I live in a house that's half slab, half crawlspace. The crawl is half basement. Typical crawlspace/basement humidity readings are:
80% RH @ 67°F 85% RH @ 68°F 78% RH @ 67°F
Where in the room above:
67% RH @ 74°F 65% RH @ 72°F 66% RH @ 75°F
In places the crawl remains visibly damp even after months of dry weather, however no water ponds. A plastic sheet placed on the ground gets wet. The basement air is too damp for storage of goods (e.g. things start to smell musty).
What normal guidelines are there for acceptable crawl humidity?
An encapsulation company has given a bid of $5000 to put heavy plastic sheets on the dirt, to hold the moisture away from the air. What are the potential pitfalls here (other than the $5000 hole in the pocket) for retrofit vapor barriers on old homes?
This particular home has no A/C, is in a mild Mediterranean climate, and sits on highly impermeable clay soil. The foundation is from 1938.