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Community and Q&A

How to insulate a partially open unvented ceiling/roof?

katie d | Posted in Green Building Techniques on

We’ve had a leak and ceiling damage in a 50 yr old house with a flat roof in zone 4A. Part of the ceiling is gone, damaged, part still intact with its fiberglass.So there are cavities with part fiberglass. There is no ventilation in the cavities. This part of the roof was re-roofed a few years ago and some sort of foam board was added on the exterior (presumably between the old rubber and new TPO) at that time. The is no condensation currently with the cavities open.
My question is how to insulate the open parts now without rotting the roof. We had thought to cut and cobble foam board (exactly what sort is also a question ) But we are unsure of how to keep moisture from going around the side of the vapor barrier of the fiberglass, presuming the part of the cavity next to it is not filled completely. (We could do that I guess??) The second issue is of possible moisture getting between the foam and the roof deck. What sort of foam would you recommend? Poliso (with its highter R value)is not available locally. Or is there something better we should use or do??

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  1. GBA Editor
    Martin Holladay | | #1

    I'm not a fan of using the cut-and-cobble approach for unvented roof assemblies.

    Here's my advice: You should install rectangles of rigid foam between your rafters to separate the section of the roof with fiberglass batts from the section of the roof that is now open from below. Each rectangle of rigid foam should be sealed at the perimeter with canned spray foam, caulk, or high quality tape so that the installation is relatively airtight.

    The best way to insulate the section of the roof that is opened up is with closed-cell spray foam, installed against the underside of the roof sheathing.

    For more information on this topic, see Insulating Low-Slope Residential Roofs.

  2. Expert Member
    Dana Dorsett | | #2

    Since you believe "...some sort of foam board was added on the exterior... " above the roof deck, it's useful to know how thick that foam board is, and what type/color. Any idea?

    How deep are the rafters?

    Per IRC 2012 Chapter 8, in zone 4A & B (but not marine zone C) as long as you have at least R15 above the roof deck, you have dew-point protection up to as much as total of (R exterior + R interior =) R49, the IRC2012 code-min for zone 4.

    That implies that any other total-R, the foam-R needs to be at lest 30% of the total.

    If you have 3" of EPS (the white beaded stuff that looks like coffee-cups and cheap coolers) you'd be at about R12-R12.5 (depending on density) with the foam , so you'd be fine up to at total up to a total of R40-R42 in zone 4A. That means you could safely install R28-R30 unfaced batts in there, with nothing more vapor retardent than standard latex ceiling paint on the interior.

    If it's only an inch of exterior the fiber portion would have to be proportionally less, or you'd have to install a "smart" vapor retarder on the interior, or paint the ceiling with a "vapor barrier latex" primer to bring the vapor redardency down to about 0.5 perms.

    Other foam boards have higher R/inch, but don't assume it's more than R5/inch if it's not EPS and you don't know exactly what it is.

    Caulking the rafters to the roof deck on the gutted section and taping over any seams of roof decking with housewrap tape or FSK tape, then painting over the tape with fiber-reinforced duct mastic to ensure long-term adhesion would be good to do, now that it's open. Any electrical penetrations in the rafters should be sealed with can-foam. With the rafter bays air-tight from one another, the rafters themselves are also Class-II vapor retarders, less than 1 perm at the 1.5" nominal thickness (no matter what species).

    If you add any more than 2" of closed cell foam to the underside of the roof deck you risk making a moisture trap, and it's expensive to boot. But from a dew-point perspective the total exterior & interior foam-R counts. As long as it all adds up to more than 30% of the total, it's fine to fill the rest of the bay with fiber insulation, and no interior side vapor retarders. Closed cell foam is both expensive, and not very green (due to the HFC245fa blowing agent, as well as the fire retardents and potential polyol outgassing), so unless it's necessary for dew point control & vapor retardency, it's better to skip it. From a thermal performance perspective most of the higher R/inch you get out of foam is wasted when installed between rafters, due to the high heat transfer through the ~R1.2/inch framing. You get much more performance out of the foam-budget applying it to the exterior, where it thermally breaks the rafters.

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