Continuous air barrier for an elevated home on pilings — Zone 3A (Charleston, SC)

This is a builder's question about a specific project, not a general news story, but the details are worth knowing if you own or are planning a house raised on pilings, which is common along the South Carolina coast.

The homeowner's builder is working through how to seal up the underside of an elevated house so outside air can't sneak into the living space from the open area beneath it, where the ground floor sits exposed between pilings. The plan is to run a sealed membrane across the bottom of the floor structure and tie it into the same sealed sheathing system used on the walls and roof, so the whole house has one continuous barrier against air leaks. On the past few houses built this way, blower-door testing (a way of measuring how leaky a house is) showed very tight results.

The open question is what to do about insulation in that same floor. The floor trusses have open, X-shaped webbing rather than solid wood, so standard batt insulation can't fully fill the cavity around the diagonal supports, leaving small gaps where heat could pass through. The builder is weighing whether batts are good enough, whether spray foam would work better despite cost and moisture concerns, or whether another approach makes more sense for a floor exposed to South Carolina's hot, humid air on one side and an air-conditioned house on the other.

If you own a coastal home on pilings, or are planning one, this is the kind of floor-and-crawlspace detail worth asking your own builder about, since gaps here can mean higher bills and a less comfortable first floor.