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  1. #1
    Join Date
    Apr 2008
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    Question Attic kneewall insulation

    I live in cold, six-month winter, upstate New York, and am looking to add some insulation to my attic. There are two rooms which are finished, but there is unfinished space behind the kneewalls. I would like to add insulation back there (I have already added some fiberglass at the space where the floor, lower wall and roofline meet). There is a wood-paneled floor back there, not exposed joists. I don't know whether I should go with blown-in cellulose under the flooring(which would leave me the storage space, but would cost more $$) or lay fiberglass on top of the floor (this would be preferred because I could do it myself).

    If I go fiberglass, is R-49 appropriate, should it be paper backed and does the paper go on the floor or on top? I also assume I would not want to put a vapor barrier in, right, since that might create a mildew situation?

    I'd appreciate some input(especially from Canuk).

  2. #2
    Join Date
    Mar 2008
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    nova scotia, canada
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    Default Re: Attic kneewall insulation

    r49 insulation is your best bet r-40 has been the standard for attic ceilings but in some regions codes have increased it to r50.

    batts wont blow around or make a mess like blown in will if you plan to use the space for storage. you should have the insulation as close to the finished ceiling as possible so to eliminate a dead air space. as for vapor barrior if its not present now it pretty tough to install one after the fact, however what you can do is install rigid styrofoam between the joists then shoot a light bead of spray foam around the perimeter and then tape the seams of the foam.. use type 2 closed cell btw. from there lay your batts on top of that

    another option is to buy vapor barrier primer for which you can reprime then paint the ceilings
    fire up the saw and make some dust

  3. #3
    Join Date
    Aug 2007
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    Default Re: Attic kneewall insulation

    To add to jkirk's post ...

    Quote Originally Posted by kbaynes View Post
    I live in cold, six-month winter, upstate New York, and am looking to add some insulation to my attic. There are two rooms which are finished, but there is unfinished space behind the kneewalls. I would like to add insulation back there (I have already added some fiberglass at the space where the floor, lower wall and roofline meet). There is a wood-paneled floor back there, not exposed joists. I don't know whether I should go with blown-in cellulose under the flooring(which would leave me the storage space, but would cost more $$) or lay fiberglass on top of the floor (this would be preferred because I could do it myself).
    First thing that comes to mind ---- what's under that flooring as far as insulation?
    Unless someone had previously filled the bays with suffient insulation before covering with the wood, there's a good chance there's not much insulation there to begin with.

    As jkirk mentioned adding insulation on top of this floor with an air space underneath is not a good thing. There will be some benifit from the extra insulation though not as much without the air space underneath --- not giving you your biggest bang for the buck. If there is an air space between the 2 layers of insulation you will end up with moving air ( stack effect ) which reduces the performance of the insulation.

    Secondly if there are pentrations from things like electrical ( ceiling fixtures in the rooms below ) and plumbing stack there will be warm humid air escaping into that void under the attic floor.

    In order to achieve R40 you will need to have at least 12 inches thick of insulation , regardless of fiber glass or cellulose.

    If it were me I would pull up the wood flooring in the attic space -- cover any electrial boxes for lights below with vapour barrier , seal any other penetrations , then fill the attic space with the insulation of choice.

    If you need the space for storage then closed spray foam in the joist bays would provide the R value needed and still allow you to apply a floor covering --- if you have the $$$$ budget.


    If I go fiberglass, is R-49 appropriate, should it be paper backed and does the paper go on the floor or on top? I also assume I would not want to put a vapor barrier in, right, since that might create a mildew situation?
    Forget about using faced batts all together since they will have potential for creating mold/mildew issues in the attic space --- use the unfaced batts.

    The issue with a vapour barrier in your case isn't much of a concern. Chances are there is are several coats of paint on the ceilings below that will retard pretty much any diffusion.
    The issue would be more from air infiltration from below --- seal any and all pentrations to stop the warm moist air from below reaching the colder attic which will condensate. This one of the most important steps before adding insulation.


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

  4. #4
    Join Date
    Mar 2008
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    nova scotia, canada
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    Default Re: Attic kneewall insulation

    as canuk says, for spray foam $$$$ very much so, spray foam costs roughly 3x that of fibreglass batts
    fire up the saw and make some dust

  5. #5
    Join Date
    Mar 2010
    Posts
    6

    Default Re: Attic kneewall insulation

    I have a cape style house with probably the exact layout you are talking about. Developed attic with 2 kids rooms upstairs with knee walls. Anyway, i fought with this for a long time and had several problems with it all. I pulled up the subfloor in the attic storage space and it was completely under insulated. I was losing heat right out of the main floor through the attic space. I tried beefing up the insulation and tried numerous things to insulate this area better. Everything didn't work very well. Finally, I saved and spent the $$ on a combination of spray foam insulation and injection foam insulation in areas where the spray foamer did not have access to. It does cost a lot, but if you can capitalize on energy rebate programs (federal and state), now is the time to do it. I saved a lot on it and so far, it's the best investment I've made in our house in a long time.

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