Throughout our 6 years inspecting properties in the Springfield Missouri area, we have found many homes where we have recommended a moisture barrier be installed in the crawlspace. However, many people don’t realize what a moisture barrier is, what function it performs, or when to use one effectively. These are excellent questions and the answers may not be exactly what you think. We thought you might like to hear an opinion of these issues from a professional who is in the business.

Sections on this page

What is a moisture barrier?
What does a moisture barrier do?
When do I need a moisture barrier?
Photos of a crawlspace that need to have a moisture barrier installed.

What is a moisture barrier?

A moisture barrier (also called a vapor barrier) is a sheet of plastic that is laid over the floor in a crawlspace. Basically, that is all a moisture barrier is, but there is more to the story. First of all, a moisture barrier must cover 100% of the floor surface leaving no gaps exposed. Secondly the plastic is lain in sections, so each section must overlap by roughly a foot. Thirdly, the plastic should be at least 6 mils thick to be durable enough to last over time (sorry, no thin painters plastic).

You can use clear or black, but we prefer clear as this allows visibility under the barrier so you can see how it is functioning, termites, or additional water problems. There is a normal assumption that the sections must be fastened together, or held down with gravel. This is not necessary as the crawlspace generally does not have foot traffic or wind that might disturb the barrier. It is perfectly acceptable to simply lay it in place on the crawlspace floor. There is an exception to this rule for moisture barriers that are also installed for radon mitigation, which is a topic for another article.

Moisture barriers are sheets of 6 mil thick plastic laid on 100% of the crawlspace floor

What does a moisture barrier do?

This is an excellent question deserving consideration as this is the most common question I get as a home inspector. A moisture barrier reduces the relative humidity in the crawlspace, which reduces the potential for organic growths like molds and rot. It doesn’t sound logical that the simple addition of a piece of plastic could prevent mold and rot, but it works!

Here’s how it functions, water vapor from the ground disperses into the air filling the crawlspace which increases its relative humidity. Once the humidity reaches 60% for a sustained period of time, mold can grow by pulling water right out of the air. Many rots can sustain growth with even less humidity. Additionally when there is a temperature differential, the water in the air will condense to form a water droplet when it contacts any surface. This could be the concrete foundation wall, piers or worse – the wooden floor joists. Once the wood becomes wet with surface water, organic growth is ready to start.

The point is, you want to reduce it to around 30% to 40% for the general health of the property. The moisture barrier’s true function is simply to act as a surface for the water vapor to condense onto instead of the wooden members of the structure. By placing the moisture barrier on the ground, water vapor emanates from the ground, condenses onto the underside of the moisture barrier, and drops back into the soil without harming anything. This is very effective at reducing the relative humidity of a crawlspace to levels that do not promote organic growths.

Moisture barriers reduce the relative humidity of crawlspace air by providing a condensation surface close to the ground.

When do I need a moisture barrier?

Our opinion is generally that every crawlspace should have a moisture barrier. There is no crawlspace that could not benefit from this relatively inexpensive feature. However, we highly recommend these barriers to be installed in properties that have signs of past chronic water intrusion, have an organic growth beginning to form on a majority of the wooden structural members, or on properties that exhibit water related problems during the home inspection.

It is important to note that a moisture barrier is completely ineffective at fixing standing water problems in a crawlspace. This is only effective against vapor intrusion from the ground due to wet soils. However, a moisture barrier can be combined with a sump pump installation to both remove standing water and reduce humidity. Read more about that solution in our next article.

Every crawlspace should have a moisture barrier to protect its long term health.

Photos of a crawlspace that needs to have a moisture barrier installed.