet will be finished with insulation and drywall. Twelve feet on both ends of that foundation interior concrete wall will be exposed in the storage rooms. Will that be sufficient for the moisture to escape?

  6. #6
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    Default Re: interior basement insulation

    Quote Originally Posted by eagleton View Post
    Thanks for the clear and thorough answer. We have the patches of plastic on the walls and no signs of moisture or condensation yet.

    A follow-up question...On the exterior of the foundation, we insulated to the top of the foundation wall, above the ground level. We don't want to remove that insulation, but do want the moisture to have a way to escape the concrete. Our basement layout is such, that some sections of the walls will not be insulated. For example, one wall is 54 feet long, but only the middle 30 feet will be finished with insulation and drywall. Twelve feet on both ends of that foundation interior concrete wall will be exposed in the storage rooms. Will that be sufficient for the moisture to escape?
    Interesting -- that's not a common method to insulate the exposed portion above grade. Is it covered over with some type of finish material ?

    The main reason it's not commonly done that way is to allow any moisture that wicks up in the concrete to drive to the outside -- this avoids the moisture having to drive into the living space where you don't want it. The insulating portion would then be taken care of from the inside.

    As it stands with what you want to do --- any moistrue that needs to escape the concrete will have to come into the basement instead of exiting to the outside. This means you should be getting yourself a hydrometer and keep a watchful eye to ensure the relative humidty ( RH ) doesn't get too high within the basement. If the RH is too high you run the risk a mold and mildew issues --- a dehumidifier may be in your cards.
    Last edited by canuk; 12-28-2011 at 11:33 PM.
    "" an ounce of perception -- a pound of obscure "
    - Rush

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