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  1. #1
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    Default no existing attic insualtion vapor barrier

    I purchased a 1952 cape cod home in the Baltimore, MD area. The house currently has 3 to 4" of loose mineral wool insulation in the attic. A vapor barrier was never installed between the joist bays and the insulation. If I want to add R30 batt insulation to the attic, should I remove the mineral wool insulation first so that I can utilize the paper vapor barrier of the batt insulation or can I just lay unfaced insulation over the existing loose fill insulation?

  2. #2
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    Default Re: no existing attic insualtion vapor barrier

    In a very similar situation myself, and would like to know too. Also, if removing the mineral wool is the right thing to do, what's the right way to do it?

  3. #3
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    Default Re: no existing attic insualtion vapor barrier

    i recommend laying the unfaced batts over your existing insulation, you dont want to trap moisture escaping from your house, i.e. bathrooms, kitchen, utility.
    HOPE THIS IS HELPFUL!

    Oklahoma City, OK

  4. #4
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    Default Re: no existing attic insualtion vapor barrier

    Make yourself a screed to level off the loose fill to a uniform thickness.

    Lay unfaced batts over the loose to fill the joist bay flush to the top....... if it projects beyond the top then peel off a layer of the batt to make it flush.

    They lay batts over top perpendicular to the joists and you should be good to go.

    Make sure you cover the top plate of the exterior walls ..... this where the joists and rafters sit on. This area is often neglected and is a cause of major heat loss and condensation issues. If there are soffit vents just make to these don't become covered over when doing this.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  5. #5
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    Default Re: no existing attic insualtion vapor barrier

    Forgot to add ...........
    before you add insulation ensure to seal any and all penetrations from the living space into the attic ...... places like the plumbing stack , electrical boxes , etc..
    This can easilly done with canned foam.

    Doing this will go a long way in preventing warm moist air migrating into the cold attic and condensing ...... also helps increasing the insulation performance.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  6. #6
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    Default Re: no existing attic insualtion vapor barrier

    One other wrinkle - I'm planning on using the attic for storage, deepening the joist bays by nailing another 2x6 edge-to-edge with the joists to make enough space for the full height of the batts below the decking. Does that make any difference as to whether to remove the mineral wool or not?

    BTW - there is a lot less than 3-4 inches of mineral wool in my attic - the home inspector estimated a value of R5 for what's currently up there, and plaster is visible through the mineral wool in many places.

  7. #7
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    Default Re: no existing attic insualtion vapor barrier

    Yikes .... you certainly have been loosing heat.

    Leave what's there and fill the joist bays as described.

    Lay your 2x6's perpendicular over the existing joists ...... fill those cavities with batts and lay the sheathing over top...... done.
    "" an ounce of perception -- a pound of obscure "
    - Rush

  8. #8
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    Default Re: no existing attic insualtion vapor barrier

    I'm confused as to how perpendicular joists would work - an R-30 batt is 10 inches thick according to the spec sheet, and due to the mineral wool, there will be less tan 6 inches of clearance.

    This government-produced flyer says:

    For many products, an insulation depth of 10 to 14
    inches is needed to achieve an R-30 to R-38 insulation
    value. Thus, a 2x4 or 2x6 extension needs to be added to
    a 2x6 joist to provide sufficient depth before installing
    decking.
    and the illustration seems to show the joists nailed edge-to-edge. I imagine one would want the seams between boards staggered rather than all in a row, however.

  9. #9
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    Default Re: no existing attic insualtion vapor barrier

    Thanks Canuk.
    You suggestion is very clear.
    As the house is a traditional Cape Cod, there are no soffit vents. There are gable vents to keep air circulating in through the attic aided by a thermostatically controlled fan at one end. Insulating up to the top plate will not be an issue.

    The only question I have is the one I started with. Wont the warm air condense when it comes into contact with the insulation? I was under the impression that that was the whole idea behind a vapor barrier. Has doing what you suggested removed the need for a vapor barrier?
    Last edited by cjwittmeyer; 10-14-2009 at 10:32 AM.

  10. #10
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    Default Re: no existing attic insualtion vapor barrier

    Canuk is telling you to fill the existing joists with insulation. this will give you roughly 6" in depth. Add additional joist perpendicular for both added strength (ie egg crate) and added depth. You can fill the new joist bays with batt for a total depth of about 10-12". Adding plywood decking on those new joists completes to whole package.

    Quote Originally Posted by jackbird View Post
    I'm confused as to how perpendicular joists would work - an R-30 batt is 10 inches thick according to the spec sheet, and due to the mineral wool, there will be less tan 6 inches of clearance.

    This government-produced flyer says:



    and the illustration seems to show the joists nailed edge-to-edge. I imagine one would want the seams between boards staggered rather than all in a row, however.

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