What Is the Best Vapor Barrier for Crawl Spaces?

Durable, Class 1 vapor barriers are the best vapor barrier for crawl space. Builders typically install the vapor barrier over the exposed soil in crawl spaces to stop vapor diffusion and control moisture intrusion and condensation.

Vapor diffusion occurs when moisture moves from a higher to a lower moisture concentration space or from a warmer to a colder place within a building product, like insulation. In contrast, vapor retarders only hinder vapor diffusion.

The ASTM E96 desiccant method measures a building product's resistance to moisture infiltration and assigns products a vapor barrier or retarder class:

  • Class I vapor barrier (0.1 perms or less)

  • Class II vapor retarder (0.1 < perm <1.0 perm)

  • Class III vapor retarder (1.0 < perm <10 perm)

Managing the ever-present water in a crawl space will help you avoid foundation failure and costly repairs due to rot, and structurally damaging, and unhealthy mold.

Code Requirements for Vapor Barriers in Crawl Spaces

The 2021 International Residential Code (R408.3) mandates covering exposed earth with a continuous Class 1 vapor barrier for unvented under-floor space. In addition, you must overlap the vapor barrier joints by 6 inches (152 mm) and then seal and tape. Extend the vapor barrier’s edges at least 6 inches (152 mm) up the stem wall, then attach and seal to the wall or insulation.

Note: All buildings prone to direct contact with water need a vapor-resistant barrier. Polyguard® suggests you contact your building inspection office for local guidance on building codes and vapor barriers for crawl spaces.

What is the Best Vapor Barrier for a Crawl Space?

The best way to stop the water and toxic gases in the ground from entering the crawl space and ultimately damaging your home or building involves installing a puncture-resistant, low-permeance Class 1 polyethylene vapor barrier with a thin poured concrete slab over it. Sufficiently preventing water infiltration into the crawl space requires completely encapsulating the crawl space, including the floor, support posts, and walls, with the best vapor barrier system.

Polyguard® TERM® Underslab Water|Termite Barrier

Fully-adhered TERM® Underslab Water I Termite Barrier offers the industry's best and thickest vapor barrier for crawl spaces. It creates a durable physical barrier against water, termites, and radon.

The high-strength TERM® Underslab Water|Termite Barrier can withstand abusive job sites. It consists of an 8.5 mils polyethylene backing laminated to a 69-mil thick TERM® barrier sealant integrated into a high strength non-woven geotextile fabric top layer for a total factory-controlled thickness of 95 mils.

The fabric has a 4-inch wide lap waterproofing adhesive covered with a removable silicone-coated release sheet that creates a 4-inch wide self-adhesive overlap seam.

Benefits and features of the TERM® Underslab Water|Termite Barrier include the following:

  • Can withstand severe abuse during slab placement with exceedingly high puncture resistance of 224 pounds

  • Puncture resistance comes from the top layer of highway-grade geotextile and the bottom layer of high-strength polyethylene film

  • A 69-mil barrier sealant in the middle of two puncture-resistant layers, gives the barrier four vital properties:
    • Termite resistance

    • Water resistance

    • Ability to absorb stress which can cause cracking

    • A self-healing sealant that will close up small punctures

  • Able to handle hydrostatic head pressure up to 231 feet

  • The only physical termite barrier with over 20 years of testing and an evaluation report by the ICC (International Code Council) AC 380 ESR 3632

  • Can withstand 30 days of UV exposure

How to Install a TERM® Underslab Water|Termite Barrier

Click here to watch a TERM® Underslab Water|Termite Barrier installation video.

Tips for successfully applying TERM® Underslab Water|Termite Barrier include:

  • Apply to dry base.

  • Apply TERM® Underslab Water|Termite Barrier in temperatures above 30°F (-1°C).

  • Applying 650 LT Liquid Adhesive can help with the barrier's initial adhesion.

  • Compact the sub-base according to project specifications

  • Remove all concrete chunks, sticks, roots, etc.

TERM® Waterproofing with Non-Chemical Termite Barrier system includes the following Polyguard® products:

Polyguard® 650 Mastic

Asphalt rubber Polyguard® 650 Mastic provides excellent adhesion to Polyguard® Waterproofing Membranes, structural concrete, masonry, and wood surfaces. Its design protects and seals detailing areas like termination edges, patches, and overlaps.

XCalif Sealant Orange

XCalif Sealant seals the porous portions of the geotextile surface and promotes adhesion of Polyguard® sealants like LM-85 SSL, LM-95, Detail Sealant PW™, and Underseal® Fabric Tape.

Underseal® Inside Corner Boot

The 60-mil Underseal® Inside Corner Boot contains rubberized asphalt bonded to polyethylene with a simple-to-use release liner. Underseal® Inside Corner Boots reinforce and seal the inside corners of the Underseal® Underslab membrane.

Underseal® Outside Corner Boot

The 60-mil Underseal® Outside Corner Boot contains rubberized asphalt bonded to polyethylene with a simple-to-use release liner. Underseal® Outside Corner Boots reinforce and seal the outside corners of the Underseal® Underslab membrane.

Underseal® Fabric Tape

Rubberized asphalt Underseal® Fabric Tape laminates to a polypropylene fabric. It seals penetrations and seams and is vital when used with the Underseal® Underslab and Underseal® Blindside Membrane waterproofing systems.

TERM® Termite Sealant

TERM® Termite Sealant blocks slab penetrations from water, termites, and radon. It also seals overlaps and gaps. Polyguard® formulated the TERM® Termite Sealant from polymer asphalt, and then upgraded it to incorporate a non-chemical termite and insect barrier.

The Best Vapor Barrier for Crawl Space - TERM® Underslab Water I Termite Barrier

Puncture-resistance, low-permeance, Class 1, fully-adhered TERM® Underslab Water I Termite Barrier and companion products are the best way to stop water, termites, and radon in the ground from entering the crawl space. Installing TERM® Underslab Water I Termite Barrier in a crawl space will protect your foundation, save money on costly repairs, and protect the health and integrity of your building or home.

For more on choosing the best vapor barriers for crawl spaces, don't hesitate to contact our team of Polyguard® professionals.