Sill Seal foam gasket is a flexible polyethylene foam gasketing strip used to reduce air infiltration between concrete foundation and sill plate or around windows. It works by compressing to seal the gaps between the foundation wall and sill plate. It can help to fill almost any gap, joint, crack or crevice through which air can enter. STYROFOAM Sill Seal foam gasket cuts heating and cooling costs by filling the gap between the sill plate and the foundation wall. Up to 40 percent of a structures heat can be lost due to air infiltration, much of it through this gap. This closed-cell polyethylene foam won’t fall apart on the job site or after years of use. It provides excellent and enduring resistance to water and water vapor.