Crawlspace Vapor Barrier: 6 Mil vs 20 Mil

If you’ve gotten quotes from multiple crawlspace contractors, you’ve probably noticed a wide spread in vapor barrier thickness: some quote 6-mil, others 20-mil. The price difference is real — but so is the durability difference. Here’s why we only install 20-mil reinforced barrier.

What ‘mil’ actually means

A ‘mil’ is one one-thousandth of an inch. So 6-mil is 0.006″ thick, and 20-mil is 0.020″. That sounds small in absolute terms, but the strength difference is exponential. 20-mil isn’t 3x stronger than 6-mil — it’s roughly 10x stronger due to thickness plus reinforcement layers.

Where 6-mil came from

6-mil polyethylene sheeting is what big-box hardware stores stock. It’s cheap (around a fair amount.10/sqft retail), readily available, and works fine for short-term applications like protecting concrete during a pour. It was never designed for permanent installation under a crawlspace.

Why 6-mil fails in crawlspaces

Three reasons: it punctures from foot traffic during install, it tears at seams within 12-36 months, and it lacks the reinforcement layer that resists UV from work lights. Most 6-mil installs we’ve seen show visible failure within 3-5 years — long before any warranty would pay out (most 6-mil warranties are 5 years and pro-rated).

What 20-mil reinforced barrier is

20-mil reinforced barrier (we use Stego Wrap or equivalent) is three layers: a 20-mil polyethylene exterior, a woven scrim reinforcement layer in the middle, and another 20-mil layer underneath. The scrim prevents punctures from spreading into tears. Manufacturer warranties run 25 years.

Cost difference on a typical job

On a 1,500 sq ft crawlspace, the material cost difference between 6-mil and 20-mil is roughly a fair quoted amount. Labor is identical — takes the same time to install either. So if a contractor is quoting 6-mil at a a fair quoted amount savings, you’re getting a barrier that needs to be re-installed in 5 years (another a fair quoted amount job). The ‘savings’ is illusory.

How to verify what you’re getting

Ask for the manufacturer name and product code on the quote — not just ‘vapor barrier.’ Stego Wrap is the gold standard; other reputable brands include Viper VaporCheck and GuardEnergy CrawlSeal. If a contractor refuses to specify a brand, that’s a red flag.

What about 12-mil or 16-mil?

Better than 6-mil but still not industry standard for permanent installation. 12-mil and 16-mil reinforced barriers exist as middle-ground products, but they don’t carry the 25-year warranty that 20-mil does. We only quote 20-mil because the cost savings on lesser barriers don’t pencil out over the life of the home.

The cheapest part of an encapsulation quote is the part you should pay closest attention to. A a fair quoted amount savings on a 6-mil vapor barrier becomes a a fair quoted amount cost in five years when it fails. Our encapsulation always uses 20-mil reinforced barrier with a 25-year manufacturer warranty.

Call (864) 362-9192 for a free inspection and a written quote that specifies the barrier brand and thickness.

Related Reading

• 7 Signs You Need Crawlspace Encapsulation in Greenville, SC
• 2026 Crawl Space Repair Cost Guide
• Crawlspace Encapsulation Service

Bottom Line

Vapor barrier thickness is the single most consequential material decision in a crawlspace encapsulation, and it’s the one homeowners are least equipped to evaluate. The honest answer: in the Upstate’s climate, 20-mil reinforced is the only thickness that delivers the lifespan and performance Google reviewers describe in their five-star write-ups. Six-mil is a short-term fix that becomes an expensive replace-and-redo job inside of five years. Don’t let a low quote tempt you into the cheaper product — the savings disappear at the first failure.

Questions to Ask the Contractor

Before you sign anything, take this list to the inspection visit:

1. Can you specify the vapor barrier brand and product code in the written quote?
2. Is the barrier reinforced or plain polyethylene?
3. What’s the manufacturer warranty on the barrier?
4. How long has this specific product been on the market?
5. Can I see install photos from a previous job using this exact barrier?
6. What’s the seam-sealing method and what tape do you use?

What Not to Do

Don’t accept a verbal description of the barrier (‘professional grade’ is not a product code). Don’t be swayed by a discount on a 6-mil install — you’re trading short-term savings for guaranteed mid-term failure. Don’t trust a quote that doesn’t specify the barrier brand by name; vague quotes are how contractors swap in cheaper material on install day.

Greenville-Specific Considerations

Upstate humidity is the variable that decides whether 6-mil is even an option in your home. In drier climates with lower foot traffic, 6-mil might last 10-15 years. In the Upstate, where vapor barriers are constantly stressed by humidity-driven contraction and expansion and where any service entry to the crawlspace risks foot traffic on the barrier, 6-mil starts failing at year 3. The cost math only works in our favor with 20-mil reinforced — and that’s why every contractor with long-term local references uses 20-mil.