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Unvented flat roof, insulating from outside?

rwinikates | Posted in General Questions on

Boston, MA area here (climate zone 5A), with a flat roof, unvented. I’m replacing my roof and fascia boards on my 1950s era house, and am hoping to do what I can to insulate between the 2×10 ceiling joists while re-roofing (as well as adding 4″ of exterior rigid foam insulation ontop of the sheating/below the epdm membrane).

From my ceiling on up, it is a typical New England assembly with sheet rock, strapping, ceiling joists, sheating, epdm currently. I have a few recessed ceiling lights (low profile LED wafers).

What options do I have for exterior-installed insulation? Here’s what I’ve gone through thus far with options:

Dense Pack Cellulose
Pros: Cheap, easy to blow in (either blown through the fascia boards pre-replacement, or even from the inside ceiling and patched)

Cons: can lead to moisture problems? Reading articles here seem to offer contradictory advice where “it can sometimes work”

Closed cell spray foam from above
Pros: higher R-value

Cons: could create double air barrier sandwich (one from the closed cell against the ceiling sheetrock, the other in the exterior foam/epdm roofing layers)

Open cell spray foam from above
Pros: cheaper than closed cell, maybe would be permeable to allow drying to the inside?

Cons: ??

Blown-in mineral wool
Pros: no concern about creating more plastics/global warming potential of blowing agents.

Cons: Literally cannot find a contractor in the area who does this.

Blown fiberglass
I’m not even considering this due to the rot potential.

So my question to you, smart folks of this forum, is: what pros/cons for these materials am I missing? Am I missing any other insulating materials that might be a better compromise?

It really feels like the only way for me to insulate this roofing assembly is to rip down all the sheetrock from my ceilings and insulate from below, but for a variety of reasons that isn’t possible at the moment.

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Replies

  1. Expert Member
    BILL WICHERS | | #1

    The usual way to insulate a flat roof in the commerical world is to use rigid foam on top of the roof deck ("roof deck" = "sheathing" here). That would be the easiest way to go, with no moisture issues. You could also do a mix, with loose fill or batts in the joist bays, then enough rigid foam to get the right ratio of interior to exterior insulation for your climate zone. This is probably your safest option.

    I would not use closed cell spray foam here due to the air space you'd end up with above the foam and below the roof sheathing. I think that would be a moisture trap and might cause trouble down the road. I would not use open cell spray foam here, unless you also put rigid foam on top of the sheathing as I mentioned earlier.

    I'm not sure why you consider loose fill (blown) fiberglass to be a rot problem. Fiberglass doesn't rot. Fiberglass can hold moisture if it gets wet, but cellulose does too. In your application, I think your best option is loose fill fiberglass between the ceiling joists, because it doesn't settle very much the way cellulose does, so you don't have to dense pack. Next, put enough rigid foam (this is what "roofing" polyiso is made for! :-) to get the right ratio of interior to exterior insulation for your climate zone. I'm going to guess (I dont' have the table handy) that you'll need 4" of polyiso here, since loose fill fiberglass in 2x10 bays is going to be around R30, so 4" of roofing polyiso is going to give you around R22 or so, for a about a 60:40 interior to exterior ratio, and about R52 total.

    Bill

  2. mr_reference_Hugh | | #2

    rwinikates, looks like Bill has this one well in hand. I strongly agree with Bill on 2 points.

    To start you stated:
    "It really feels like the only way for me to insulate this roofing assembly is to rip down all the sheetrock from my ceilings and insulate from below, but for a variety of reasons that isn’t possible at the moment."

    I think that what you believe is the optimal solution is possibly a very bad solution. Why? I was said it quite eloquently above: "The usual way to insulate a flat roof in the commercial world is to use rigid foam on top of the roof deck ("roof deck" = "sheathing" here). That would be the easiest way to go, with no moisture issues."

    I strongly agree with the othe comment above that using loose fill in between the joists would require you "to get the right ration of interior to exterior insulation for your climate zone". Unless there is some problem adding more polyiso over the roof deck, I think that using loose fill insulation means you are really complicating the retrofit process.

    By only using polyiso over the deck (if it is physiccally possible),
    - you don't have to make holes in the facia, or your interior ceiling;
    - you follow the well established methods used for commercial building and you should have an easier time finding trades/contractors to do the job;
    - you and the contractor can visually inspect the insulation that is being installed over the roof deck, where as you are filling those roof joist cavities blind;
    - loosed filled insulation is cheaper than polyiso but there would be an extra time and expense making holes, addressing issues you find when you are making the holes (who knows what), and then fixing the holes properly afterwards;
    - keeping it simple usually means also saving money.

    If you want to read more, the GBA site has a few articles on unvented roof assemblies.

    https://www.greenbuildingadvisor.com/article/a-researcher-looks-at-insulated-roof-assemblies
    https://www.greenbuildingadvisor.com/article/the-bs-beer-show-unvented-roofs-and-fluffy-insulation
    https://www.greenbuildingadvisor.com/article/sanity-check-on-an-unvented-roof-assembly

  3. rwinikates | | #3

    Thank you both, Bill and mr_reference!

    To be clear, yes the plan is to definitely have 4" of polyiso above the sheathing/decking for sure, and then tapered insulation ontop of that (reaching a peak of an additional 4-5" at the high point).

    I think I definitely need something in between the joists because otherwise wouldn't the warm air just escape out my uninsulated fascia? I agree that it is more complication to have to insulate there, but without something "fluffy" on the interior I won't get to my goal of at least R50 (without adding significant amount of exterior polyiso which might become problematic), plus the aforementioned thermal escape route out the fascia.

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