Insulating cathendral ceiling


What's the best way to insulate cathedral ceiling? Where I live (central NJ) attic R49 insulation is recomended. I don't see how to achive this with fiberglass insulation even if using 2 X 12 ceiling joists. Of cause it is possible to use rigid foam insulation but it seems to be expensive and also will create thermal bridging where joists are. I wonder what's the best and optimal way of insulating cathendral ceiling to required level>
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pmmr had written this in response to http://www.thestuccocompany.com/maintenance/Insulating-cathendral-ceiling-419255-.htm :
ls02 wrote:

------------------------------------- If you are building a new construction, You can use the rigid foam,the brown Isocyanurate kind is best. You would use the Engineered Truss type framing material to minimize the bridging.
Patrick\//.
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Thermal bridging? What do you mean. If you mean the difference in R value Polyurethane or Polyisocyanurate sheet settle at about R 6.5 cured, wood might be R 3.5 per inch or more, I dont see a problem except you need 8" of foam and spraying would be cheapest, but some spray foams are only R 5 per inch, some are alot more so you have to pick and research what locals sell. What is the construction now, you have shingles, decking, then what. The only thing I think is important is an air gap to the peak with ridge vent, so roof decking and shingles dont cook in sun and fail in 5 years, unless you have a metal, slate or tile roof. But im no pro at this, just learning.
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By thermal bridging I mean wood rafters uncovered by insulation. They have way less insulation then whatever insualtion is in cavity thus creating thermal bridging. When insulating attic you cross second layer (ussualy R19) fiberglass insualtion over ceiling joists that have R30 in cavities thus eliminating thermal bridging.
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Are you sure wood rafters, wood in general, is less R value than fiberglass per inch. Last I read about it they were the same at about R 3.75", am I missled.
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This is what I got
"The R-value for wood ranges between 1.41 per inch (2.54 cm) for most softwoods and 0.71 for most hardwoods. Ignoring the benefits of the thermal mass, a 6-inch (15.24 cm) thick log wall would have a clear- wall (a wall without windows or doors) R-value of just over 8. "
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So ceiling beams are 12" or R 12, there wont be any condensation, that will occur on less insulated areas if it even happens like on your R 3.3 dual pane glass, Consider fiberglass it looses R value the colder it gets, spray foam is still the best.
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When you don't know the species of lumber, figure an R value of 1 for design purposes.
You seem to be implying that the framing area is minimal, but that is not the case. Typical wood framing makes up 10 to 15% of the wall/ roof area. That is not an insubstantial number and definitely affects the overall R value of the building envelope. If you see any energy calculation that ignores framing area, it is a faulty calculation - a lie.
R
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On 1/16/2010 2:59 PM, ransley wrote:

on top of the roof joists then reshingle.
I live much further up north. At the time I built my house the recommendation was R-40 with rumors of that changing to R-50. I maxed out and put in R-60. (Parallel chord trusses).
One of my neighbors did the opposite. He put in R-32 at the same time I did mine. His house costs more to heat than mine and I have 600 sq ft more. Both houses are one story bungalows with R-20 walls. I also have close to double the amount of windows that he has. Mine are triple pane, his are double.
Seems to me that the way energy costs are going additional insulation is a good investment. It will pay for itself then start putting money back into your pocket.
LdB
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This is new addition to be built in spring.
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ransley wrote:

Not saying this is what happened here, but lots of people hear something like "wood is one of the best insulating building material". Yes, this is true for "building materials". But fiberglass isn't a "building material". Well anyway I've never seen a house built out of fiberglass insulation.
In my last house the attic was finished and it had a low angled ceiling. Outside it was quite obvious to see the frost or light snow melt in small strips.... right where there were studs. In between the studs were it had fiberglass insulation, the heat was better kept in the house and the snow stayed on the roof.
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What I don't get is the nit picking over tiny numbers when just having a cathedral ceiling is such a big waste of heat in and of itself....Especially if they are very high.....Yea , I know people love them but they are such a waste of space and heat....Not to mention how big a PITA it is to change light bulbs or re-paint ect......I'ld rather have the attic space for storage or another room or two upstairs..JMHO.........
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I do not understand why "cathedral ceiling is such a big waste of heat" if they are properly insulated and ventilated?
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I do not understand why "cathedral ceiling is such a big waste of heat" if they are properly insulated and ventilated?
Because of ALL the extra space you need to heat to feel warm at floor level...Wasted space and wasted heat....
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