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Community and Q&A

What is the best way to retrofit insulation in an unventilated flat roof?

KeithH | Posted in General Questions on

The house has a flat EPDM roof, open chord roof trusses, no ventilation, and R-30 fiberglass batts with dubious installation quality (wiring interrupts, can lights, swamp cooler ducts). I’d like to add insulation to this truss space. I’m wondering if there are any effective solutions that don’t rely on closed cell foam.

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  1. GBA Editor
    Martin Holladay | | #1

    If you don't want to install spray foam, your roof can be insulated by installing several layers of rigid foam on top of the roof sheathing. Of course, that means that you will also need to install new roofing.

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

  2. KeithH | | #2


    Thanks for the link to that great article. I read that one and several others as well as some pdf studies on line. This study is very apropros (linked off one of the other GBA discussions):

    I also did some investigation into my structure by removing some drywall and existing fiberglass insulation. It has R-30 of fiberglass batts on top of drywall (with PolyE between but with a lot of penetrations) with at least a 6" gap above the batts. I found some mold on top of the fb batts in little droplet style patterns near the northern wall. The truss ends are uninsulated and have Celotex sheathing (except the corners) which I understand to be pretty air/vapor permeable though the 1" XPS under stucco should be retarding vapor pretty well.

    At this point, I'm trying to determine how to proceed. It sounds like, based on that article you referenced as well as others, that the gold standard is to stay unventilated, keep the interior vapor barrier (but perhaps upgrade it to a smart barrier), insulate the interior cavity with air permeable batts of to be determined R value, and add R-20 to the roof deck. As you mention that will require replacing the roof. Obviously the building will perform much better from a thermal and energy perspective if I move the insulation in the ceiling from batts 24" o.c. @ R30 to continuous R20 and batts 24" o.c. in the R23 to R46 range. I'm using Roxul Comfortbatt so achievable values are 23, 30, 38, and 46, given my minimum truss depth of 16" and a need to leave some clearance from wiring at the drywall plane.

    For the benefit of other readers, I'll point out that adding ~3" to 6" of insulation to the top of the roof will also precipitate a chain reaction of the following items:
    - the parapet will need to be heightened and the cover likely replaced
    - the new parapet will need siding
    - the skylight will need to be recurbed (and replaced since it is old and not efficient)
    - the existing scupper cuts will probably need to be covered and resided
    - new scupper cuts will need to be cut
    - surface mounted electrical (a/c, whole house fan, swamp cooler), that electrical will need to be re-done or removed. I'm going to remove my swamp cooler.
    - if you have an adjacent structure (multi-roof, penthouse etc) with fenestrations at the existing parapet level, they will need to be replaced/removed etc. In my case, a window will need to be resized or decommissioned.

    I'm going to get some pricing for this nightmare, but suffice it to say that the project has the potential to be prohibitively expensive (at least at this time).

    So the questions are:
    - Is there interior air-sealing and vapor barrier work that could make air permeable batts work under an unventilated low slope roof? The existing setup apparently worked reasonable well for 35+ years with <4 sf of plywood deck showing moisture damage and very modest spotting on the fiberglass batts.
    - I don't think there is a smaller amount of insulation that would precipitate less work for my project but would it be beneficial to install a modest amount of insulation if that is a more manageable project? With a black epdm roof in climate zone 5b, the pdf study mentioned above appears to suggest that even ~R-6 (1" of closed cell foam) would eliminate most condensation producing circumstances.

    Well, thank you for the information. I'm mostly providing this information to benefit other readers considering similar projects.

  3. GBA Editor
    Martin Holladay | | #3

    It sounds as if installing closed-cell spray foam on the underside of your roof sheathing would be less expensive and less of a hassle.

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