The liner and its connections form one system
Building codes commonly establish a 6-mil Class I vapor retarder as a minimum for exposed earth in an unvented crawl space. Full encapsulation contractors often use reinforced 12- to 20-mil liners where people will crawl over the surface or service equipment. Thickness improves durability, but an unsealed heavy liner still leaks at every lap and pier.
Compatible tapes, mastic, or manufacturer-approved sealants close seams and penetrations. Termination bars or other mechanical fasteners hold the liner at foundation walls. Access doors need weatherstripping, and old vents are blocked and sealed when the design is unvented.
Water and humidity need separate controls
The liner slows ground moisture. It cannot redirect a roof valley that dumps beside the foundation or remove water rising through a low crawl space after storms.
- Exterior grading, gutters, and downspout extensions to keep bulk water away.
- Interior perimeter drainage or a sump pump where groundwater or seepage requires it.
- A code-compliant dehumidifier, conditioned-air supply, or mechanical exhaust strategy.
- A condensate drain routed to an approved location instead of an open bucket.
- A humidity sensor placed where it can be checked after the work.
Insulation and safety details depend on the house
Closed crawl spaces are often insulated at the foundation walls with code-approved rigid foam, with rim joists air sealed where appropriate. Pest-inspection gaps, ignition barriers, flood-zone rules, combustion appliances, and radon mitigation can change the allowed materials and layout.
Ask the proposal to name liner thickness, product, seam method, wall attachment, drainage, insulation R-value, humidity-control capacity, and warranty. A phrase such as premium encapsulation is not a material specification.
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Product details, health guidance, and code references were checked against these sources.
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