Vapor Barrier Installation in Palo Alto, CA

Local Context

A November Storm Charges the Soil Under Your Floor and It Stays Charged Until July

Palo Alto takes 14 to 16 inches of rain a year, almost all of it between November and April. Under a raised-floor house that water does not simply drain away. Soil holds it, and open ground in a sub-area evaporates it upward all summer — into the joists, subfloor, girders and sill plate directly overhead. Wood that stays above roughly twenty percent moisture stops behaving like structural lumber and starts behaving like habitat.

The creeks decide who gets it worst. San Francisquito Creek along the San Mateo County line, plus Matadero, Adobe and Barron creeks, run through and past the older residential neighborhoods, and properties near those corridors hold a higher water table well past the last storm. That is the same moisture supporting subterranean termite (Reticulitermes) activity in this part of the city, and where a plumbing or roof leak has run alongside it long enough, dampwood termites (Zootermopsis) in the framing itself.

The houses that need this most are the ones built before sheeting was standard. Raised-floor construction in Palo Alto runs from the pre-1930 blocks of Professorville and Downtown North through the early post-war streets, and that era frequently left bare soil, buried form boards and construction scrap under the floor. Add sub-area vents blocked by decades of landscaping and you get a space that never dries — which is exactly the condition that fills the Section 2 column of a WDO report.