Wayne Whitney wrote:
On 2009-08-20, Aaron Fude wrote:

Just received a quote for roof insulation with closed cell foam
between the rafters. The quote is for 3" and I am being told that 3"
is 90% as good as 6". Do I believe them?

Are you insulating between the rafters because you'll be turning the
attic into habitable space?

As to your question, 6" of foam will be about twice as good an
insulator as 3" of foam. If your concern is about reducing the
heating and cooling costs of the entire house, then the useful of the
extra 3" of foam depends on how well everything else is insulated.

Here's a simplistic example: suppose the roof represents 20% of the
surface area of the house, and lets call F-1 the insulation value of
1" of foam. If the rest of the house is a perfect insulator
(impossible), doubling the roof insulation will halve the energy loss.
If the rest of the house is insulated to F-6, then the F-6 roof will
reduce the system energy loss by 17% over the F-3 roof. If the rest
of the house is insulated to F-3, then the reduction is only 10%. And
if the rest of the house is F-1, the reduction is only 4%.

But, the energy saved by adding attic insulation is the same, regardless of the
other insulation in the house.