On Thu, 20 Aug 2009 15:16:18 -0500, "HeyBub"
wrote:

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.

Don't think so. Resulting efficiency rate drops off as thickness increases.
If you're at 50% with 3", you might be at 70% with 6" and 80% at 9" and 85%
at 12" (numbers are made up).

You're spot on about insulating the rest of the house. It won't do much good
to insulate the attic if you leave the front door open.


Although that is very true, it's also true that more heat escapes
upwards than outward. Overhead insulation tends to be the most
important. Better to have r50 in the attic and r20 in the walls than
the other way around.