Crawl Spaces: Encapsulation, Vapor Barrier and Drying
How to seal a dirt or block crawl space, choose between venting and encapsulation, and keep it dry without creating a new problem.
A crawl space is a basement that nobody visits, which is exactly why problems there run for years. Bare soil releases moisture continuously, humid summer air condenses on cool surfaces, and the result sits directly below the floor you walk on. This hub covers sealing the ground, controlling the air, and the safety rules that apply to a space you have to crawl into.
Seal the ground first
Exposed soil is the dominant moisture source in most crawl spaces. A sealed liner over the ground, run up the walls and fastened, stops that at source. EPA’s radon-resistant construction guidance describes the same principle for new build: heavy duty plastic sheeting, 6 mil polyethylene, or a vapor retarder laid over gravel to stop soil gases entering. The Vapor Barrier Material Calculator works out liner, overlap, tape and rolls from your measurements.
Vented or encapsulated?
Traditional practice vents a crawl space to the outside. In a humid climate that can make things worse, because warm moist outdoor air entering a cool crawl space condenses on the surfaces it meets. Encapsulation takes the opposite approach: seal the ground and the vents, then control the air inside. Which suits your house depends on your climate and your existing construction, and it is worth getting right before spending.
Drying the air
Sealing the ground usually is not quite enough on its own. A crawl space dehumidifier holds the space at a target humidity, and EPA suggests keeping indoor humidity below 60 percent, ideally 30 to 50 percent. Size it with the Dehumidifier Size Calculator and set a realistic target with the Basement Humidity Target Tool.
Insulation and the rim joist
Where insulation goes depends on whether the crawl space is inside or outside the building envelope. In an encapsulated crawl space the walls and rim joist are usually insulated and the floor above is left alone. ENERGY STAR publishes recommended R-values by climate zone, and air sealing the rim joist is often the highest-value small job in the whole space.
Safety before you go in
A crawl space can meet the definition of a confined space. Reference 29 CFR 1910.146 rather than treating it as an ordinary room, never work alone in a tight space, and take ventilation and air quality seriously. Never run a generator, pump engine or unvented heater in one.
Tools for this topic
- Vapor Barrier Material Calculator – liner, overlap, tape and rolls
- Dehumidifier Size Calculator – pints per day for the space
- Basement Humidity Target Tool – dew point and condensation risk
- Basement Dry-Up Plan Builder – where the crawl space fits in your plan
Latest Crawl Spaces guides
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Sources we rely on
- EPA: A brief guide to mold, moisture and your home
- EPA mold course, chapter 2: why and where mold grows
- EPA: Radon-resistant construction basics and techniques
- ENERGY STAR: Recommended home insulation R-values
- eCFR: 29 CFR 1910.146, Permit-required confined spaces
Frequently asked questions
It depends on your climate and construction. In humid climates, venting can bring in warm moist air that condenses on cool surfaces. Encapsulation seals the ground and vents and controls the air instead. Thickness is quoted in mil. Thicker liner resists punctures from crawling and debris, which matters in any space someone will enter to service equipment. Usually yes. Sealing the ground removes the largest moisture source, but a dehumidifier holds the space at a target humidity through humid weather. Treat it as a potential confined space. Reference 29 CFR 1910.146, never work alone in a tight space, and stop if the air seems poor.Should a crawl space be vented or sealed?
How thick should a crawl space vapor barrier be?
Do I need a dehumidifier in a sealed crawl space?
Is it safe to work in my own crawl space?