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Old 10-16-2011, 02:09 PM   #1
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Cathedral Ceiling Insulation


I am building a cabin with a roof design for Valdez Alaska. 12/12 pitch with BCI 60 I joists. (2 5/16" x 14") on 16" centers. Vented at the top and bottom of all spaces. From looking on the net I've found R38 to be 10 1/2" or 12" thick. I'd like to use a kraft face to avoid any sliding issues. Will 3 1/2" vented space be to much if I use a 10 1/2" thick insulation. Not a carpenter, this place built by a safety man, truck driver and a dirt worker.
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Old 10-16-2011, 04:16 PM   #2
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Cathedral Ceiling Insulation


3.5" is fine. Make sure you use a HD batt in this application.

I would also like to see some insulation webbing or housewrap between to the joist bay and the venting space to prevent the wind washing of the insulation.

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Old 10-16-2011, 05:53 PM   #3
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Cathedral Ceiling Insulation


Are you suggesting something like this.
http://www.brentwoodindustries.com/s...l-Ceiling.aspx
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Old 10-16-2011, 08:37 PM   #4
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Yes, but you can do it with Tyvek just he same.
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Old 10-16-2011, 08:54 PM   #5
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Cathedral Ceiling Insulation


Here in WA State, our energy code requires R-38 in cathedrals, minimum, with only -5k Heating Degree Days. You have -10k HDD for your location. I’d use 1-1/2 inch air space and foamboard, then cellulose, drywall, latex paint as on page 31, here; http://www.buildingscience.com/docum...-climate-zones

Or the fiberglass with foamboard under the drywall as in the next picture, for a thermal break from the cold rafters (though they won’t transmit much as engineered-not solid wood). If the slope was shallow, then bigger airspace is recommended.

PS- check out their recommendations for your zone 7 on page 11.

I would not use any baffles that don't advertise the net free air venting rate on the spec. sheet. Example- At H.D.- http://www.bergerbuildingproducts.co...sAccuvent.html

And O.C.- http://www.owenscorning.com/around/v...s/VentSure.pdf
Notice the gap between baffle ends for condensation relief....

Gary
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Old 10-16-2011, 09:23 PM   #6
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I like Gary's roof approach better.

Rigid foam against the inside edge of the outer flange, insul web the joist bays, blow cellulose, and then a layer of rigid foam against the inside surface.

You would have a vented near R-50 insulation layer at that point.
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Old 10-16-2011, 10:40 PM   #7
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Cathedral Ceiling Insulation


I think I should add that this is a recreational cabin off the road system. Access by snow machine, atv or walking.

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