Are you talking about the Dynamat or the Dynapad?
I agree that the Dynamat type stuff is probably not much of an issue on heat retention, as it is not that thick.
I was talking about using the Dynapad. That is about 1/2 inch thick and weighs about a pound a square foot. There is a lot of thermal mass there.
Sometimes in my long winded dissertations I tend to obscure or miss the point....
