BackerBoard is manufactured from Raylite® brand expanded polystyrene insulation. Raylite® is a superior quality insulation that is extremely lightweight. Raylite® resists water absorption, will not mold, rot or decay and will not create a second moisture barrier. Raylite® BackerBoard cuts easily with a knife or can be scored and snapped over a sharp edge. Raylite® Backerboard's high compressive strength and R-value of 3.85 per inch 75°F. Make it an excellent siding underlayment and insulator.