Closed Cell Spray Insulation

We are building a low sloped roof over our new kitchen addition (roof pitch will be 2/12 pitch or lower. We will be using a solid rubber roof , but this post is about the insulation and I digress....
We were at the home show this weekend and I talked to a spray insulation company. We have considered using closed cell spray insulation since this attic space will be limited in room and it seems to offer some benefits.
The spray insulation company indicates 1" to 1.5" of closed cell foam is enough. At 6 "R Factor" per inch .... it doesn't make sense to me that 1" to 1.5" inches of spray foam is enough. But the company indicates that air infiltration is the key with spray foam .... and that "R factor" isn't as relevant...???
Is this accurate .... This ceiling is in our home in NE Oklahoma. Normally, ceilings have R-30 or better. But at 6 R per inch (for spray foam closed cell) ... do I need 5" to get up to R-30??
Thanks!
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Hambone,
You might seriously consider using polyisocyanate ( 2" or more is usual) on the top of the roof with either the EPDM, modified bitumen, or a standing seam metal roof. You could still use foam, blow in, or batts inside to increase the R. We deal with lay in ceilings and getting tradesmen to put insulation back in place leaves much to be desired, so we have gone to insulation on top for all reroofs..
Here is an interesting discourse on R value; http://www.advanced-roofing.com/productsservices/r-fairy_myth.htm
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The direct answer is no. While it's a good idea to spray closed cell foam directly under the roof sheathing as it will greatly reduce the opportunity for condensation based moisture damage, you still need R- value to reduce heat transfer from the hot roof. At 150F on the roof and maybe 70F on the inside, you've got an 80F temp. difference. At R-10, that's still transmitting 8 BTU/sq. ft into your living space. Increase that to R30 and you're down to 2 1/3 BTU/sq ft.
Another option to consider is a medium thickness layer of foam with a radiant barrier underneath. Those can be very effective in reducing the radiative component of heat transfer.
See the building science website references on roof design for more info.
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We were going to use closed cell and we were going to spray straight under roof deck - However we were advised by the building inspector to build sort of traditionally, and use the open cell to allow moisture to escape and also leave an air gap under the roof deck for long life of exterior roof. We sprayed open cell Poly-Iso R5 to a depth of 6 inches (or more, they were generous sprayers) and left the 2" air gap so that shingle Co can't complain if anything happens to the shingles - and we now save 70% on heating bills! The closed cell also costs more and is harder to spray, we used that on verticle gable ends only as we didn't want the walls to stick out too far.
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