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What is best way to upgrade insulation in sloping ceiling with rubber roof directly above it?

BB Minc | Posted in Energy Efficiency and Durability on

I’m about to take out a small room that encroaches on my living room to enlarge my liv rm/dining rm area. The living room is in an addition to the building with a gently sloping rubber roof directly above it. This room is always cold, the walls and ceiling are not very well insulated. We will have to cut open the ceiling to place a support beam down the middle perpendicular to the joists. While we have the ceiling open, we are thinking it would be a good time to put in additional insulation in the roof. Right now it looks like the roof is vented under the eves but probably not along the top. Would blowing in a solid pack of cellulose insulation be the right option for us? We have to shovel the roof occasionally when we get a heavy snowfall and sometimes the rubber roof has gotten damaged, so I’m a little worried about mold with the cellulose. If we blow in a solid pack of cellulose will roof still need to be vented? Can we get the cellulose packed tight enough with a blower rented from Home Depot or is this a job for a professionals?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    BB,
    If this is a rubber roof, I'm guessing it's a low-slope roof. It would help to know the slope.

    Assuming it's a low-slope roof, you have only one option: seal the ventilation openings and install closed-cell spray foam on the underside of the roof sheathing. Here is a link to an article with more information:
    Insulating Low-Slope Residential Roofs

    If the roof has a substantial slope, the next question is: Can you conceive of any way to install a ridge vent? If the upper end of the slope terminates in a wall, this is tricky -- but there are a few proprietary venting products to address this dilemma. Whether to even bother depends on whether you have your heart set on a vented roof assembly. The unvented solution is probably easier.

  2. BB Minc | | #2

    Thanks for your reply! Yes, it is a low slope roof. and the roof terminates in a wall. but the ceiling is a cathedral ceiling without any attic space. I'm hoping to avoid the expense of taking down all of the sheet rock and completely redoing the ceiling, by just adding dense-pack cellulose to the existing fiberglass bats. My understanding is that if you completely fill the space between the rafters that would eliminate the buildup and movement of vapors within the the roof cavities? We will be cutting open a 12" or so strip of drywall across the middle of the ceiling to put in a horizontal support beam, and am thinking we could cover the opening with a heavy plastic or fiberglass cloth and blow the cellulose through openings in it, before installing the beam. Does that make sense?

  3. GBA Editor
    Martin Holladay | | #3

    BB,
    Q. "I'm hoping to avoid the expense of taking down all of the sheet rock and completely redoing the ceiling, by just adding dense-pack cellulose to the existing fiberglass bats. My understanding is that if you completely fill the space between the rafters that would eliminate the buildup and movement of vapors within the the roof cavities."

    A. Your approach violates building codes and is risky. Building codes don't allow unvented roof assemblies to have their rafter bays packed solid with an air-permeable insulation like cellulose unless there is an adequately thick layer of rigid foam above the roof sheathing. If you ignore this advice, the roof sheathing is likely to accumulate moisture and rot.

    All of these facts are explained in this article: Insulating Low-Slope Residential Roofs.

    If one article isn't enough to convince you, here is a link to another: How to Build an Insulated Cathedral Ceiling.

    You need to either pull down the ceiling and install closed-cell spray foam, or you need to add rigid foam above the existing roof sheathing, plus new roofing.

  4. BB Minc | | #4

    Thank you for clarifying!

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