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Venting flat roof with two thermal boundaries

I wanted to get your opinion on this, as I've heard differing thoughts. As an energy auditor in Phoenix (8 inches of rain / year, relatively low outdoor RH), I see a number of "flat" roofs with two different thermal barriers (foam w/elastomeric atop roof deck, and unfaced batt (or even faced batt) in the attic space atop the ceiling).

The attic space is usually between 20 and 30 Pascal @ CFM50, and many times, the attic is vented.

It would seem that the vents render the foam relatively ineffective. In most cases, capping the roof vents would lift the air barrier to the roof deck, but then there are the batts, as well as the space between. Is capping the vents harmless (in the desert or otherwise) if the ceiling is sealed fairly tight? Also, if the vents are not capped, is there any point in spray-foaming the roof rather than simply painting it white with roof coating?

Asked by Jason Owsley
Posted Jan 13, 2013 4:52 AM ET


5 Answers

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Your instincts are correct; the roofs you describe have several flaws.

In my opinion, flat roofs (or low-slope roofs) should never be vented. Period -- end of story. When vents are installed in a flat roof, they cause more problems than they solve. It is more likely that rainwater will enter these vents than the vents will ever contribute to drying.

You can't used fiberglass batts alone to insulate an unvented roof, so you'll need either rigid foam or spray foam to make these roofs work. More information here: How to Build an Insulated Cathedral Ceiling.

Once you have installed foam on top of the roof sheathing -- and doing so is an excellent approach -- then any insulation installed under the roof sheathing needs to be installed tight to the sheathing, without any intervening air gap or attic space.

Answered by Martin Holladay
Posted Jan 13, 2013 8:22 AM ET


Seems like the thing to do here is ignore the roof and focus on the attic floor. Air sealing and more insulation. If you aren't seeing signs of moisture problems in the attic (mildew on the sheathing, etc) then I probably wouldn't alter the ventilation, if any.

If you're going to bring in a spray foam rig, then I suppose you could move the thermal barrier to the roof, although then you do need to cap any venting, and make sure it's not re-installed when the roof is eventually replaced.

How much headroom is there in these attics?

Answered by David Meiland
Posted Jan 13, 2013 11:22 AM ET


Thanks, Martin and David.

David, the headroom -- well, the space between the top of the batts, which are on the attic floor, and the roof sheathing is anywhere from a foot to 18 or so inches. They're Santa Fe-style homes popular here in Phoenix.

Answered by Jason Owsley
Posted Jan 24, 2013 6:48 AM ET


Martin, I've been recommending sealing inside the roof jacks (the ones that look like mini-quonset huts) between the bathroom exhaust vent duct and the jack itself or, if accessible, between the duct and sheathing, as in this photo. I don't know if that's overkill, but some of the homes will have five or more, with the sheathing cut fairly large.

Answered by Jason Owsley
Posted Jan 24, 2013 6:54 AM ET


It's hard to know where the significant air leaks are located in this type of roof assembly. In most cases, the roofing installed on flat (low-slope roofs) is an air barrier. (For example, EPDM roofing or spray-foam roofing.)

A roof vent is an obviously an interruption of the air barrier created by the roofing. But the gap between an bathroom exhaust duct and the roof sheathing may not matter. These questions can be confirmed with a blower-door test.

Answered by Martin Holladay
Posted Jan 24, 2013 9:52 AM ET

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