What Is a Crawl Space Vapor Barrier?
A vapor barrier is a heavy-duty polyethylene sheet installed on the floor (and often walls) of your crawl space to block ground moisture from entering. In North Carolina, where soil moisture and humidity are high year-round, a vapor barrier is the first line of defense against wood rot, mold, pests, and poor indoor air quality.
Why NC Homes Need Vapor Barriers
North Carolina sits in a humid subtropical climate zone. The ground beneath your home constantly releases moisture vapor — up to 12 gallons per day in a typical crawl space. Without a barrier, that moisture:
- Rots floor joists and subfloor from below
- Creates ideal conditions for mold and mildew
- Attracts termites, carpenter ants, and other pests
- Pushes indoor humidity above comfortable levels
- Forces your HVAC to work harder, increasing energy costs
- Causes musty odors throughout your living space
Types of Vapor Barriers
6-mil Polyethylene (Minimum Code)
NC building code requires a minimum 6-mil vapor barrier in crawl spaces. This thin, basic plastic sheeting meets code but tears easily, doesn't last long, and provides minimal protection. We don't recommend it — it's like putting a band-aid on a broken pipe.
12-mil Reinforced Poly
A step up — reinforced with string or fiber for better durability. More resistant to tears and punctures. Adequate for some applications but still not our first recommendation for full encapsulation.
20-mil Encapsulation Liner (Our Recommendation)
The professional standard for crawl space encapsulation. A 20-mil reinforced, antimicrobial liner covers the entire crawl space floor and walls, creating a sealed environment. This is what Sterling Blue installs on most projects. It's puncture-resistant, lasts 20+ years, and makes a dramatic difference in moisture control.
Vapor Barrier vs. Full Encapsulation
Vapor barrier alone: A sheet on the floor of a vented crawl space. Blocks ground moisture but doesn't control humidity from air exchange through open vents. Cost: $1,000–$3,000.
Full encapsulation: Vapor barrier on floors AND walls, sealed vents, sealed rim joist, and typically a dehumidifier. Creates a completely controlled environment. Cost: $5,000–$15,000. Learn more about our encapsulation service →
Our recommendation: For NC homes, full encapsulation is almost always the better investment. A vapor barrier on the floor of a vented crawl space only solves half the moisture problem.
Installation Best Practices
- Overlap seams by 12+ inches and seal with waterproof tape
- Run the barrier up foundation walls at least 6 inches and secure mechanically
- Seal around all penetrations — piers, pipes, supports
- Ensure proper drainage before installing — the barrier should never sit on standing water
- Add a dehumidifier in sealed/encapsulated crawl spaces to maintain 50–55% humidity
Cost of Crawl Space Vapor Barriers in Raleigh
- Basic 6-mil barrier (DIY): $0.50–$1/sq ft
- 12-mil reinforced (installed): $1–$2/sq ft
- 20-mil encapsulation liner (installed): $2–$4/sq ft
- Average 1,000 sq ft crawl space: $2,000–$4,000 for liner installation
DIY vs. Professional Installation
While you can lay a basic vapor barrier yourself, professional installation ensures proper sealing, overlap, and integration with your overall moisture control strategy. Improper installation can trap water and make problems worse. For encapsulation, professional installation is strongly recommended.