Crawl Space Insulation in Tacoma, WA
Removing the wet, fallen, rodent-fouled batts under a Tacoma house and putting back insulation at the value the state energy code sets.
What Washington Requires and What Most Tacoma Houses Have
Washington's energy code sets R-30 as the prescriptive insulation value for a floor over a crawl space. Replacing wet or rodent-damaged batts with new R-30 under a Tacoma house runs about $2,000 to $5,000, and Tacoma Power pays $750 toward it on electrically heated homes.
The energy code (WAC 51-11R, Table R402.1.3) lists R-30 for floors in Washington's marine climate zone, and Section R402.2.7 requires the insulation to be in substantial contact with the underside of the subfloor, to run the full height of the perimeter framing, and to sit above the foundation vents or behind a baffle so vent air passes beneath it. A house built in Tacoma in 1955 had, at most, a couple of inches of something between the joists, and whatever is there now has usually spent a decade on the ground.
Why the old insulation failed
Fiberglass batts hold water. In a vented Tacoma crawl space the winter air condenses on the cold subfloor, drips into the batts, and adds weight until the staples or wire supports let go. Rodents, which are a fixture of crawl spaces in every older Tacoma neighborhood, tunnel and nest in the same batts. Wet, fouled fiberglass insulates poorly and keeps the joists above it damp, which is why it comes out before any moisture work and never goes back in.
What goes back in
R-30 fiberglass or mineral wool batts, sized to the joist spacing and held tight to the subfloor with rigid supports rather than string, is the standard replacement and what the utility rebate is written around. Closed-cell spray foam on the subfloor is used in some encapsulations and costs more. Insulating the stem walls instead of the floor is a design some contractors propose for a sealed crawl space; the floor section is what the state code's prescriptive path lists, so a wall-insulation plan should come with the contractor's explanation of how it complies.
The Tacoma Power Rebate
Tacoma Power's residential insulation incentive pays $750 for each of three areas, crawl space floor, attic, and walls, up to $2,250 in total. The crawl space rebate applies when the existing floor insulation is R-11 or less and the new installation reaches R-30, or a full cavity fill with air sealing. The house has to be served by Tacoma Power and heated primarily with electricity, and the work has to be done by a Tacoma Power participating contractor; new construction, manufactured homes, do-it-yourself jobs, buildings of five or more units, and homes that have already taken the incentive are excluded.
Participating contractors handle the paperwork and take the rebate off the invoice. Much of Pierce County outside the city is served by a different utility, and the contractor will know which program, if any, the house qualifies for.
Removal and Replacement
- Photograph and moisture-test the joists before anything comes out, so the wood's condition is on record.
- Bag and remove the old batts, hangers, and any rodent material, with respirators in a fouled crawl space.
- Deal with the moisture: a vapor barrier at minimum, drainage if there is water, encapsulation if the vents are being closed.
- Install R-30 batts tight to the subfloor with rigid supports, full height at the rim, air sealed at the perimeter.
- Confirm the vents are clear beneath the insulation, or baffled if they sit above its lower face.