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
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??
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;
Keep the whole world singing . . . .
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
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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