Drop Soffits / Open Cavities

Drop soffits are large open cavities that run along the exterior wall. They can be found in areas where the ceiling height changes, IE. above stairways, bathrooms, above closets. Check for these area during the walk-through of the home.

There are two ways to properly air seal these areas:

  1. Treat same as wet wall, install rigid foam board or similar material along the same plane as the rest of the attic ceiling height.

  2. Air seal inside the cavity along all four sides of the bottom and any areas where there may be air bypasses.


Large open cavity above a stairway


Should be sealed along the bottom of the joists with rigid foam board and expanding foam


Example of how to seal an open drop soffit

drop 4.jpg

When installing foam board, make sure to seal underneath both joists on both sides to complete the air barrier. It was not completed yet in this photo