Crawl Space Insulation
Fiberglass batts between floor joists are standard in Puget Sound homes with crawl spaces, and they fail in predictable ways: they get wet, they sag, they fall, and rodents nest in them. A cold floor in a room above a crawl space is the usual first sign. We remove the old insulation, fix what caused it to fail and install new insulation that stays where it belongs.
Why floor insulation falls down
| Cause | What we see |
|---|---|
| No supports, or supports rusted out | Batts drooping between joists, gaps at the subfloor |
| Wire supports too far apart | Batts held only at the ends, sagging in the middle |
| Moisture | Heavy, wet batts pulling away from the subfloor |
| Rodents | Tunnels, nests and torn insulation lying on the barrier |
| Plumbing or HVAC work | Batts removed to reach a pipe and never put back |
Insulation that sags even an inch away from the subfloor leaves an air gap that cold air moves through freely. At that point it is doing little.
What we install
- Unfaced fiberglass or mineral wool batts at R-30, the level current Washington energy code calls for in floors over unconditioned crawl spaces, cut to fit tight between joists and against the subfloor
- Support at close spacing: spring-steel rods, or a full-coverage mesh or netting where joists are irregular, so the batt cannot sag
- Sealed rim joists: the band of framing at the top of the foundation is sealed and insulated with rigid foam or spray foam
- Insulated ducts and pipes: heating ducts and water lines that run below the insulation get wrapped
- Kraft facing, if used, toward the warm side — against the subfloor, not facing the soil
Before we insulate
Insulating a damp crawl space just creates new wet insulation. During the inspection we take moisture readings on the floor framing. If they are high, the proposal includes a vapor barrier replacement or encapsulation. If you are encapsulating, insulation may move to the foundation walls instead.
Floor insulation replacement cost
| Crawl space area | Remove old + install R-30 with supports |
|---|---|
| Under 900 sq ft | $2,200 – $3,200 |
| 900 – 1,400 sq ft | $2,800 – $4,100 |
| 1,400 – 2,000 sq ft | $3,500 – $5,000 |
Insulation replacement is commonly done alongside a new vapor barrier, which shares setup and lowers the total.
Crawl space insulation questions
Will this make my floors warmer?
Usually yes, especially over a vented crawl space in winter. Rooms on the main floor above failed insulation are often several degrees colder at foot level.
Should I use spray foam on the underside of the floor?
We rarely recommend it for the full floor. It is hard to inspect plumbing and wiring through, and it complicates future repairs. We use spray foam at rim joists, where it performs best.
Can I just push the fallen batts back up?
If they are dry, clean and undamaged, with proper supports, sometimes. Most fallen batts we see are compressed, soiled or damp, and the supports are what failed.