Why Venting A Crawl Space Backfires Here
The theory behind crawl space vents is that outside air dries the space out.
That works where outside air is drier than the space. Here it frequently is not. With something like seventy fog days a year, air arriving off the bay is close to saturated for long stretches, and when it meets ground that stays cool all summer it drops its moisture rather than carrying any away.
So the vents run a slow supply of damp air into a cool space. The joists above take it on, and the floor of the house is the boundary between the two.
Sealing It Instead
The approach that works is to stop treating the crawl space as outdoors.
A ground sheet goes down over the earth and gets taped at the laps and run up the walls. Closed cell foam goes onto the perimeter walls and across the rim joist. The vents get closed.
The space is now inside the building envelope. It stays within a few degrees of the rooms above, it is dry, and the floor stops being cold.
That last part is what people actually notice.
Walls Or The Underside Of The Floor
There are two places the insulation can go, and they are not equivalent.
Insulating the underside of the floor leaves the crawl space itself outside the envelope, still cold, still holding whatever is in it, with services running through it that now need their own protection.
Insulating the perimeter walls brings the space inside. Pipes and ducts down there stop being at risk, and the assembly is far simpler.
We insulate the walls in almost every case. Where the underside of the floor genuinely is the right answer, we will say so.
Access, And What It Does To The Price
This is the service where access moves the number more than anything else.
A crawl space with a decent hatch and eighteen inches of headroom is straightforward. One entered through a cupboard floor, with ductwork in the way and a foot of clearance, is a different job at the same square footage.
Existing material has to come out first, and in a wet crawl space that is often unpleasant work. We price it honestly rather than discovering it on the day.