Moisture in crawlspaces can cause harm to the home by promoting mold and rot along floor joists and rim joists. Some moisture in crawlspaces occurs from water vapor in soil. To prevent water vapor in the soil from entering the crawlspace, the crawlspace floor should be covered with a heavy plastic that is sealed to the walls. This plastic ground cover should be installed regardless of whether the crawlspace is vented or unvented. This plastic covering will also help keep soil gases from entering the home. This task should be included in the contract for the appropriate trade depending on the workflow at the specific job site.
Other Considerations: Moisture can enter a crawlspace from sources other than vapor from the ground. In vented crawlspaces, the dominant source of crawlspace moisture is bulk water intrusion from improper grading of the site, lack of perimeter drainage, improper irrigation practices, etc.; see Final Grade. In humid climates, water vapor in warm outdoor air can enter through crawlspace vents and condense on cooler framing. In humid climates, building scientists recommend building insulated, unvented crawlspaces (see Basement Walls). Because radon accumulation in a home cannot be measured until after the home is built, as an added precaution consider installing a passive radon venting system, which collects soil gasses under the plastic and vents them through the roof via a vent pipe (see Radon Resistant Construction).
How to Install a Polyethylene Ground Cover
- Spread one layer of 6-mil or thicker polyethylene across the entire ground surface. Edges should lap up side walls at least 8 inches.
- Overlap all seams by 12 inches and tape them. Ensure that surfaces where tape will be applied are clean and dry.
- Attach the polyethylene to the walls at least 6 inches up the walls or to a height at least equal to ground level. Attach the polyethylene with pressure-treated wood furring strips or other mechanical fasteners. The edges can be sealed to the wall with fiberglass mesh tape and duct mastic.
Figure 1 - A continuous layer of polyethylene covers the crawlspace floor and is attached to the wall with wood nailing strips
4. Lap the vapor retarder up the sides of any interior columns at least 4 inches above the crawl space floor.
5. For added durability, consider one of the following:
a. Lay one polyethylene groundcover at the beginning of construction; when construction is completed, install a second sheet over the first sheet to cover any rips in the first sheet (make sure the first sheet is dry and free of organic matter). Seal the top sheet to the walls.
b. Lay a vinyl runner or extra plastic over any areas that will get traffic, such as a service path to the furnace.
c. Cover the single, sealed layer of polyethylene with a concrete slab 2 or more inches thick.
Figure 2 - Polyethylene completely covers the floor of this crawlspace and is attached to the walls and piers as well
Figure 3 - Polyethylene is being attached to the crawlspace floor and walls with plywood furring strips