What are the considerations for crawlspace encapsulation barriers / humidity control?

Question

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.

Answer 1

I use a standalone humidity reducer in the wettest room of my house. I've used one in a finished basement too, when it got too damp down there we ran it. For about $100 USD you can get a decent dehumidifier that plugs into a regular wall socket. It works by pulling water from the air. They also produce heat, which will raise the room temperature a little when it's running, helping the drying process. Most of them have a way to attach a hose and run the water directly to a drain if you don't want to manually empty the bucket. Otherwise you'll have to empty the basin a couple times a day when the room is very wet. I opted to buy a larger machine with a bigger collection basin so I don't have to empty it as often.

$5000 to throw heavy plastic over the crawlspace floor sounds ridiculous. We had our whole basement cleaned, insulated, and sheeted over dirt floors for less than half that cost. You might want to get a few more bids or consider a DIY.

Answer 2

Is there any way you get some air moving through there? I would see if you can vent some of the air out of the area, or bring some drier air in. I am not sure of your DIY level, but you could also buy some of the heavy plastic sheeting and line it yourself. You can use bonding agents to make the joints at the seems, something along the lines of a plastic glue compound.