On 2009-08-20, HeyBub wrote:

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).

I'm not sure what kind of efficiency you are talking about. If the
R-value of 3" of foam is R-15, then the R-value of 6" of foam is about
R-30.

Cheers, Wayne