Crawl space vapor barrier quotes

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Liner only, or full encapsulation?

A vapor barrier is the plastic sheet over the dirt floor. On its own, it keeps ground moisture from rising into the wood above, and the vents stay open. Encapsulation goes further: the liner runs up the walls and is sealed, the vents are closed, and the space is dried by a dehumidifier or conditioned air (see encapsulation).

Both approaches are in use. UT Extension's termite publication describes the older vented approach: about 30 inches of clearance, evenly spaced vents, and polyethylene over 80% of the soil. The 2018 building code (R408.3) allows an unvented crawl space only with a continuous Class I liner, 6-inch sealed overlaps, edges 6 inches up the wall, and a drying method. In the Advanced Energy field study, vented crawl spaces that had a full liner still stayed above 70% humidity 98% of the time in summer 2003, while sealed ones were above 70% only 5% of the time.

What does a vapor barrier cost?

Angi's 2026 range for vapor barrier installation is $1,000 to $3,000. HomeGuide puts replacement at $2 to $4 per square foot, or $1,200 to $4,000. Material is a small part of the bill; HomeGuide's liner prices per square foot:

Angi lists 20-mil liner at $0.50 to $0.70 per square foot. Ask which thickness is quoted and whether it's reinforced.

Before you replace the plastic

• If water stands on the old liner after rain, a new liner won't fix that. Deal with gutters, grading or drainage first (standing water and drainage).
• Ask whether the old plastic and any debris are removed, and whether that's in the price.
• Ask how seams, piers and plumbing penetrations are sealed, and how far the liner goes up the walls.
• Ask your pest control company how it will inspect for termites once the liner and any wall insulation are in. UT Extension warns that termites can tunnel through foam board insulation below grade, and that liquid termiticides are hard to apply where foam is in the way.