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  1. #1
    Join Date
    Apr 2012
    Location
    Princeton NJ
    Posts
    125

    air seal attic: plywood ok?

    Hi,
    In the name making my HVAC more effective, may i ask a question about plywood in the attic. The house has a 10x6 x4 foot deep sized built in wooden display case, with an area for a minifridge. To spare you details, the built is connected to openings to the attic with no insulation. It causes a great deal of "ventilation" to leak all around and between the entire built in. The opening from the display case, to the attic is irregular--parts of rafters are in the way. Its not like i can easily just throw a roll of insulation over the area. I would need cut plywood into strips and squares, nail them in place, then lay insulation over it. Can i do this with plywood, or do i have to use drywall which i am less familiar with? I'm specifically worried about risk of fire? There is a recessed light in the built in, but the plywood i would overlay would be about 2 feet above it/then insulation.

    I am a white collar type guy, and I would hire someone to do the job, but i'm not sure who would do it right---I have a feeling by the time i spent talking and interviewing people, it would be easier to do myself.

    Thanks
    Steve

  2. #2
    Join Date
    May 2004
    Location
    south louisiana
    Posts
    3,187
    what you are describing is called a thermal bypass.
    in the attic you can see the top of the 'built in' at a lower
    level than the rest of the attic floor.

    I use ductboard to seal between ceiling joists.
    measure & cut to fit area to be filled. cut long enough to cover opening
    and overlap onto attic floor. cut it to fit tightly between joists.
    caulk ductboard to attic floor, secure with button cap nails.
    caulk to joists to make air tight.
    once the opening is closed & sealed then put batt insulation
    on top of ductboard.

    over the years I've tried many methods. plywood & sheetrock
    were difficult to install, because sometimes you need to cut to
    fit as you put the plywood or sheetrock in place. ductboard
    is much easier to work in place.

    from inside the house you can do additional air sealing by
    using clear caulk to seal inside the built in.

    with the recessed light in the ceiling of the built in and the
    ductboard at attic floor there is no worry about overheating
    of fixture. 2' is plenty of space.

    it isn't uncommon to see batts covering openings like this.
    I've learned to move batts around to look for these thermal
    bypasses. fireplace inserts, dropped ceilings at showers,
    closets and like you have...built ins.

    have to agree..by the time you find someone to do the work
    you could have done it yourself. thats how I wind up doing
    these types of things!

    best of luck.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  3. #3
    Join Date
    Apr 2012
    Location
    Princeton NJ
    Posts
    125
    energy rater-----U ROCK!!!! That is exactly what it is, and thanks for the careful explanation.

    I was a little hesitant of plywood, thinking that drywall would be more of a fire stop, but at least the ductboard can't burn. The area is tight to work in, so the ductboard seems much more doable.

    I'm feeling pretty lazy--probably won't get around to it for several weeks!

    Happy New Year.

    Steve

  4. #4
    Join Date
    May 2004
    Location
    south louisiana
    Posts
    3,187
    put foil facing into attic space so that when caulked it will be
    air tight.
    Happy New Year back at ya!
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  5. #5
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,339
    Quote Originally Posted by central nj View Post
    I was a little hesitant of plywood, thinking that drywall would be more of a fire stop, but at least the ductboard can't burn. The area is tight to work in, so the ductboard seems much more doable.
    Be sure you catch the part where energy rater suggested working from the attic vs. working from below (if I interpreted what you described in the OP correctly). Your challenge as a homeowner may be finding duct board...Home Depot used to carry it but I'm not sure they still do. It is sold in HVAC supply houses, many of them may not sell direct to the public since they are wholesale. You do want duct board vs. foam sheathing, as most foam sheathing requires some form of fire resistant barrier between it and the rest of the building.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  6. #6
    Join Date
    Apr 2012
    Location
    Princeton NJ
    Posts
    125
    Thanks for the help. As Shophound suggested, I cannot easily find duct board.

    Back to plywood---is there a code (or more importantly safety) issue with plywood covering this sort of thing, then insulation? I know drywall has flame resistant properties but i'm not familiar with working with it.

    Thanks, any comments appreciated.
    Steve

  7. #7
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    7,041
    5/8 Sheetrock = ~1hr fire rating. You should be able to find an hvac supply house that will sell you duct board.

  8. #8
    Join Date
    May 2004
    Location
    south louisiana
    Posts
    3,187
    http://www.homedepot.com/buy/master-...6-dboard1.html

    if your local HD doesn't have it in stock...they can order it for you.
    much much easier to work with compared to sheetrock.

    to work with sheetrock, you use a straight edge & a razor knife.
    first you score the sheetrock on both sides, then chose a side
    to make a deep cut.
    turn the sheetrock on its side & crack along
    cut. scoring the sheetrock keeps paper backing from tearing.

    you may have to practice a bit first..but sheetrock is cheaper
    by the sheet & you'll have excess.

    caulk under sheetrock to attic floor when ready to install piece.
    you can't caulk edges because sheetrock dust keeps it from
    making a seal.

    best of luck.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  9. #9
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,081
    Moved to General Discussion forum.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  10. #10
    Join Date
    Apr 2012
    Location
    Princeton NJ
    Posts
    125
    thanks very much! Very helpful.

    Steve

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