On 2009-08-20, Bob F wrote:

Wayne Whitney wrote:

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.

I guess my main point was that you may be better off directing your
dollars towards insulating elsewhere in the house.

As to your comment, it is mostly true. Adding attic insulation may
change the mechanism of heat loss that the house experiences. So you
need to know something about the other insulation in the house to
figure out the impact of your added attic insulation.

Cheers, Wayne