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Old 02-04-2015, 11:44 PM   #16
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Because you have a low pitch roof angle, 2" air space under the roof sheathing is better than our code required minimum 1" air space. You can add rigid foam board to the bottom of the horizontal ceiling joists (eliminating thermal bridging of the wood) with cavity fill above that- up to some baffles made of rigid foam board as here- separating the air space from the cavity; http://www.buildingscience.com/docum...n-roof-venting

Never let cavity insulation (other than rigid FB) abut a cold sheathing to transmit warm temps to cold roof- conduction loss of R-value.

Gary
Interesting!

Unfortunately I can't use the rigid FB between the ceiling joists and drywall because I can't afford to lose any head room, as it's a pretty low ceiling.

Here's my next question. Should I leave a 1-2" space between the insulation and roof boards all the way to the roof edge? I was thinking of going right up against the roof boards at the very sides of the roof, but I think I could run into ice damming problems in the future. Better to be slightly less thermal efficient than causing other problems I suppose.
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Old 02-05-2015, 05:08 AM   #17
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You have to provide for unobstructed ventilation pathway. If the roof pitch is steep, the smaller gap is less worrisome, however, you really should have about a 2" continuous gap to allow for the space to vent.
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Old 02-05-2015, 09:54 AM   #18
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You have to provide for unobstructed ventilation pathway. If the roof pitch is steep, the smaller gap is less worrisome, however, you really should have about a 2" continuous gap to allow for the space to vent.
Even if that means the very edges of my garage will have almost no ceiling insulation???
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Old 02-05-2015, 10:42 AM   #19
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I think Gary referenced it earlier, when it was mentioned about putting rigid foam over the top of that outer top plate.

Caulk, foam, or otherwise seal that top plate connection against any air leakage. After that is completed put the highest R-value foam, rigid board, that you can in that location.

If you use something like a polyiso bored, you get the maximum R-value you can per inch at that location. Be sure to seal that phone where it connects to the framing into the attic floor as well.
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