What is the purpose of a sill seal at exterior masonry openings sitting on a foundation?

• A. To improve security.
• B. To reduce air leakage and moisture transfer at the sill, and to provide space for flashing.
• C. To increase insulation value.
• D. To decorate the sill.

Answer: (B)

Sealing the gap around the sill to stop air leakage and moisture moving into the wall, while providing a spot for flashing to sit, is the main purpose. The sill seal fills irregular gaps between the masonry opening and the foundation so air carried by wind or convection can’t easily travel into the wall cavity, which helps control drafts and reduce moisture problems. It also creates a proper space for flashing to shed water away from the opening, preventing water from wicking into the masonry or framing. While a sill seal can have a small supporting effect on insulation and isn’t meant for security or decoration, its primary role is to block air and moisture paths and to accommodate flashing.