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Old 09-27-2010, 12:30 AM   #1
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Insulating vaulted ceiling

Have a vaulted ceiling, rafters are 12" engineered I beams. I can get 2" polyurethane foam boards at a good price. I was going to place the foam boards between the rafters leaving 1 1/4 " space between board and roof sheating, under the foam R35 fiberglass insulation, then vapour barrier then drywall.
Would this be a good way of insulating the ceiling?
Do I make the foam board layer air tight, or do I allow the insulation to breath?
My concern is the fiberglass between two non breathable layers....foam and vapour barrier.

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Old 09-27-2010, 07:31 AM   #2
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Does this cathedral roof have continuous ventilation flow through the 1/4 inch gap? IE: soffit inlet and a vented ridge cap. Its sometimes recommemded to make the gap 2-3 inches. The 1/4 inch could be too restrictive. Is it asphalt shingles with tarpaper underlayment? Depending on your climate, you may want to omit the vapor retarder below for a more permeable air barrier instead.


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Old 09-27-2010, 08:30 AM   #3
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R35 might be too much batt insulation for that situation. The batt stuff needs room to fluff up.
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Old 09-27-2010, 09:46 AM   #4
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I'm located in the eastern township of quebec, canada

the air space is 1 1/4 (one and a quarter).
The 2 " of polyurethane (not polystryrene) I understand gives me a R value of around R15 (The foam is foiled faced on both sides.

I was trying to acheive the upper R40's in the ceiling.

The roof has a continuous ridge vent with venting along the soffits.

The roof in covered with metal over 15/32 OSB.

I could drop the fiberglass down to an R30 9.5 inches.

Plus I will have strapping attached to the bottom of the I beam for the drywall, that could give me an extra 3/4 - 1 inch of space for the fiberglass.
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Old 09-27-2010, 01:02 PM   #5
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You don't want air gaps at the batt insulation, behind or in front of to give convective loops.
Especially in the channels created by the strapping.

Need the v.b.:

I would use foam or a sill sealer as a thermal break to the rafter bottoms from the heated room.

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Old 09-27-2010, 03:03 PM   #6
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So if I as per my drawing. leave the 1 and 1/4 inch air gap above the foam board to vent.
install the foam board making it air tight.
install fiberglass
cover with vapour barrier
install strapping
install drywall onto strapping.

Foam board R -15
R30 fiberglass
total r = 45

Would this work?


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