Vapor Barrier Installation in Hayward, CA

Local Context

Eighteen Inches of Rain in Five Months Goes Somewhere, and Under a Hayward House It Goes Up

Hayward takes roughly 18 inches of rain a year, with more than 80 percent of it falling between November and March. On the western flats the ground is low, level and slow to drain, and the creek corridors — San Lorenzo, Sulphur, Ward and Mt. Eden — hold water in the soil well past the last storm. Under a raised-foundation house that water does not simply sit. It evaporates upward into the crawl space, condenses on cool framing, and keeps the wood damp through the spring.

Damp sub-area framing is exactly what subterranean termites (Reticulitermes) look for, and it is what wood-decay fungi need to get going. The two show up together often enough that a WDO report will list both in the same paragraph: shelter tubing on a pier, and fungus damage at a joist end where the moisture never left. Cool marine air over Hayward slows the drying further, since the morning cloud does not clear until early afternoon for much of the year.

A lot of the raised-foundation stock here predates the practice of laying a barrier at all. Homes in the older parts of Mt. Eden, Longwood and downtown often have bare soil under the floor, sometimes with construction scrap left from the original build still sitting on it — cellulose already inside the crawl space. Sealing that soil is the most basic structural improvement those houses have available.