Crawl Space Vapor Barrier Cost & Installation Guide

A vapor barrier is the foundation of any crawl space moisture control system. Whether you’re doing a basic installation or a full encapsulation, the barrier you choose and how it’s installed determines how well your crawl space stays dry for the next 15–25 years.

This guide covers the specific types, costs, and installation considerations so you can make an informed decision — and spot contractors who are cutting corners.

What Is a Vapor Barrier?

A vapor barrier (also called a moisture barrier) is a sheet of polyethylene plastic that blocks ground moisture from evaporating into your crawl space. In an unprotected crawl space, the soil beneath your home can release 10–15 gallons of water per day as vapor — moisture that saturates wood framing, feeds mold, and degrades insulation.

The barrier covers the crawl space floor and, in a full encapsulation, also covers the foundation walls. It’s sealed at seams, around piers, and at the foundation-wall junction to create a continuous moisture shield.

Vapor Barrier Types and Thickness

Not all vapor barriers are equal. The thickness (measured in mils, where 1 mil = 0.001 inch) directly affects durability, tear resistance, and longevity.

6-Mil Polyethylene

Cost: $0.05–$0.15 per square foot (material only)
Use case: Minimum code requirement in many jurisdictions
Durability: Low — tears easily, degrades in 3–5 years
Recommendation: Not suitable for encapsulation. Acceptable only as a temporary ground cover or where code mandates nothing more.

A 6-mil barrier is essentially construction-grade plastic sheeting. It serves as a minimal ground moisture retarder but provides no real encapsulation benefit. If a contractor proposes 6-mil for an “encapsulation” job, they’re either underqualified or underbidding.

12-Mil Reinforced

Cost: $0.25–$0.50 per square foot (material only)
Use case: Standard for residential encapsulation
Durability: Good — resists punctures, 15–20 year lifespan
Recommendation: The minimum acceptable thickness for any encapsulation project.

A 12-mil reinforced vapor barrier has a woven polyester cord reinforcement layer that prevents tears from spreading. This is what most reputable contractors use as their standard offering. It withstands normal crawl space traffic (inspections, HVAC maintenance) without damage.

20-Mil Reinforced

Cost: $0.50–$1.00 per square foot (material only)
Use case: Premium encapsulation, high-traffic areas, commercial
Durability: Excellent — highly puncture-resistant, 20–25+ year lifespan
Recommendation: Best long-term value for homeowners who want a set-it-and-forget-it solution.

A 20-mil barrier is significantly more robust. The material is thicker, heavier, and nearly impossible to tear during normal use. It’s particularly worth the upgrade if your crawl space has rough terrain, exposed aggregate, or frequent access for HVAC/plumbing maintenance.

Total Installation Costs

The full cost of a vapor barrier installation includes materials, labor, and any prep work. Here’s what to expect:

Basic Vapor Barrier (Floor Only)

Materials: $200–$800 (12-mil, 1,000 sq ft crawl space)
Labor: $800–$1,500
Total: $1,200–$2,500
What you get: Barrier on floor, taped seams, secured with pins

This option reduces ground moisture but doesn’t address wall moisture, humidity control, or vent sealing. It’s significantly less effective than full encapsulation but can be a reasonable first step if budget is tight.

Full Encapsulation (Floor + Walls + Sealing)

Materials: $800–$2,500 (12–20 mil, includes tape, fasteners, sealant)
Labor: $2,200–$4,500
Dehumidifier: $800–$2,000
Total: $3,000–$8,000 for a 1,000 sq ft crawl space

This is the complete solution. The barrier covers the floor and extends up the foundation walls, all vents are sealed, and a dehumidifier maintains target humidity. This is what contractors mean when they say “encapsulation.”

The national average cost is approximately $5,500 for a typical residential project, with significant variation by region. Homeowners in San Francisco might pay $7,000–$9,000 while those in Memphis could get the same work done for $3,500–$4,500.

Installation Process

Understanding the process helps you evaluate contractor proposals and spot shortcuts.

Step 1: Inspection and Preparation

The contractor inspects the crawl space for mold, structural damage, standing water, and pest issues. These must be addressed before barrier installation. Debris, old insulation, and any existing damaged vapor barrier are removed.

Step 2: Drainage (If Needed)

If the crawl space has water intrusion, a drainage system (interior French drain, sump pump) is installed before the barrier goes down. The barrier goes over the drainage system, not under it.

Step 3: Floor Installation

The vapor barrier is rolled out across the entire floor, overlapping seams by 6–12 inches. Seams are sealed with specialized seam tape rated for below-grade use. The barrier is secured to the soil with landscape pins or weighted to prevent shifting.

Step 4: Wall Installation

For full encapsulation, the barrier extends up the foundation walls to within 3 inches of the top (leaving a termite inspection strip as required by code in most states). It’s mechanically fastened to the wall with termination bars and sealed with polyurethane caulk.

Step 5: Penetration Sealing

Every pier, column, pipe, wire, and duct that penetrates the barrier must be individually sealed. This is where quality workmanship separates good contractors from bad ones. A single unsealed penetration can allow enough moisture through to compromise the entire system.

Step 6: Vent Sealing and Dehumidifier

Foundation vents are sealed with foam board and caulk. A properly sized dehumidifier is installed and connected to drain (either gravity drain or condensate pump). The dehumidifier maintains relative humidity between 45–55%.

DIY vs. Professional Installation

A basic floor-only vapor barrier is one of the few crawl space projects that’s reasonable for DIY. Full encapsulation, however, is best left to professionals.

DIY-friendly (floor-only barrier):

Material cost: $200–$600
Time: 4–8 hours for one person
Skill level: Moderate — working in confined spaces is physically demanding
Savings: $800–$1,500 in labor

Professional recommended (full encapsulation):

Wall installation requires proper fastening to masonry
Vent sealing affects home ventilation and must be done correctly
Dehumidifier sizing requires load calculations
Drainage work requires plumbing knowledge
Mistakes trap moisture and make problems worse

How Long Does a Vapor Barrier Last?

Lifespan depends primarily on material quality and installation:

Material	Expected Lifespan	Key Risk
6-mil basic	3–5 years	Tears, UV degradation
12-mil reinforced	15–20 years	Seam failure, punctures
20-mil reinforced	20–25+ years	Minimal (most durable)

The barrier itself doesn’t “wear out” — it’s protected from UV light and weather underground. Failures are typically from physical damage (someone crawling through and tearing it), seam tape failure (cheap tape dries out), or installation errors (unsealed penetrations, inadequate overlap).

Annual visual inspections catch small problems before they become big ones. A 5-minute check each year can extend your system’s life significantly.

What to Ask Your Contractor

Before signing a contract for vapor barrier work, ask these specific questions:

What mil thickness are you using? Accept nothing less than 12-mil for encapsulation.
What brand of vapor barrier? Reputable brands include Stego, Americover, and Crawl Space Ninja. Generic “contractor grade” is a red flag.
How are you sealing seams? Proper seam tape rated for below-grade moisture is essential. Duct tape is not acceptable.
What’s the warranty? Material warranties should be 15–25 years. Workmanship warranties vary but should be at least 5 years.
Are you including a dehumidifier? A vapor barrier without humidity control is incomplete in all but the driest climates.

Ready to get started? Get free quotes from licensed contractors who can assess your crawl space and recommend the right vapor barrier system for your home and budget.