Chases and soffits are raceways for air leaks
Older houses often have several big holes in the attic floor. Many are around open chases for ducts, electrical cables, or flues, often running unimpeded from the basement to the attic. When installing plumbing chases in new houses, install air-barrier sheathing first, and seal the gaps between intersecting walls with caulk. As the framing shrinks, these gaps become large. The top of an open plumbing chase in an existing house can be sealed with a variety of sheathing materials as long as seams and edges are sealed with caulk or tape.
If any chases for plumbing pipes, ducts, or flues originate in the basement, be sure to seal the chases at the bottom with the same techniques you used to seal the chases in the attic.
While you’re in the attic, check for any unsealed kitchen soffits. Such soffits are often built above a row of wall cabinets. In new construction, the ceiling drywall should be installed and taped before the soffit is framed. The first clue to a leaky soffit is often a piece of discolored fiberglass insulation. The discoloration is caused by escaping air that has carried dust upward, often for years.
In many existing houses, the sides and top of kitchen soffits are open to the framing cavities. In this case, you can install the neces-sary pieces of air-barrier sheathing from above. Don’t forget to seal the perimeter of the sheathing and any seams with durable tape, caulk, or spray foam. Once the soffit is airtight, don’t forget to replace the insulation.
Cabinet soffit Chase walls (plan view)