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Community and Q&A

Venting a flat roof properly

James DeSalvo | Posted in Energy Efficiency and Durability on

I have an existing house with a flat roof with an an addition that has a shed roof on one side that blocks the vents on one side. The shed roof addition stands 8ft above the flat roof. The original construction had fascia vents on both sides of the roof, but when they built the addition they blocked one side. I am renovating the addition portion of the house and plan to use closed cell on the walls and ceiling. my plan is to drill holes through the sill plate of wall that sits on the flat roof close to the exterior face of that wall and use a baffle to create an air path that would go from the fascia vent on one side and up the wall that extends above the flat roof and out the ridge vent in the shed roof. once the baffles are in place then the closed cell would seal the envelope. Does this sound like it would work? The wall is 2×6 so even with the baffle I should get enough R value. I was also going to create an air space on the shed roof prior to spray foam. Any comments would be appreciated. Thanks

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Does this sound like it would work?"

    A. No. Here is a link to an article that explains your options: Insulating Low-Slope Residential Roofs.

  2. James DeSalvo | | #2

    Ok, So I guess that I was not being clear enough. I am faced with the following:
    1.) leave the fascia vents on one side only
    2.) create a path by wich natural convection would be made by the connection of the shed roof with the flat roof via the vertical wall
    3.) close the fascia vents off
    The flat roof only has fiberglass batts with an R22 value, so one day in the future the roof will need to be replaced and rigid foam can be placed on top but that will have to wait. I just don't do anything that will make the situation worse than it already is. I have read the link that you mentioned and thought that the reason flat roofs are hard to vent was do to the fact that hot air rises and in a flat roof there is no pull from lower colder air to higher hotter air. your best guess is all I can ask for.

  3. GBA Editor
    Martin Holladay | | #3

    There are a lot of poorly vented flat roofs out there that are insulated with fiberglass batts, and it sounds like you have one of them.

    I'm reluctant to advise you to violate the rules for insulating low-slope roofs, just because you don't want to "make the situation worse than it already is." (After all, the title you chose for this Q&A thread is "Venting a flat roof properly." Your title, not mine.)

    If you want to know what to do with this poorly ventilated low-slope roof insulated with fiberglass batts, the answer is, "Either ventilate it properly with one or more dog-house style vents in the center of the roof, plus adequate ventilation inlets at the perimeter, or seal up all vents and install an adequate thickness of rigid foam above the roof sheathing."

    These approaches aren't cheap, but they are the correct approach. Good luck.

  4. Charlie Sullivan | | #4

    I like your idea better than Martin does. I think the 8-foot wall could work better than a typical dog house vent, given that it's taller. But I do wonder about the climate and the orientation of that wall. A little sun on it could help drive the stack effect convection.

  5. James DeSalvo | | #5

    I'm it climate zone 5 and the wall faces west. It is also sits high up on a hillside so it gets a lot of afternoon sun. The dog house vents wouldn't work because the roof deck is attached to the ceiling joists. It's more like a cathedral ceiling. It was built with just an 1 or 2 pitch overall from the center out. they must have added extra insulation on the addition side because water doesn't pool there. Martin is correct in saying it was poorly designed though!

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