Vapor Barrier Installation in Ben Lomond, CA

Local Context

Bare Crawl-Space Dirt Under a Ben Lomond Cabin Evaporates Into the Floor Above It

Soil gives off water constantly, and a crawl space traps it. A Ben Lomond house from the 1950s or 1960s typically sits on piers over open ground with no barrier at all, because nothing at the time required one and nobody added one since. Around fifty inches of rain a year saturates the hillside from November through April, the ground under the house stays wet long after the storms pass, and that vapor rises into the joists, the subfloor, and whatever insulation is left in the bays.

The canopy removes the drying half. A house on a hot valley floor bakes its ground moisture off each summer; a cabin under second-growth redwood on a north-facing slope off Alba Road never gets that chance. Add the marine fog that fills the San Lorenzo Valley on summer mornings and the crawl space simply never resets. That is the mechanism behind the three things we get called about most on these parcels — springy subfloors, fungal decay in the joists, and dampwood termites in the mudsill.

A barrier is the straightforward half of the fix. It will not remove water arriving from a downspout, a broken grade, or a seep on the uphill side, and those get corrected first. What it does is cut off ordinary evaporation from bare soil, which on most Ben Lomond parcels is the bulk of the moisture load. Crawl spaces we cover come through the following winter noticeably drier.