With the exceptional thermal protection offered by CM and SL Insulation Board, it's easy to see why GreenGuard is the leading brand of underlayments for residing. With its high R-Values of 3.0 (1/2") and 5.0 (1"), CM and SL Insulation Board protects homes against cold and heat, saving the homeowner money. Because it's made of extruded polystyrene (XPS), the board has no more than 1/10% water absorption by volume, giving it long-lasting performance while retaining its R-Value. Since the board is easy to cut and install, you'll save time and money on every job. Plus, it provides a solid base for new siding, so every installation will look smooth and professional.