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  1. #1
    Join Date
    Mar 2009
    Posts
    237

    Default Insulating existing flat roof

    Im in southeast pennsylvania which i have mapped to be a zone 4 and i'm looking to insulating an existing flat roof. Roxul R30 batts will give me good coverage for the existing space which doesn't have any insulation at all. The space between the finished ceiling and the actual roof is almost 2 feet so the R30 batts will leave plenty of ventilation room left.

    I plan on cutting more access holes in the closet spaces and then laying the batts within the space. My question is about vapor barrier. Some say that because it is an asphalt roof, it already counts as a vapor barrier but if there are roof vents, the moisture will equal the outside so i must still need a vapor barrier. I was planning on getting 6 mil plastic sheeting and laying that down to act as the vapor barrier and laying the batts on top but it doesnt smeem like much of a vapor barrier to me if the edges arent sealed.

    Is this n acceptable vapor barrier solution? If so, I'm not sure how I would seal the edges.
    My advice and opinions come from hands on knowledge...and This Old House Hidden Content

  2. #2
    Join Date
    Jun 2010
    Location
    Chicago
    Posts
    30

    Default Re: Insulating existing flat roof

    Will there be enough space above the batts and below the rafters for air to circulate? The vapor barrier needs to be on the warm side, the roofing membrane doesn't really count (especially with roof vents as you point out). Are you going to use batts with an integral barrier, that might be your best bet rather than a sheet of plastic beneath them.

  3. #3
    Join Date
    Mar 2009
    Posts
    237

    Default Re: Insulating existing flat roof

    There will be sufficient space above the insulation for ventilation. The problem is tga the 2 methods i have chosen for insulation do not have vapor barriers. Roxul product has a higher R-value per inch than fiber glass but they dont make it with a paper facing. Blown-in cellulose obviously does not have a vapor barrier so in botg cases i would assume i have to add one. The only thing i can think of is a big sheet of plastic but im not sure if i can just lay this down first or if i have to attach/seal the edges somehow.
    My advice and opinions come from hands on knowledge...and This Old House Hidden Content

  4. #4
    Join Date
    Dec 2007
    Location
    Tennessee
    Posts
    1,745

    Default Re: Insulating existing flat roof

    Most houses have a peaked roof with plenty of ventilation so they don't need a vapor barrier in the ceiling at all. The wall have much less ventilation so a vapor barrier is necessary.

    But in your case, it depends on how much ventilation you get in the remaining space above the insulation. If you have adequate ventilation, then you could get by without the barrier, but if you don't, then I think the best bet is to take down the ceiling, put in the batts, then put a vinyl barrier on the bottoms of the joists and put up a new ceiling. But that is a lot of work.

    The alternative is to look into adding more ventilation, without creating more opportunities for leaks when it rains.

  5. #5
    Join Date
    Dec 2007
    Location
    Tennessee
    Posts
    1,745

    Default Re: Insulating existing flat roof

    "but if there are roof vents, the moisture will equal the outside so i must still need a vapor barrier."

    This is actually your goal here, to get the humidity and moisture level inside your attic down to that of the outside air. The reason for the vapor barrier is to keep moisture generated inside the home from migrating into the attic and increasing the moisture level up there from going above the dew point.

    If you have enough ventilation to carry any moisture that migrates to the attic before it reaches that point, then you can get by without the vapor barrier. You could limit your vapor barrier to the rooms that typically generate the most humidity, the bathrooms, laundry and kitchen. Just pull down these ceilings, add insulation and a vapor barrier and a new ceiling.

    There are also some paints that have a high vapor barrier rating. So if you are up to painting the ceiling, that could help as well.

    You also have another issue to tackle and that would be "wet walls", those are walls that your plumbing runs in. Often these are double walls and are open to the attic. These are like chimneys for heat and humidity to go from your interior to the attic. If you can't address these, then you are going to be very disappointed in the results from adding all this insulation. You will probably only see about half the savings you were expecting.

  6. #6
    Join Date
    Mar 2009
    Posts
    237

    Default Re: Insulating existing flat roof

    i've thrown up some insulation and there is still a good 18 inch minimum left of space for ventilation. Out of about 700sqft of roof and only 2 tubular roof vents, I think I would have to have a roof cut in a new vent or 2. Also since the rooms have drop ceiling i have placed the 6mil plastic between the plaster&lathe and the drop ceiling to provide the vapor barrier.
    My advice and opinions come from hands on knowledge...and This Old House Hidden Content

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