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Community and Q&A

Dealing with inside humidity that is creating damage to a flat roof

dianeStuff | Posted in Energy Efficiency and Durability on

I have a TPO membrane-based flat roof on a 3 story above ground townhouse. My roof shows signs of deterioration that has been attributed to condensation from high interior home humidity meeting the cold air and therefore causing damage from underneath vs. damage to the membrane. There are signs of interior damage to walls or condensation/moisture issues. Since I have not been given a definitive source of the humidity I am wondering why this would occur, how I can find out the root cause of the problem, and what is the right way to fix it. Thanks for any insights.


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  1. Expert Member
    Dana Dorsett | | #1

    Location/climate and a better description of the roof assembly would make this a bit easier.

    It's hard to properly ventilate a flat roof, but is there a vent space between the roof deck and any insulation under the roof deck?

    Is there any insulation (such as rigid foam board) above the roof deck and the TPO membrane?

  2. GBA Editor
    Martin Holladay | | #2

    Here's how the condensation happens: warm, interior air hits a cold surface. In this case, the cold surface is probably your roof sheathing (plywood, OSB, or boards).

    To prevent the problem, your home needs (a) an interior air barrier like drywall, without leaks, (b) adequate insulation, and (c) in some but not all cases, a ventilation space between the top of the insulation and the underside of the roof sheathing.

    Here is a link to an article that describes how to insulate a flat roof: Insulating Low-Slope Residential Roofs.

    High indoor humidity can make these problems worse. Here is a link to an article that discusses indoor humidity problems: Preventing Water Entry Into a Home.

  3. dianeStuff | | #3

    Thank you for your helpful responses. In response to Dana - I live in Northern Virginia in a townhome built in the late 60s. There is a small ~12" space between the interior ceiling and the roof sheathing (plywood). Don't know if there is any insulation under the sheathing. Sloping insulation board is installed under the membrane. The only vents on the roof are those used to vent bathrooms.

    Diane M

  4. GBA Editor
    Martin Holladay | | #4

    You have told us that your roof assembly has sloping insulation board (probably some type of rigid foam) above the roof sheathing. But you didn't tell us how thick the insulation is. And you also seem to be uncertain about whether there is any insulation between the ceiling and the roof sheathing.

    It's hard for us to advise you unless you can provide more information on the types of insulation you have, the the thickness of the existing insulation.

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