· Michael Blair
Fibreglass batts pushed up between floor joists are the standard older approach and they fail reliably here. They fall because they have absorbed moisture, and once they have they are doing harm rather than nothing.
Why batts fail here
Fibreglass takes on moisture from humid crawl air, gains weight, compresses and loses R-value. Wet batts then hold that moisture against the subfloor, which is exactly where you least want it.
They also give rodents nesting material, and once they have moved in the insulation becomes a contamination problem as well.
Where insulation belongs after encapsulation
In an encapsulated crawl, the thermal boundary moves from the floor above to the crawl walls. Rigid foam board against the perimeter walls insulates the now semi-conditioned space.
Insulating both the walls and the floor is redundant and can trap moisture between them.
- Remove wet or fallen batts entirely, do not re-fix them
- Rigid foam on the perimeter walls after encapsulation
- Seal and insulate the rim joist — a major air leak
- Insulate any ductwork in the crawl
- Insulate water lines against freezing
The rim joist matters most
The band around the perimeter where the floor structure meets the foundation is one of the largest air leaks in a house, and it is almost always uninsulated in older Nashville homes.
Sealing and insulating it with rigid foam or spray foam has an outsized effect for the area involved.
Ducts in the crawl
Most Nashville houses run HVAC ducts through the crawl space. Uninsulated or leaking ducts there lose conditioned air into a space you are not trying to condition.
Sealing duct joints with mastic and insulating the runs is frequently the highest-return work available in a crawl space.