On the popular Double Hung windows, the sill is a true sloped design. The fusion-welded, compound cut overlapping sill features five insulating chambers. These chambers help prevent bowing and add superior strength and rigidity. The sloped design provides quick water runoff during heavy rains. The sill dam weatherstripping with additional sash seals help make it weathertight as well. In addition, the sill does not require weep-holes – making it very easy to wipe clean.