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Old 09-22-2009, 07:52 AM   #1
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cathedral ceiling


What do you think about using 2x10 rafters for cathedral ceiling with a horizontial span of 18'6" and a length of approximatly 26' from ridge to outside wall? What is the max size insulation for this and still have air gap to prevent moisture? Thanks for any replies

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Old 09-22-2009, 07:56 AM   #2
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#2 Doug Fir can easily make that span.

As for insulation, you can easily fur the rafters down with 2x2's or 2x4's to gain extra insulation depth. That's very common.

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Old 09-22-2009, 08:07 AM   #3
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Thanks for the reply, most of the lumber we can get in this area is Southern Yellow Pine. We have moderate winters here maybe several times up to 8" snow. What size insulation would you suggest without putting any fur downs on bottom of rafters? Could go with 2x12 but expense begins to play into that.
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Old 09-22-2009, 08:22 AM   #4
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Southern Yellow Pine will certainly make the span as well.

You need to determine the minimum require R factor in your specific area/jurisdiction, which is often a local requirement. A call to the building codes office will let you know what they'll require. Then you'll know how far you need to fur down, which is easy and inexpensive to do.
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Old 09-22-2009, 01:16 PM   #5
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Check the Pink Panther's web site for R-30c and R-38c, special denser glass specifically for cathedral ceilings that give you the ratings plus 1" air space. Also here are some comparisons after you find your local requirements: http://hes.lbl.gov/hes/makingithappe...omparison.html
And if you can afford it, an R-65: http://www.lowes.com/lowes/lkn?actio...065&lpage=none
Be safe, Gary
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