Vaulted Ceiling Insulation and extruded polystyrene

Old 09-14-04, 10:29 PM
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Vaulted Ceiling Insulation and extruded polystyrene

I'm adding a room with a vaulted ceiling that calls for 2X10 rafters. The city is asking for R-38 insulation factors, which is not possible in 10" with standard batting.
Is it possible to use extruded polystyrene in multiple layers to achieve this rating in the rafter cavity?
Old 09-17-04, 02:38 PM
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You can achieve the R38 using standard batts by increasing the depth of the rafter to 12", by using scissors trusses or by installing rigid foam between the rafters to a depth of about 6 inches using polyisocyanurate foam.

You'll have to do local price comparisons to see which method is most economical.
Old 09-17-04, 03:49 PM
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like homebild said go with the scissors truss there to get good insulation and vents in the attic . Thats all we go with now when it calls for a vaulted ceiling.

Old 09-17-04, 05:28 PM
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Thanks for the comments. I'm pretty committed to the 10" rafters, due to some other constraints I'm having to deal with. However, rigid foam board as you've described appears to be the way I'll go. I can acheive the high R-value I'm looking for, and still have a 2" air gap for ventilation. Might be a little more costly, but worth it for what I'm attempting to accomplish.

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