Here is an interesting discussion over at Green Building Advisor.
For hot climates like mine, he recommends a 3-5-10-20-60 rule, i.e. R60 for the ceiling, R20 for the walls. For cold climates, he changes the walls to R40.
I have roughly and nominally 1.5 inches of R7 closed cell spray foam, combined with 4 inches of R4 open cell spray foam, on all walls and roof deck. (Possibly a bit more on ceiling since all 2x6's are fully covered.) That computes to 26.5. So I'm a bit above recommendation for walls, but a bit below recommendation for roof.
I had to struggle to get this much, the contractor insisted that 1.5 inches of the closed cell foam was enough, and claimed that was all they had agreed to (I had thought we had agreed to 5.5 inches of closed cell foam). Finally we worked out the closed and open cell combo deal for a small added cost, and I was happy with it for sound reduction reasons.
On the plus side, I also have white coated metal roof supported by 3/4 plywood. Foam also prevents heat transfer through air leakage, and I have a sealed attic (or it will be sealed when I'm done sealing it). The white coating reflects about 70% of solar energy right away while most dark roofs only reflect about 25% of the energy or less. The walls have siding, sheathing, and sheetrock, which all add some thermal benefit. Exterior walls are painted a very light color. All and all, from energy performance standpoint, I think I've done quite well, which is what I intended. But the point is how hard it was to get to this point. Though it isn't specifically itemized, the contractor claimed I was getting $4000 of insulation, including $3000 for the very costly first 1.5 inches of closed cell foam.
The discussion brings up that we should really be dealing with UA, total energy loss, rather than numbers like R, to set targets. Have R60 roof and R20 walls does little if the building is covered with windows and doors. Speaking of which, I put no doors or windows on the south and west sides.
For hot climates like mine, he recommends a 3-5-10-20-60 rule, i.e. R60 for the ceiling, R20 for the walls. For cold climates, he changes the walls to R40.
I have roughly and nominally 1.5 inches of R7 closed cell spray foam, combined with 4 inches of R4 open cell spray foam, on all walls and roof deck. (Possibly a bit more on ceiling since all 2x6's are fully covered.) That computes to 26.5. So I'm a bit above recommendation for walls, but a bit below recommendation for roof.
I had to struggle to get this much, the contractor insisted that 1.5 inches of the closed cell foam was enough, and claimed that was all they had agreed to (I had thought we had agreed to 5.5 inches of closed cell foam). Finally we worked out the closed and open cell combo deal for a small added cost, and I was happy with it for sound reduction reasons.
On the plus side, I also have white coated metal roof supported by 3/4 plywood. Foam also prevents heat transfer through air leakage, and I have a sealed attic (or it will be sealed when I'm done sealing it). The white coating reflects about 70% of solar energy right away while most dark roofs only reflect about 25% of the energy or less. The walls have siding, sheathing, and sheetrock, which all add some thermal benefit. Exterior walls are painted a very light color. All and all, from energy performance standpoint, I think I've done quite well, which is what I intended. But the point is how hard it was to get to this point. Though it isn't specifically itemized, the contractor claimed I was getting $4000 of insulation, including $3000 for the very costly first 1.5 inches of closed cell foam.
The discussion brings up that we should really be dealing with UA, total energy loss, rather than numbers like R, to set targets. Have R60 roof and R20 walls does little if the building is covered with windows and doors. Speaking of which, I put no doors or windows on the south and west sides.
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