When air sealing a house, begin with the biggest holes and work toward the small ones
One big hole occurs in ceilings where dropped soffits are built for ductwork, can lights, or exhaust fans.
Many dropped soffits provide a direct path between the walls and ceiling, inside and outside.
Action items for tight soffits:
- Seal the section of the wall that will have soffit. This can be done with drywall and caulk, plywood and adhesive, our similar.
- Cover the ceiling with drywall before framing the soffit. If retrofitting, use rigid foam (or any rigid sheet stock) to cover the opening and seal the edges with canned foam.
—This video of Rick Arnold is from Building America's Solutions Center, an excellent resource for builders who want to build better.