# Cathedral Ceiling Framing and R-values



## Jobsaver (Nov 1, 2010)

I am in the habit of requiring a minimum 8.5" insulation cavity for portions of rafter cavities formed where the rafter acts as the ceiling joist, (cathedral ceilings). In the examples of a bonus room where a portion of the ceiling is cathedral, or a tray ceiling incorporates rafters to form portions of the tray, I am allowing a furr-out of a 2X6 rafter with a 2X4 to achieve the 8.5" cavity.

(I require 8.5" to achieve minimum R30 and room for baffles).

Any objections? And, what is the best way to secure the 2x4 furring to the rafters?


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## JMORRISON (Nov 1, 2010)

Trade the labor to furr out and buy 2x12 for R-30.  Most R-30 is 10".  Leaves 1 1/4" for air space


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## Robert Ellenberg (Nov 1, 2010)

9" minus 8.5" = .5" air space. R806.3 Vent and insulaiton clearance, calls for a minimum of 1".  Have them add 2x6 furring (assuming it is already in place) and wrap flat straps around in a U shape and nail them on both sides.


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## Jobsaver (Nov 2, 2010)

Oops. 2X6 (5.5") + 2x4 (3.5") = 9" cavity. I am allowing 8" for insulation and 1" for baffles. Thanks.


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## Rio (Nov 2, 2010)

From the Johns Mansville website:

R Value Dimensions Sq. Ft. Per Bag Best For... K1242 R-30 Kraft-Faced Batts 031979110407 30 10.25" D x 16" W x 48" L 58.66 attics and ceilings .

 We always spec out 2x12's when using normal R-30 batts.  We've heard about another product that has high R-values without the bulk but haven't researched it yet.  We have a R-38 roof we're calling out due to Title 24 and are going to see if we can find it and use it in a 2x12 flat roof.


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## Jobsaver (Nov 2, 2010)

Rio: High density batts. For example, Certainteed's R38 high density batt calls for 10.25" dedicated space for the insulation, then add minimum 1" air space, you are there!


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## Rio (Nov 3, 2010)

Thanks Jobsaver.


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## bldginsp (Nov 3, 2010)

I built a house once that the plans called for 2x4 to be installed on the edge of the 2x8 rafters.  I counter bored the 2x4 and used 3 1/2" screws to secure the "furring" to the rafters.  Worked great and gave me plenty of space.  I believe it was designed by Gardner in SC.


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## DRP (Nov 3, 2010)

We've nailed 2x2's both above and below 2x12 rafters in different situations as called for by the plans. We've also used long oly screws to attach 2x4's and scraps of ply to gusset deeper lumber on.


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## Jobsaver (Nov 4, 2010)

Here are two related questions. For the purposes of determining R-value requirements, at what point off vertical does a wall become a ceiling? Also, I have heard that some ahj's allow an allowance, as a percentage of cathedral portion of ceiling to total square feet of ceiling, that the R value can be reduced?


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## TimNY (Nov 4, 2010)

Jobsaver said:
			
		

> Here are two related questions. For the purposes of determining R-value requirements, at what point off vertical does a wall become a ceiling? Also, I have heard that some ahj's allow an allowance, as a percentage of cathedral portion of ceiling to total square feet of ceiling, that the R value can be reduced?


Hmm.  I never had a situation where there was a question as to whether it was a wall or a ceiling.

The percent/sf you are talking about is in 2006 IRC N1102.2.2 and limits the reduced r-value to 500sf.  In the 2009 I think it also limits it to a percentage of total roof area- 20%?


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## Jobsaver (Nov 4, 2010)

Thanks. This is correct. Note that a minimum of R30 is required even in the reduced area.


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## Mac (Nov 4, 2010)

Here's a random thought - if it has shingles, it's a roof. If it has siding........


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## technomaze (Jan 27, 2011)

R Value Dimensions Sq. Ft. Per Bag Best For... K1242 R-30 Kraft-Faced Batts 031979110407 30 10.25" D x 16" W x 48" L 58.66 attics and ceilings .

We always spec out 2x12's when using normal R-30 batts. We've heard about another product that has high R-values without the bulk but haven't researched it yet. We have a R-38 roof we're calling out due to Title 24 and are going to see if we can find it and use it in a 2x12 flat roof.

This is the right way to do it confirmed by different experts.


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## Yankee (Jan 27, 2011)

Mac said:
			
		

> Here's a random thought - if it has shingles, it's a roof. If it has siding........


Hence the development of the "Mansard" roof . . .


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## Mac (Jan 27, 2011)

And lest we forget - the "A" frame house.


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## Francis Vineyard (Jan 27, 2011)

Compliance in climate zone 3 can be easily achieved in a 10 inch cavity with R806.4 unvented attic assemblies using a minimum 1 inch (R-5) foam board and R-25 (8 inch) fiberglass batt.


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## Pcinspector1 (Jan 27, 2011)

Your answer is use sissor trusses.

Paying some framer to add 2x4's attached to 2x8's with 3-1/2" screws may need an engineered design, this is on the plans you've approved correct? Theres drywall or bead board hanging from the 2x4 right?

pc1


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## Daddy-0- (Jan 29, 2011)

How about using R-38C? You get R-38 value in a reduced thickness....or is that only allowed in the attic floor?


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