Crawlspace Vapor Barrier: What to Know Before Installing

Written By Adam Paul - Waterproofing Expert

A crawlspace vapor barrier is a plastic or polyethylene sheeting system designed to stop moisture from seeping into the subfloor area from the ground. Crawlspace vapor barriers are available in multiple standard widths, lengths, and thicknesses. The main purpose of installing the crawlspace vapor barrier is to prevent the water vapor transmission through the soil to the crawlspace.

Crawlspace vapor barrier can control the moisture, prevent the mold and mildew growth, increase the energy efficiency, protect the crawlspace structure, and improve the indoor air quality in the home.

You should install a vapor barrier in the crawlspace when you see condensation, musty odor, standing water, mold and mildew growth, wood rotting, wood warping, and pest infestation in the crawlspace. When you choose a crawlspace vapor barrier, you should consider size, thickness, structure design, ASTM standard, color, and water resistance of the material.

As waterproofing experts, Aqviz follows the steps below when installing a vapor barrier in the crawlspace. If you do not install it properly, it may cause to enter the vapor into the crawlspace and lead to many water problems and damages over time. So we highly recommend you install a vapor barrier in the crawlspace and maintain and conduct relevant repairs when needed.

What is a Crawlspace Vapor Barrier?

A crawlspace vapor barrier is a plastic or polyethylene sheeting system designed to stop moisture from seeping into the subfloor area from the ground. The primary function of the crawlspace vapor barrier is to prevent water vapor transmission through the soil, which often leads to mold growth, wood rot, insulation damage, and musty odors in the crawlspace. Crawlspace vapor barriers are available in multiple standard widths (3 ft, 6 ft, 10 ft, 12 ft) and lengths (50 ft, 100 ft, 200 ft). Its thickness ranges from 6 mil to 20 mil.

At Aqviz, we install crawlspace vapor barrier systems across a variety of residential and commercial properties, especially where the basement-like void is prone to dampness. In homes with high humidity levels, the crawlspace vapor barrier acts as the first line of defense, maintaining a dry, healthy underfloor cavity space. We’ve seen how effective a properly installed crawlspace moisture barrier is in preventing long-term damage to HVAC systems, plumbing, and floor framing located in the plenum space.

What are the Benefits of Installing a Crawlspace Vapor Barrier?

There are 5 critical benefits of installing a crawlspace vapor barrier that directly protect the structure, air quality, and energy efficiency of any home or building.

Moisture control: A crawlspace vapor barrier stops ground moisture from evaporating into the underfloor space, reducing condensation, humidity, and water damage in the crawlspace.
Mold and mildew prevention: Installing a crawlspace moisture barrier helps prevent biological growth on insulation, wood joists, and pipes, which commonly occurs in humid crawlspace cavities.
Energy efficiency: A sealed crawlspace vapor barrier reduces HVAC system load, leading to lower heating and cooling costs, often saving up to 15% annually.
Structural protection: A crawlspace vapor barrier limits water-related wood rot, rust, and corrosion, preserving the long-term integrity of subfloor supports, ductwork, and foundation walls.
Improved indoor air quality: A properly sealed crawlspace moisture barrier stops the upward flow of contaminated air, reducing allergens, odors, and airborne spores entering the living space.

What are the Signs you need a Vapor Barrier in the Crawlspace?

There are 6 common signs that indicate the urgent need for a crawlspace vapor barrier to prevent escalating moisture-related damage and health risks.

Visible condensation: Water droplets forming on pipes, insulation, or subfloor joists in the crawlspace require a vapor barrier in the crawlspace.
Musty odors: Persistent damp or moldy smells rising from the subfloor area indicate uncontrolled humidity in the crawlspace.
Standing water or wet soil: Pooled water or saturated ground in the crawlspace void is a major warning sign that requires a crawlspace moisture barrier.
Mold or mildew growth: Black or green patches on wooden beams, insulation, or ductwork typically point to the lack of a protective crawlspace vapor barrier.
Warping or cupping floors: Wood flooring above the crawlspace may deform due to excessive moisture, and it requires a crawlspace moisture barrier.
Pest infestation: Termites, carpenter ants, and rodents thrive in damp environments in the crawlspace requires a vapor barrier for the crawlspace.

What are the Specifications of a Crawlspace Vapor Barrier?

Crawlspace vapor barrier specifications vary depending on the moisture severity, building type, and installation method. At Aqviz, we always assess site-specific conditions before recommending the proper material dimensions and characteristics.

Size: Crawlspace vapor barrier rolls are available in multiple standard widths (3 ft, 6 ft, 10 ft, 12 ft) and lengths (50 ft, 100 ft, 200 ft). We typically use 12 ft × 100 ft rolls for large subfloor areas to reduce seams and speed up installation.

Thickness: Crawlspace vapor barrier’s thickness ranges from 6 mil to 20 mil. For vented crawlspace voids, we often recommend 10 mil for basic moisture control. In high-moisture plenum spaces or where durability is critical, we use 15 mil or 20 mil reinforced polyethylene vapor barriers.

Structure design: Most crawlspace moisture barrier products have multi-layer construction with woven fiber reinforcement sandwiched between polyethylene layers. This increases tear resistance and puncture strength, especially for rough underfloor surfaces.

Color: White and black are the two most common crawlsapce vapor barrier colors. We use white crawlspace vapor barriers in dark cavities to reflect light and improve visibility. Black barriers are preferred for UV resistance and thermal absorption in semi-exposed areas.

Water resistant: Every crawlspace vapor barrier we install must meet ASTM E1745 Class A standards for water vapor permeability, offering perm ratings below 0.1 perms. This ensures long-term resistance to water transmission even under hydrostatic pressure.

What You Should Consider When Selecting a Crawlspace Vapor Barrier?

Selecting the right crawlspace vapor barrier depends on understanding both the moisture conditions of the crawlspace void and the technical specs of the barrier. At Aqviz, we evaluate factors like water intrusion level, soil type, seasonal humidity, and mechanical activity within the subfloor area before making any material recommendation.

Choose a crawlspace moisture barrier with a minimum thickness of 10 mil for moderate moisture levels, and at least 15-20 mil reinforced polyethylene if standing water, pests, or high foot traffic is present.

Always verify that the vapor barrier meets ASTM E1745 Class A for water resistance and has a perm rating under 0.1 perms.

For darker cavities, we prefer white barriers to reflect light; black is better for exposed or sunlit sections. Also, consider using reinforced multi-layer designs for rough or rocky cavity spaces to avoid tearing during installation or future maintenance.

How to Install a Crawlspace Vapor Barrier?

At Aqviz, we follow a proven step-by-step method to install a crawlspace vapor barrier that ensures long-lasting moisture control and structural protection. Based on our 15+ years of experience, this is how we professionally install a vapor barrier in any crawlspace cavity:

Step 1: Inspect and clean the crawlspace
Remove all debris, sharp objects, old insulation, and standing water in the crawlspace. Clear access to foundation walls and smooth out the soil in the subfloor area for even barrier placement.

Step 2: Address drainage and plumbing issues
Fix any leaking pipes or clogged drainage in the crawlspace before placing the crawlspace moisture barrier. Install a sump pump or perimeter drain system if groundwater intrusion is active.

Step 3: Lay out the vapor barrier on the ground
Use a 10 mil to 20 mil polyethylene barrier, unroll it across the entire crawlspace void, overlapping seams by at least 12 inches (30.48 cm) and extending 6-12 inches (15-30 cm) up the foundation wall.

Step 4: Seal seams and edges
Secure all overlapping sections with waterproof tape or double-sided butyl tape. Use mechanical fasteners or adhesive to attach the crawlspace vapor barrier to the wall and prevent gaps or curling.

Step 5: Seal penetrations and columns
Cut and fit the vapor barrier around piers, footings, and mechanical components. Use spray foam or mastic sealant to close off air gaps around those intrusions.

Step 6: Final inspection and documentation
Check all seams, wall attachments, and overlaps for proper sealing in the crawlspace. At Aqviz, we document each crawlspace moisture barrier project with photos and humidity readings to ensure the system meets the expected performance standard.

What if the Crawlspace Vapor Barrier is Damaged?

A damaged crawlspace vapor barrier compromises the entire moisture protection system, allowing water vapor to rise from the soil into the underfloor space.

The most common problems caused by a damaged crawlspace vapor barrier include elevated humidity, which promotes mold growth on insulation, floor joists, and ductwork. More than that damaged crawlspace vapor barrier can cause wood rotting, warping, and metal corrosion in the crawlspace.

We’ve also found that damaged vapor barriers attract pests like termites and rodents seeking moisture-rich environments in the crawlspace.

Over time, damaged crawlspace vapor barriers cause indoor air quality to deteriorate, increase energy costs, and reduce the lifespan of the structural framing above the crawlspace cavity. Immediate repair or full replacement is critical to restore moisture control and prevent costly repairs.

Is Vapor Barrier Installed During Crawlspace Waterproofing?

Yes, installing a crawlspace vapor barrier is a fundamental part of crawlspace waterproofing. At Aqviz, we always include a vapor barrier to block ground moisture as the first layer of defense in our crawlspace moisture control systems.

While the vapor barrier doesn’t stop liquid water like drainage or sump systems, it prevents water vapor transmission, which is essential for protecting the subfloor area, reducing humidity, and preventing mold, rot, and air quality issues.

Should you use a Dehumidifier in a Vapor Barrier Installed Crawlspace?

Yes, using a dehumidifier in a vapor barrier-installed crawlspace is often necessary, especially in high-humidity climates or poorly ventilated crawlspace cavities. At Aqviz, we recommend pairing a sealed crawlspace moisture barrier with a properly sized crawlspace dehumidifier to maintain optimal relative humidity between 45% and 55%.