Somebody may have software that can do an exact analysis of thermal problems like this.
Getting this answer by experimenting is probably out of the question for most people.
So. . .
First, try your local permitting dept. for info of this type.
My copy of the International Residential Code talks about insulation for this purpose; you might take a peek at the latest version of this book in a tech. bookstore.
Lastly, Border's may have a copy of the fat little handbook that Civil Engineers use to solve problems like these. Take a peek.
Here's a link that brings up several factors affecting this problem