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  1. #1
    Join Date
    Dec 2008
    Posts
    2

    Default Extra Attic Insulation - Over / Under Plywood

    I live in a town home in Maryland.
    Our attic is constructed with Trusses with only 2" x 4" floor (top level ceiling) joists.
    The attic floor appears to have been originally insulated with 6" fiberglass batt.
    At some point there was a plywood floor / deck installed in the attic. This floor compressed the batt down into the 2" x 4" void space.
    I understand that the performance of the 6" batt is now compromised by its compression.
    I would like to add insulation to the attic (probably blown in cellulose) to help lower my high heating & cooling bills.

    The advantage of pulling up the deck / floor is that this would enable me access to seal the top plates of interior walls and any plubming / chase penetrations as they enter the attic. I have read that these areas provide heat loss through convection and are addressed by sealing well. I suppose that removing the floor would also improve the performance of the existing fiberglass batt.

    On the other hand, it is quite a bit of work to remove the decking which appears to be installed tightly. I'm not sure if in the end I'm going to be any better off thermally by removing the floor given that I'm going to blow a layer of cellulose over the whole attic.

    QUESTION:
    Should I....
    1. Pull up the plywood floor / deck, seal penetrations as they enter the attic, and finally blow in cellulose on top the existing fiberglass batt? OR

    2. Leave the plywood floor / deck in place, seal any penetrations through the plywood, and finally blow in cellulose on top of the plywood floor / deck?

    Advice / Options are welcome. Thank you.

  2. #2
    Join Date
    May 2008
    Location
    Pacific Northwet
    Posts
    1,661

    Default Re: Extra Attic Insulation - Over / Under Plywood

    You could seal the penetrations in the attic floor, AND board up the joist bays (where the 6" insulation becomes 4") at the edge of the plywood, sealing that well. Then just add insulation on top of the floor. I think this would be a lot easier. What are other peoples' thoughts on the possibility of trapping moisture under the plywood, thus soaking that insulation and potentially damaging the ceiling underneath?

    Where the insulation has been compressed, the effective R-value will be about the same as for 4" (OK, 3-1/2") insulation uncompressed (R-13).

  3. #3
    Join Date
    Aug 2008
    Posts
    54

    Default Re: Extra Attic Insulation - Over / Under Plywood

    I would leave the decking in place and dense pack cellulose over the existing fiberglass. This is a very beneficial method that adds R-value, while air sealing the attic quite well. My best air infiltration reductions in homes were obtained in these circumstances, my highest being an 85% reduction determined by use of a blower door. It also leave the floored attic intact if you choose to use it for storage.


    Quote Originally Posted by spollin View Post
    I live in a town home in Maryland.
    Our attic is constructed with Trusses with only 2" x 4" floor (top level ceiling) joists.
    The attic floor appears to have been originally insulated with 6" fiberglass batt.
    At some point there was a plywood floor / deck installed in the attic. This floor compressed the batt down into the 2" x 4" void space.
    I understand that the performance of the 6" batt is now compromised by its compression.
    I would like to add insulation to the attic (probably blown in cellulose) to help lower my high heating & cooling bills.

    The advantage of pulling up the deck / floor is that this would enable me access to seal the top plates of interior walls and any plubming / chase penetrations as they enter the attic. I have read that these areas provide heat loss through convection and are addressed by sealing well. I suppose that removing the floor would also improve the performance of the existing fiberglass batt.

    On the other hand, it is quite a bit of work to remove the decking which appears to be installed tightly. I'm not sure if in the end I'm going to be any better off thermally by removing the floor given that I'm going to blow a layer of cellulose over the whole attic.

    QUESTION:
    Should I....
    1. Pull up the plywood floor / deck, seal penetrations as they enter the attic, and finally blow in cellulose on top the existing fiberglass batt? OR

    2. Leave the plywood floor / deck in place, seal any penetrations through the plywood, and finally blow in cellulose on top of the plywood floor / deck?

    Advice / Options are welcome. Thank you.

  4. #4
    Join Date
    Dec 2008
    Posts
    2

    Default Re: Extra Attic Insulation - Over / Under Plywood

    Shubi,

    The "floor" joists are only 2 x 4s.

    1. I'm not certian how much (if any) space exists between the top of the fiberglass and the bottom of the flooring. The batt is 6" compressed into the 3.5" space.
    2. I'm not sure I can achieve sufficient R value in only 3.5". (short of removing the batt and replacing it with foam).

    Thoughts?

  5. #5
    Join Date
    Aug 2008
    Posts
    54

    Default Re: Extra Attic Insulation - Over / Under Plywood

    If you have R-19 (6" fiberglass) compressed in a modern 2x4, your R value is already quite reduced. Fiberglass loses a lot of thermal resistance if compressed, cellulose does also but to a lesser extent. Under these circumstances, I would remove the flooring, seal any bypasses, and six to 8 inches of cellulose over the fiberglass, giving you an approximate R-38. You can use unfaced R-19 over the existing, but it is more expensive than cellulose and sometimes difficult to fill voids effectively. If you choose this method, you might want to install the additional rolled fiberglass at a right angle over the existing. Make sure your attic is properly vented.

  6. #6
    Join Date
    Jan 2008
    Posts
    731

    Default Re: Extra Attic Insulation - Over / Under Plywood

    keep in mind that the plywood might be serving another purpose as well such as protecting portions of your electrical system (for example electrical cable especially near the scuttle opening to this area). So you may have some other projects/concerns to address as you prepare for your project.

    buildingscience has a very good website that you may find helpful. it also has specifics for different regional areas in the US and Canada.

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