· by Michael Blair, Nashville's Insulation
A crawl space is either inside the building envelope or outside it, and where that boundary sits decides which surface gets insulated. Doing both traps moisture between them.
Vented crawl space
The boundary is the floor above, so insulation goes between the floor joists. Fiberglass batts are traditional and in Middle Tennessee humidity they absorb moisture, sag and fall.
That is why so many crawl spaces here have insulation hanging down doing nothing.
Encapsulated crawl space
Sealing the vents, laying a vapor barrier and dehumidifying brings the crawl inside the envelope. Insulation then goes on the perimeter walls rather than the floor.
This is the approach that suits our climate, because it addresses the humidity that defeats floor insulation.
- Vented crawl: insulate the floor above
- Encapsulated crawl: insulate the perimeter walls
- Never insulate both — it traps moisture between them
- Rim joist is a major air leak in either case
- Remove wet or fallen batts rather than re-fixing them
The rim joist either way
The band where the floor structure meets the foundation is a substantial air leak and is almost always uninsulated in older Nashville houses.
Rigid foam or spray foam there has an outsized effect for the area involved, and it applies whichever approach you take.
Ducts and pipes
In a vented crawl, ductwork and water lines are outside the envelope and need insulating and, for pipes, freeze protection.
In an encapsulated crawl they are inside it, which is one of the practical advantages of the approach in a Middle Tennessee house with ducts down there.