Vapor Barrier Installation in South San Francisco, CA

Local Context

Fog Drip Keeps the Ground Under Winston Manor Damp in a Month With No Rain

Moisture decides what lives under a South San Francisco house. The San Bruno Gap funnels marine air across this city through most of the summer, which is why July and August stay between 68 and 74°F while inland towns bake. That same air keeps humidity high and sub-area soil damp in months when no rain has fallen since March. Bare soil evaporates upward all year, and the vapor condenses on joists, subfloor and the underside of the sill plate.

The consequences are structural before they are ever a pest problem. Sustained sub-area humidity softens framing, feeds wood-decay fungi, and raises the moisture content of wood into the range subterranean termites (Reticulitermes) prefer — and into the range where dampwood termites (Zootermopsis) can establish. It also makes the space attractive to rodents and to widows. Then roughly 20 inches of rain arrives between November and March, and any drainage running toward the foundation keeps all of it going.

Age is the other half of it. Plenty of raised-foundation houses here predate 1980 and were built with no ground cover at all, or with a thin sheet that decades of plumbers, electricians and HVAC crews have torn through. A vapor barrier only works if it is continuous, overlapped at the seams and carried up to the piers and foundation walls. A shredded sheet lying on dirt does nothing.