A vapor barrier is a plastic liner on the crawl space floor that blocks moisture rising from the soil. Encapsulation includes a vapor barrier but also seals the foundation vents and walls and adds a dehumidifier, so the crawl space is closed off from humid outside air. In the humid Upstate, a vapor barrier alone usually isn't enough if the vents stay open.
Side by side
| Vapor barrier only | Full encapsulation | |
|---|---|---|
| Covers the dirt floor | Yes | Yes |
| Runs up and seals to the walls | Sometimes | Yes |
| Piers wrapped, seams taped | Sometimes | Yes |
| Foundation vents sealed | No | Yes |
| Dehumidifier | No | Yes |
| Controls humid summer air | No | Yes |
| Typical liner | 6–12 mil | 16–20 mil (Cobb) |
Where moisture comes from
A crawl space gets wet two ways: from the ground, and from the air. A vapor barrier handles the ground. In the Upstate, though, the air is the bigger problem in summer. Humid outside air comes in through the vents and condenses on the cooler wood and ducts. A barrier on the floor can't stop that. Sealing the vents and running a dehumidifier can. In a U.S. Department of Energy-sponsored study, closed crawl spaces stayed "substantially drier" than vented ones in humid weather.1
When a vapor barrier alone is enough
- The vents are already sealed and humidity stays below 60%
- The crawl space has no mold, standing water or wet insulation
- You're replacing a torn or missing liner in an otherwise dry crawl space
When you need full encapsulation
- Humidity in the crawl space runs above 60% for weeks at a time
- The house smells musty
- There's mold on the framing, or insulation is falling
- Ducts sweat or rust in summer
Cost difference
Every Cobb package is a full encapsulation, not just a liner: vents sealed, walls and piers sealed, and a dehumidifier included. See the cost guide for prices by size, or the vapor barrier page for liner details.
How to compare quotes
When you compare companies, ask four questions: What liner thickness? Are the walls and piers sealed? Is a dehumidifier included, and what size? How long is the warranty? Two quotes that both say "encapsulation" can be very different jobs.