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Cathedral ceiling reinsulation

user-1085163 | Posted in Green Building Techniques on

I am in zone 8. One cathedral ceiling I have to rebuild consists of 2″x6″ rafters with 2 layers of 1.5″ of white expanded polystyrene sandwiching a layer of aluminum foil paper attached to the underside ,or interior side.Old pest damaged fiberglass insulation is between the rafters. Plywood and shingle is on the exterior of the rafters. There is no other structure to the interior of the rafters other than the polystyrene.
The plan is to remove the shingles, plywood,old fiberglass insulation and reinsulate.
I assume that placing 6″ of polyisocyanurate above 5.5″ of newly placed fiberglass between the rafters would create a double vapour barrier , and therefore be problematic.
Alternately I could spray 6″ of closed cell urethane between the rafters, directly in contact with the most exterior layer of polystyrene, with an air space above; then plywood and shingle, and arrive at close to r-50. Would this work ?

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  1. GBA Editor
    Martin Holladay | | #1

    Your second option (spraying 6 inches of closed-cell spray foam between the rafters) would certainly work.

    If I understand correctly, though, the finish ceiling consists of white expanded polystyrene. That's a no-no. It's a fire hazard -- remember that awful nightclub fire in Rhode Island? You need to cover up the white polystyrene with a layer of gypsum drywall.

  2. user-1085163 | | #2

    You understand correctly, I have already informed the homeowner of the fire hazard , and he's ok with it, as he's been there 15 years and no problem ; bad decision, and I will try to convince him otherwise.
    Just to add , the 2"x6' rafter would be extended by 1"x2" strips of xps to create a thermal break and a space of 6.5" for the spray urethane, capped by a 2"x3" for the vent space of 1.5", and then the ply. Sorry, my 2.7mb diagram was't uploaded.

  3. user-1085163 | | #3

    Slight problem . Just checked with my urethane spray applier. He tells me that the urethane will not adhere to the polystyrene. Hence the urethane in time will contract , and form as much as a 1" gap away from the 2"x6" framing . He suggests either removing all the styrene and installing a wood or gyprock ceiling, or placing a plywood barrier between the polystyrene and spray foam.
    I'm thinking it might be easier to layer 3-2" foil faced polyisocyanurate boards between the rafters, foam sealed at all joints. More labour, but slightly cheaper than the spray foam in the end. Any thoughts ?

  4. GBA Editor
    Martin Holladay | | #4

    There may be a good reason why the spray foam installer doesn't want to spray against expanded polystyrene, but the explanation doesn't make a lot of sense.

    Closed-cell foam usually sticks tenaciously to almost anything, and I'm surprised to hear that it won't stick to expanded polystyrene. I'm especially surprised that the spray foam installer expects that the spray foam will "form as much as a 1-inch gap away from the 2x6 framing." That makes no sense. Properly applied spray foam will stick to framing lumber.

    In any case, it's certainly possible to install a stack of rigid foam rectangles in each rafter bay, and to seal the perimeter of the rectangles with canned foam. But that is slow, fussy, work, and I certainly wouldn't want to do it from above, with the roof open to the weather.

  5. user-1085163 | | #5

    So here's the scoop.Meeting with a second contractor, he explained the problem to me. Apparently it resides in the application. Applier #1 shoots all 6" all in one pass, which takes less material and therefore more profit. This causes overheating of the foam, and hence contraction. The second applier shoots 2" at a pass, leaving 15 minutes to set before a second, and then a third pass. It seems there is no problem with the bond to polystyrene, as he has done this before without incident. He is now preparing me a test sample to verify the bond. I will inform of any complications.
    As you rightly pointed out, I was not looking forward to that much time on an open roof.
    Cheers, Brian

  6. GBA Editor
    Martin Holladay | | #6

    Clearly, Contractor #2 is following the correct procedure, and Contractor #1 is to be avoided.

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