Vapor Barrier Installation in Sunnyvale, CA

Local Context

Sunnyvale's Older Raised Foundations Sit on a Valley Floor That Drains Slowly

Most of Sunnyvale is slab-on-grade, which is why crawl spaces get overlooked here. The housing that predates the big post-war tracts is the exception — older blocks near the Heritage District, along Sunnyvale Avenue, and scattered through Lakewood Village sit on raised perimeter foundations with bare dirt under the floor. The city takes 14 to 16 inches of rain between November and March on a flat alluvial valley floor with very little fall, and a good share of that water leaves by evaporation rather than drainage.

That evaporation goes straight up into the crawl space. Damp soil under an unprotected floor raises humidity against joists, subfloor and sill plate all winter, and the tule fog that settles into low spots on cold Sunnyvale mornings slows the drying. Softened, damp wood is what subterranean termites (Reticulitermes) and wood-decay fungi need to get established. Houses near Calabazas Creek, Saratoga Creek and the Sunnyvale East and West Channels sit closest to that moisture and tend to show it at the sill first.

A vapor barrier is a physical break between soil and the air under the floor. It will not dry out a crawl space that has standing water — a plumbing leak, a downspout dumping at the foundation, or grade sloping toward the house has to be corrected first, or the sheeting simply traps the water beneath it. On Sunnyvale lots the usual culprits are lawn irrigation set against the stem wall and a downspout that ends a couple of feet from a foundation vent.