How to Install a Vapor Barrier in a Crawlspace

Many homes and additions are built over a crawl space to save time and money. Unlike traditional basements, crawl spaces are often only a few feet tall, built above grade, and many are unfinished with dirt floors. In these settings, moisture must be managed a little differently.

Vapor barriers are a crucial part of any moisture management system in a crawl space and are not difficult to install. If you’re considering adding a vapor barrier to your crawl space, read on to learn tips and advice from industry pros Michael Church from Crawl Space Ninja and Ted Towne from T3 Inspections.

Problems Caused by Moisture in Your Crawl Space

While moisture in a crawl space is common, managing it is possible (and necessary). Moisture left unchecked can lead to a host of issues. “Excess moisture will rust mechanical [parts] in the area or deteriorate the wood itself,” Towne says. High-moisture areas also attract wood-eating insects like termites and carpenter ants.”

Chronic dampness can also impact a home’s air quality. “Moisture affects more than the crawl space,” Church says. “It can also enter the living space and create high dust mite populations,” a leading cause of childhood asthma. Moisture can cause mold and mildew to grow out of sight under the home, which then circulates through the living space.

What Causes Moisture to Build Up in a Crawl Space?

Moisture in a crawl space comes from several sources. Like a traditional basement, dampness from the ground seeps into the space. “Moisture constantly comes up from the ground and is always present in the air,” Towne says. This issue is intensified in a crawl space with a dirt floor.

Surface water can also infiltrate the crawl space from the outside through improper grading, overflowing gutters or air vents. While ventilation openings are designed to flush out moisture from crawlspaces, they often don’t do the job, except in dry climates. In fact, during humid summer conditions, they often make matters worse.

Plumbing leaks are another common source of water in a crawl space. As it evaporates, moisture is released into the air and settles as condensate on surfaces.

If unmanaged, moisture has nowhere to go and builds up in the crawl space over time, causing the abovementioned problems.

How To Prevent Moisture in a Crawl Space

A good first step to managing moisture in your crawl space is to start on the outside. Clean gutters and add downspout extenders. Make sure your property is properly graded. Sloping the soil away from your home at about 1/2 inch per foot is usually the best way to reduce crawlspace moisture. Cover any outside air vents or skip them in new construction.

Inside a crawl space, cover exposed foundation walls with 1-1/2 inches of rigid, moisture-proof insulation. Cover the floor and foundation walls with a plastic vapor barrier, and use a dehumidifier to remove remaining moisture from the air.

All of these steps work together to control moisture levels.

Why You Should Have a Vapor Barrier in Your Crawl Space

Vapor barriers repel moisture infiltration from the ground into the crawl space. “It does not stop it, but slows it down so that an active humidity control device, like a dehumidifier, can remove the moisture from the air,” Church says. A dehumidifier alone is not enough.

What is a vapor barrier?

A vapor barrier consists of plastic sheeting installed on the floors and foundation walls in a crawl space. Typically 10 to 20 millimeters thick, vapor barriers are installed in slightly overlapping sheets across the floor and partially up foundation walls.

How To Install a Vapor Barrier in a Crawl Space

Cover the crawlspace floor with a single layer of plastic moisture polyethylene vapor barrier. Overlap any seams and tape them with waterproof seam tape. “Make sure you overlap the plastic at least 12 inches,” Church says.

Bring the plastic at least 6 inches up the wall and fasten it there with rubberized butyl tape, not seam tape.

“Stake the plastic down with landscape fabric stakes to keep it in place,” Church says. Chances are that someone will have to slide around in the crawlspace later to fix a pipe or run a new cable. A few holes won’t make any difference in performance, and they’ll drain puddles if heavy rain or a leaky pipe leaves water on the plastic.

FAQs

What kind of vapor barrier is best to use?

“A puncture-resistant, single-layer vapor barrier is the best type,” Church says. This is a Class 1 vapor barrier, which is the most impermeable and offers the best moisture protection.

Can mold grow under the vapor barrier?

“Molds are present everywhere,” Towne says, even under vapor barriers. However, a vapor barrier in a crawl space prevents mold or mildew from circulating into the living space air. “That is why we use a plastic vapor barrier to keep everything out of the space above,” Towne adds.

How do you dry out your crawl space?

Use a shop vac or pump to remove excess water, then use fans or a dehumidifier to dry out the crawl space before adding a vapor barrier and other moisture-mitigating systems.

How long does it take to dry out a crawl space?

Depending on the amount of moisture in the space and the air humidity, a crawl space can take 10 hours to several days to dry out.

About the Experts

• Michael Church founded Crawl Space Ninja, a crawl space encapsulation company, in 2005. He is also host of the popular YouTube Channel, Crawl Space Ninja, offering how-to tutorials and advice for homeowners. Mr. Church is the author of Crawl Space Repair Myths: Busted! Avoid Bad Advice, Bad Decisions, and Bad Repairs, released in 2023.
• Ted Towne owns T3 Inspections, a full-service home inspection company in Easthampton. Massachusetts. He is a licensed and insured home inspector with 10 years of experience.