see http://www.k-mac-plastics.net/data%20sheets/ATG-014S1.pdf i believe it is 1/3 for .325 inch thick
Usi= Uip*5.678263
As in pie r squared? r = radius.
R value is a measure of the ability of a material to retard, or slow down, the movement of heat.
The R value or R-value, the measure of thermal resistance (K·m²/W) used in the building and construction industry