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Community and Q&A

Insulating a flat roof: ventilated or unventilated?

Yf9Setezmi | Posted in Energy Efficiency and Durability on

I have a (very sligthly sloped) flat roof deck held up by 2×8’s to which the ceiling for the upstairs room is attached. Therefore, the space between the ceiling and the roof deck is about 7 inches – much like a cathedral ceiling, except that it’s basically flat.

In each 16″ wide channel there is a pitiful layer of R13 fiberglass batt attached to vapor barrier paper laid against the ceiling, with cold air blowing continuously over and arround it from openings at either end (“vents”).

Because taking down the ceiling is not an option and I’m not keen to re-roof, I need to work with the 7″ cavity. However, I’m finding strongly differing opinions about whether the solution should (a) retain the ventilation under the roof deck, or (b) seal it off, making the roof assembly unventilated.

For ventilated roof: the best option appears to be blown-in loose fill fiberglass on top of the fiberglass batt, which will cut down on the air infiltration but (apparently?) still allow the roof to ‘dry out’. This would be a marginal energy efficiency improvement.

For unventilated roof: (1) dense-packed cellulose on top of the fiberglass batt would create an air barrier; (2) injection foam (tripolymer or light weight closed-cell foam) to fill up the space completely, which would be impermeable and it would also have the highest R-value.)

One contractor in my area strongly advises against injection foam for this application “because the roof has to breathe.” He also says that the injection foam application makes it impossible to trace roof leaks for repair.

Question 1: Is it really inadvisable to convert the 7″ cavity in the roof assembly to an unventilated space by filling it up completely with insulation? (Any experience out there with this?)

Question 2: Is it appropriate to blow-in dense-packed cellulose in this application?

Thank you for any insight from experience.

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  1. GBA Editor
    Martin Holladay | | #1

    H. Hummel,
    Flat roofs (or low-slope roofs) should be unvented. Period.

    Your two best options are spray foam insulation on the underside of the roof sheathing -- an installation which will require the ceiling to be removed, at least partially -- or the installation of rigid foam on top of the existing sheathing. The second option will require new roofing and roof sheathing, of course.

    In either case, plug up those vents.

  2. yonyonsin | | #2

    "Flat roofs (or low-slope roofs) should be unvented. Period." Martin, is that because not enough air can be made to move through the rafter/joist bays?

  3. GBA Editor
    Martin Holladay | | #3

    There are several reasons why you don't want to vent a flat roof:

    1. Even when a roof is sloped, the ventilation isn't able to do many of the things that ventilation proponents expect -- for example, lower roofing temperatures significantly.

    2. Not much air moves through vents installed in flat roofs.

    3. Experiments with venting flat roofs have resulted in more problems (especially water leaks) than occur in unvented roofs.

  4. yonyonsin | | #4

    Thanks, Martin. If I may ask another, related question, Would it work to use Roxul, rather than rigid foam, over the roof sheathing in an unvented assembly, as long as the sheathing is adequately sealed and functioning as an air barrier?

  5. GBA Editor
    Martin Holladay | | #5

    Yes, manufacturers of mineral wool insulation make insulation panels that are suitable for such an installation. Be sure to check with the mineral wool manufacturer to be sure you are buying a product appropriate for such locations.

  6. yonyonsin | | #6

    One last question (I promise!) while I've got you on the line. Various articles about unvented roof assemblies, on this site as well as Building Science, prescribe air barriers both above and below the cavity insulation. In your article, "One Air Barrier or Two?", concerning walls, you argue convincingly that one, properly constructed, barrier will suffice. Is there something about a roof that would require two barriers?

  7. GBA Editor
    Martin Holladay | | #7

    This issue is complicated, but briefly, if you are installing fluffy insulation like fiberglass batts, it's certainly best if the fiberglass batts are enclosed with an air barrier on all 6 sides. In my article, I pointed out that most American builders can't manage a single air barrier, much less two. The best builders, of course, install an air barrier on both sides of fluffy insulation in walls.

    The code and the Energy Star program have traditionally given insulation installed on attic floors a pass on the top-side air barrier, basically because of years of builder habit. In fact, wind-washing (especially near soffit vents) degrades the performance of insulation installed on attic floors. The best solution to this problem is good detailing to address wind-washing near soffits, and, in addition, making the insulation deeper than it ordinarily would be to compensate for the lack of the top-side air barrier.

    Basically, it's cheaper to install a thicker layer of cellulose than to install Tyvek on top of the insulation.

  8. yonyonsin | | #8

    I should have specified that I'm talking about an unvented flat roof. It would look something like the attached detail (except horizontal, obviously). The seams of the roof sheathing would be sealed, creating an air barrier above cellulose cavity insulation. I would take care to make the drywall tight, too, but would prefer not to rely on it as a be-all end-all barrier. (The space will be an art studio with many light fixtures and a couple skylights poking holes in the ceiling plane.)

  9. Andrew1962 | | #9

    Follow up question: I audited a 1984 flat roof home in the DC area. There are FG batts in the joists -probably 8". The soffits are open and I get that we should block them off, but do I need to dense pack the cavities? Will the voids in the insulation cavity without any ventilation create a different problem like trapped moisture? I don't think the customer will want to go through all the ceiling repair and cost especially since she already needs to replace the 28 year old heat pump. Thanks.

  10. GBA Editor
    Martin Holladay | | #10

    If you are talking about a flat roof, my answer to you is the same as the answer I gave H. Hummel: "Your two best options are spray foam insulation on the underside of the roof sheathing -- an installation which will require the ceiling to be removed, at least partially -- or the installation of rigid foam on top of the existing sheathing. The second option will require new roofing and roof sheathing, of course."

    Fluffy insulation like fiberglass should not be used on the underside of a flat roof.

    1. huumadvisor | | #11

      my question for Andrew's case above would be if rigid were installed on top of the existing sheathing plus fluffy fiberglass on the underside, should this fill the cavity entirely, should it minimally be in contact with the bottom of sheathing if it doesn't entirely fill the void? Asking because it seems this maybe the mos cost efficient way to go if one wanted to avoid the spray foam install on the underside. thanks.

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