Vapor Barrier Installation in San Leandro, CA

Local Context

Bare Dirt Under a Raised San Leandro Floor Pumps Water Into the Framing

Soil gives off water vapor continuously, and in a crawl space with an open dirt floor that vapor has one direction to go: up into the joists, the subfloor, the sill plate and whatever insulation sits above them. San Leandro sits low on the Bay shore, with tidal marsh at Oyster Bay and four creeks — San Leandro, Indian, Redwood and Buckhorn — running through town, and roughly 20 inches of rain lands between December and March. The water table under the western flats does not have far to travel.

The marine layer takes away the drying season. Summer highs here only reach 70 to 75°F and overnight humidity stays high under the Bay fog, so a sub-area that absorbs water all winter never gets the hot, dry stretch that would give it back. Framing that holds moisture is what dampwood termites (Zootermopsis) and wood-decay fungi need, and it is also the condition that pulls subterranean termites (Reticulitermes) up out of the soil toward the structure.

It shows on the report before it shows in the house. On raised-foundation homes in Mulford Gardens, Marina Faire, Old San Leandro and the older Broadmoor blocks, the Section 2 list reads much the same way over and over: excessive sub-area moisture, cellulose debris on the soil, inadequate ventilation, earth-to-wood contact at a pier or a form board. A vapor barrier is the standard correction for the first of those, and usually the simplest item on the list to put right.