Replies

1. GBA Editor
   Martin Holladay | Mar 26, 2012 04:21pm | #1

   Brian,
   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 | Mar 26, 2012 06:15pm | #2

   Martin,
   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.
   Brian

3. 
   user-1085163 | Mar 28, 2012 12:40pm | #3

   Martin,
   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 ?
   Brian

4. GBA Editor
   Martin Holladay | Mar 28, 2012 01:41pm | #4

   Brian,
   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 | Mar 30, 2012 10:39am | #5

   Martin,
   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 | Mar 30, 2012 10:46am | #6

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