Vapor Barrier Installation in Burlingame, CA

Local Context

Bare Dirt Under a Raised Floor Is a Humidifier Running All Winter

An open crawl space with a bare soil floor behaves like a humidifier. Ground moisture evaporates upward all day, and in a Burlingame winter — about twenty inches of rain between November and March, with ambient humidity near 72 percent even in summer — that vapor has nowhere useful to go. It condenses on joists, subfloor and ductwork and raises the moisture content of the framing. Once structural wood holds enough water, wood-decay fungus and dampwood termites (Zootermopsis) can establish in the sill plate and the joist ends.

Burlingame's geography concentrates the problem in specific places. The pre-1940 raised-foundation houses through the Easton Addition, Burlingame Park and Burlingame Terrace sit on lots draining toward Mills Creek, Easton Creek, Sanchez Creek and Terrace Creek. The flat blocks east toward California Drive and Lyon-Hoag are close enough to the tidal mudflats that the water table stays high through the wet months. Hillside properties below Trousdale Drive get the opposite version — slope drainage that ends up under the house instead of around it.

Moisture is also what makes a crawl space attractive to everything else. Damp, dark and undisturbed suits Argentine ants, sowbugs, silverfish and western black widows, and a wet sub-area corrodes vent screens and softens the wood access door until rodents can push through it. A vapor barrier does not repair a drainage failure — standing water has to be corrected first — but it cuts the vapor path so the framing dries out and the conditions that show up as Section 2 items on a WDO report actually go away.