Most Seattle houses sit over a vented crawl space with fiberglass batts pushed up between the floor joists, and in this climate that assembly has a limited life. The batts get damp, they sag, they drop out of the joist bays, and rodents finish the job. What holds up here is usually the opposite approach: stop treating the crawl space as outdoors. Seal it, insulate the foundation walls and rim joists with closed-cell foam, and lay a proper vapor barrier over the dirt.
One thing has to be true before any of that. If you have standing water, a spring under the house, or a drainage problem, foam is not the answer and we will say so on the first visit. Insulation over a wet crawl space is an expensive way to hide a grading or plumbing problem, and we would rather lose the job than bury one.
The short version
In a Puget Sound crawl space the usual best answer is to seal the vents, insulate the foundation walls and rim joists with closed-cell foam, and lay a sealed ground vapor barrier, rather than stuffing insulation between the floor joists. Clean-out, rodent exclusion, and drainage come first. Always.
Why a vented crawl space struggles in this climate
Crawl space vents come from an old idea: let outside air wash through the space and it will carry moisture away. In some climates that does roughly what it promises. Here it mostly does two other things. It keeps the space cold nine months of the year, which is why your floors are cold in January with the furnace running, and it gives rodents, insects, and wind-driven rain a way in.
The Southeast advice does not transfer, and neither does the Southeast problem
Most crawl space advice online was written by contractors in Texas, Florida, or the Carolinas, and their argument is that summer air is the enemy: outdoor air with a dew point in the mid seventies enters a crawl space whose surfaces sit near ground temperature, that moisture condenses on the joists and ducts, and the place grows mold every August. In a humid climate that is real, and it is why encapsulation took off down there.
Puget Sound summers do not work that way. Our summer dew points generally sit well below typical indoor temperatures, so summer air moving through a vented crawl space here is usually not depositing water on cool surfaces the way it does in Atlanta. Our problem is the ground and the season. Soil under a Seattle house stays wet most of the year and evaporates moisture upward continuously, and a high winter water table, gutters discharging at the foundation, or a lot that drains toward the house all make it worse. Then the vents pull in cold, near-saturated winter air and hold the space at outdoor temperature. Cold, damp, and still is exactly what rot and mold want. Same conclusion as the Southeast, different reason, and the reason changes the scope of work.
More on the mechanism in why Seattle crawl spaces get moldy and does spray foam cause moisture problems in the Pacific Northwest, which is the fair question to ask before sealing anything up.
What we actually find when we open the hatch
- Batts hanging out of the joists or lying on the ground. Fiberglass held up with wire rods or twine eventually loses. Once a batt is on the dirt it is a wet sponge and the floor above it is bare.
- Rodent runs and contamination. Rats and mice tunnel through batts and nest in them. That material is a health issue and has to come out, not get covered over.
- Surface mold on joists and subfloor. Usually worst near the vents and under bathrooms, and often a symptom of something simpler like a slow supply leak or a dryer venting under the house.
- A torn plastic sheet, half buried, and a duct off its boot. A loose sheet thrown over the dirt twenty years ago now does almost nothing, and on ducted systems we regularly find a duct heating the crawl space instead of the bedroom above it.
- Standing water or a mud line. Sometimes puddles, more often a tide mark on the footings showing how high it gets in February. This one changes the whole conversation.
Floor or walls: the decision that decides everything else
Every crawl space job comes down to one question: where is the thermal boundary of your house? You can put it at the floor, which leaves the crawl space outside the heated envelope, or at the foundation walls, which brings the crawl space inside. Both are legitimate, they lead to completely different scopes of work, and in this region they do not perform the same.
| Insulate the floor | Seal and insulate the walls | |
|---|---|---|
| Crawl space stays | Vented, cold, damp | Sealed, close to house temperature |
| Pipes and ducts | Outside the envelope, freeze risk, duct losses | Inside the envelope, protected |
| Air leakage | Continues unless every penetration is sealed | Handled at the perimeter |
| Durability here | Batts sag, absorb moisture, get nested in | Foam is bonded on and is not nesting material |
| Cold floor complaint | Helps if the work is genuinely airtight | Usually the bigger improvement |
| Cost | Lower upfront, less prep | Higher upfront, includes cleanout and barrier |
Why the wall approach usually wins here
Three reasons, in order. Durability: foam on a foundation wall is bonded in place, does not absorb water, and is not nesting material, so it is still working in fifteen years, which is more than we can say for batts under a Puget Sound house. Mechanical systems: nearly every Seattle crawl space holds water lines and many hold the ducts for the whole house, and bringing that space inside the envelope protects them. Air: warm air rises out of the top of your house and pulls replacement air in at the bottom, and in a vented crawl space that replacement air is cold damp crawl space air. Sealing the perimeter fixes the supply side instead of chasing every gap in the subfloor.
The foam for this is almost always closed-cell, because it insulates and controls vapor in one step and tolerates the conditions. If you are weighing foam against a full encapsulation package, we compare them in should I spray foam or encapsulate my Seattle crawl space.
When we will still insulate the floor instead
It is not always the wall. We will recommend floor insulation when the crawl space has a water problem we cannot economically solve, when the foundation is odd enough that sealing it properly is not realistic, when a combustion appliance down there has to keep its air supply, or when the budget does not stretch and real R-value under the floor today beats nothing for three more years. If we go that way, we say plainly what you are and are not getting.
The parts of the job people forget
Rim joists
The rim joist is the band of wood on top of the foundation wall that closes off the ends of the floor joists. It is one of the leakiest parts of a house and one of the smallest areas of work, which makes it the best value in the whole crawl space. A couple of inches of closed-cell air seals and insulates a spot fiberglass never sealed well. On older Seattle houses that band is often where the cold comes from.
The ground vapor barrier
Bare soil evaporates moisture into the space continuously, so the ground barrier is not optional in a sealed crawl space, it is half the system. Done properly that means a heavy sheet run wall to wall, seams lapped and taped, edges carried up the stem wall and sealed, and piers wrapped. A sealed crawl space with a sloppy barrier is a worse idea than leaving the vents open.
Combustion appliances and dryer vents
If a gas water heater or furnace draws its combustion air from the crawl space, sealing that space changes how it gets air, and that is a safety issue rather than a preference. We check before we quote. We also look for dryer vents and bath fans that were never run all the way outside, because any of those dumping under the house defeats everything else you pay for.
What has to be fixed before any foam goes in
Water and drainage first
Standing water, a wet spot after rain, or a tide line on the footings means gutters, grading, sometimes a sump. Insulation does not fix that and should not go in over it.
Clear out and clean up
Old wet batts, fallen debris, and rodent contaminated material come out and go in the truck. That is insulation removal, priced separately and honestly.
Seal the rodents out
Vents screened or closed, gaps at pipe and wire penetrations sealed, the hatch made to fit. If they can still get in, they will be back in the new work.
Fix the leaks and the ducts
Slow plumbing drips, disconnected ducts, and bath fans venting into the crawl space get corrected before we seal anything up.
We mean that first step. If we get under your house and find a drainage problem, our recommendation will be to spend your money on water management and call us back afterwards. Foam over a wet crawl space is the kind of job that looks fine for two years and then is not.
What crawl space insulation costs around Seattle
Closed-cell commonly runs about $1.00 to $2.00 per board foot installed, a board foot being one square foot at one inch thick. As a rough whole-project figure, 1,000 square feet at three inches often lands around $2,200 to $5,400. Crawl space work spreads widely inside that range for reasons specific to working under a house:
- Headroom and access. An 18 inch crawl space with a small exterior hatch is slow, hard work. A 4 foot space with a basement door is a different job at the same square footage.
- How much has to come out first. Removal and rodent cleanup is its own line, and on a bad one it can rival the insulation.
- Walls versus floor. Wall area is far less than floor area, but the wall approach adds the ground barrier and the sealing work.
- Obstructions. Ductwork, plumbing runs, and low pipes slow coverage and change how the foam has to be applied.
Seattle trade labour generally runs around $95 to $175 per hour, which is the other reason access matters so much here. Anyone who quotes your crawl space firmly over the phone without going under the house is guessing, and we will not do that. Full ranges are on the spray foam insulation cost in Seattle page.
Common questions about crawl space insulation
Should I close my crawl space vents in Seattle?
Closing vents is part of a system, not a standalone fix. Close them and do nothing else and you have turned a cold damp space into a still damp one, and possibly changed the air supply to a combustion appliance. Vents get closed as part of a package: cleanout, a sealed ground vapor barrier, insulated and sealed walls and rim joists, rodent exclusion, and any drainage work the house needs. As a package it works well here. Alone it can make things worse.
Is spray foam better than fiberglass batts under the floor?
For durability in a Puget Sound crawl space, yes, and it is not close. Batts depend on supports to stay up, absorb moisture, and get nested in. Closed-cell foam is bonded in place, does not hold water, and is not nesting material. That said, the more useful comparison is floor versus walls, not foam versus batts. Moving the insulation and the air barrier out to the foundation walls is usually the better answer here whichever material you would have used in the joists.
Do I need a vapor barrier if I am spraying foam on the walls?
Yes, they do different jobs. Foam on the walls handles heat loss, air leakage, and vapor through the concrete. The ground barrier handles moisture evaporating out of the soil, which is the largest single moisture source in most Seattle crawl spaces. Insulating the walls without covering the dirt leaves the main problem in place.
My crawl space has rodent damage and mold. Can you just foam over it?
No, and nobody should. Contaminated insulation and debris has to come out, the surfaces have to be cleaned, and the entry points sealed before new material goes in. Foaming over rodent contamination traps it in your house permanently and seals the smell in with it. Removal is a real cost and we price it as its own line so you can see what you are paying for. See insulation replacement for how the sequence works.
Will sealing my crawl space cause a moisture problem instead of fixing one?
Fair question, and the honest answer is that it depends on whether the work was done completely. A sealed crawl space with a proper ground barrier, sealed walls, no bulk water entry, and nothing discharging into it generally runs drier than the vented version it replaced, because you have cut off both the soil evaporation and the outside air. A half sealed one with a torn barrier and a dripping supply line is worse than leaving it alone. That is why the prep list is not padding.