To get the most R-value with the least thickness, use batt insulation with higher R-value per inch in combination with rigid foam insulation. Rigid foam sheeting has an R-value ranging from R-5 to ...

ACH Foam Technologies' Foam-Control MAX graphite polystyrene (GPS) is a rigid foam insulation well ... Developed to achieve R5 (R-value) at 1 1/16th in. thick without decrease in R-value ...