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Community and Q&A

Insulation of a flat roof

susanlane | Posted in GBA Pro Help on

I am considering having my flat roof insulated using blown in fiberglass insulation. One contractor I’ve consulted accesses the space by cutting holes through the roof deck and then installing roof vents to cover the holes once the fiberglass is installed. My building is approx. 25′ x 60″. The contractor recommends the addition of 2 roof vents. Currently, my roof is not vented and because the building is approx. 90 years old, I am assuming that there is no insulation in the space between the roof deck and the interior ceiling.

Our local gas supplier is now offering a rebate for attic insulation which has prompted me to consider this work being done. However, after reading another Q and A on your site, I’m wondering if i should just leave well enough alone. Please comment.

S. Lane

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  1. GBA Editor
    Martin Holladay | | #1

    It would be useful to know your location or climate.

    However, it's safe to say that in all U.S. climates (with the possible exception of some parts of Hawaii), roofs should be insulated.

    I'm not a fan of vents in flat roofs. These vents usually cause more problems than they solve. In general, a vent in a flat roof is more likely to introduce moisture than release moisture.

    The best way to insulate a flat roof is with a layer of rigid foam on top of the existing roof sheathing, followed by new roofing. Of course, this costs more than your plan, because it requires you to replace the roofing.

    Installing blown-in fiberglass insulation in your roof assembly will probably be an improvement over the existing situation, even if it's not as good as rigid foam. Is there any way that your contractor can access the attic by cutting holes in the ceiling?

  2. etrochez | | #2

    I don't mean to hijack this thread and I apologize if I am. Martin, will blowing fiberglass insulation without any vents in the roof cause condensation problems in the assembly? I'm in a similar situation as the OP, I have a flat roof and I'm trying to figure out the best way to insulate. The difference is that I have removed the ceiling in 2 of the 4 rooms in the area that needs insulation, I'll be replacing the Sheetrock in these rooms. I'm located in NYC, do you have any advise?

    Thanks in advance,

  3. GBA Editor
    Martin Holladay | | #3

    I don't have enough experience insulating this type of roof to have an opinion. My gut tells me that it's risky to do as you propose, but I imagine that many people have done it, so the question is: how risky is it?

    Readers: care to chime in?

  4. etrochez | | #4

    Martin, what I meant to ask is if there's any type of insulation that I can use now that the ceiling is gutted in part of the house? Sorry, I wasn't specific. Most contractors in NYC recommend doing what Susan's contractor told her. I don't like the idea of making holes in my roof. So, my two questions are: 1. With the ceiling removed, is there a type of insulation that I can use? And 2. Will blowing in fiberglass insulation cause moisture problems is there aren't vents in the roof?

    Thanks again,

  5. GBA Editor
    Martin Holladay | | #5

    Q. "With the ceiling removed, is there a type of insulation that I can use?"

    A. Yes. Install closed-cell spray polyurethane roofing against the underside of the roof sheathing.

    Q. "Will blowing in fiberglass insulation cause moisture problems if there aren't vents in the roof?"

    A. Maybe -- but as I said before, I'd like to hear from other GBA readers on this point.

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