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  1. #1
    Join Date
    Dec 2008
    Posts
    1

    Default foam insulation board over fg insulation

    My husband and I are remodeling an old house. We took out all the slats and plaster on the ground floor. To make up the distance we lost with the slats and plaster we are insulating with kraft backed insulation. Then we want to put up 1/2" foil back foam insulation board then 1/2" drywall. My question is whether or not the Kraft paper up against the foil will create moisture?

  2. #2
    Join Date
    Aug 2007
    Location
    The Great White North
    Posts
    4,045

    Default Re: foam insulation board over fg insulation

    In a sense yes this could potentially trap moisture between the foil backed foam board and the kraft paper faced batts .... considering both are vapor retarders.
    In the case warm moist air was able to get in behind the drywall through penetrations from electrical or plumbing for example .... this may be trapped between the 2 layers and not allowed to breath out.

    If you decide to use the foil backed foam then use unfaced batts .

    In my opinion save yourself a few bucks and use extruded foam sheets without the foil .... for the method you're using it this isn't really benificial.

    Regardless .... ensure to seal the butt joints where the sheets come together with house wrap tape . Also where the edges of the foam sheets meet the ceiling , corners and floor leave a gap of around 1/4 inch and fill with canned foam.

    Having the foam sheets completly covering the studs directly behind the drywall will definately help the insulation performance.

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

  3. #3
    Join Date
    Jan 2008
    Posts
    731

    Default Re: foam insulation board over fg insulation

    Which way are you facing the foil, what is on the outside of the framing? where are you located?

    Insulation and vapor barriers, drainage planes: best practices vary by construction type AND geographic/climatic location. For example, arid mostly warm climate where heating days are few but cooling days are high, marine warm climate few heating days, but great need to dehumidify and cool throughout the year versus Frigid cold winters large number of heating days with humid warm summers.Most important is to consider that any fluid water accumulations as well as pockets of humidity from inside or outside can wick/drain/dry out rather than cause issues.

    You may want to consider draft blocking/smoke/fire suppression while you have the walls opened up - this generally also assists in your "insulation" project's effectiveness. You may want to consider a thermal break before you install wall board.

    You may find the web site buildingscience.com very helpful. You'll find information specific both to construction types and to climatic regions for insulation, air barrier, vapor barriers and moisture barriers; avoiding mold, rot and mildew problems; and some good information on avoiding radon accumulation. Depending on where you're at and the type of construction - you may also want to beef up protection from insect (like termites and ants - ants seem to like foam) invasion and rodent proofing - projects and less damaging to complete while the walls are open then after they are closed.

    Hope that helps and good luck with your project.

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