Crawl space vapor barriers
A ground vapor barrier is the simplest moisture fix under a house. Here's when it's enough and when it isn't.
A vapor barrier, or ground cover, is a sheet of polyethylene laid over the dirt floor of the crawl space. Soil gives off moisture constantly, and on Wayne County ground that holds a high water table through the winter, a lot of it can rise into the crawl space and up toward the floor. A sound barrier slows that down.
When does a vapor barrier make sense on its own?
If you want to keep the crawl space vented and the main problem is a bare dirt floor or old, shredded plastic, replacing the ground cover is a smaller, simpler job than full encapsulation. It's also common before listing a house, since a missing barrier is an easy thing for a home inspector to flag. What it doesn't do is stop humid outdoor air from coming in through the vents in summer. If ducts sweat and insulation sags even with a decent ground cover, that's usually a sign the air is the bigger issue.
What separates a good installation from a poor one?
- Debris, sticks and sharp rubble cleared so the plastic isn't punctured
- Sheets overlapped generously at seams, and taped if the space will be closed later
- Coverage right up to the foundation walls and around piers, not just down the middle
- The barrier held in place so it doesn't bunch up when someone crawls across it
- Any standing water dealt with first, since plastic laid over puddles traps moisture
How thick should it be?
For a closed crawl space, the North Carolina Residential Code sets a minimum of 6-mil polyethylene or equivalent. Many companies use thicker or reinforced material because it holds up better when plumbers, HVAC techs and pest control crews crawl over it. Ask what thickness is being proposed and whether it will be sealed.
Thinking about encapsulating later?
Tell the company. If you might close the crawl space down the road, it can make sense to use a liner and seam method that can be extended up the walls and sealed as part of encapsulation, which needs a permit in North Carolina.
What affects the price?
- Crawl space size and clearance
- Material thickness and whether seams are taped
- Removing old plastic, insulation or debris
- Water or grading issues that need attention first
Pricing comes from the crawl space company after someone has been under the house with a flashlight and a tape measure. We don't quote prices, and a figure offered over the phone before that visit is only a ballpark.
Where we connect you with help
Common questions
Is a vapor barrier the same as encapsulation?
No. A vapor barrier covers the ground. Encapsulation also seals the vents and adds one of the moisture-control methods listed in Section R409.5 of the NC Residential Code.
Will a new vapor barrier stop my ducts from sweating?
Not always. Sweating ducts usually come from humid outdoor air entering through vents. Closing the crawl space or other moisture control may be needed.
Related services
More background: Goldsboro crawl space guide.
Damp smell, sweating ducts or a floor that's started to dip?
Call and we'll put you in touch with a crawl space company that works in Wayne County.