Are Crawl Space Vapor Barriers Necessary?

If you're trying to revamp your crawl space, you probably have many questions that need answering. What type of insulation do you need? Is there a way to make the crawl space less musty? Does the crawl space need a vapor barrier, or does that depend on how you approach the insulation?

South Central Services has insulated hundreds of homes with spray foam. In southern PA towns like Gettysburg, there are a lot of houses with musty crawl spaces. The short answer is that your crawl space does need a vapor barrier, but it is crucial to understand why.

By the end of this article, you will know:

• Why crawl spaces need vapor barriers
• What products can create a vapor barrier
• How to use these vapor barrier products in your crawl space

Don't have time to read right now? Check out everything you need to know at a glance.

Crawl Spaces Are Damp Environments.

If you've ever gone hiking or on a nature walk, you may have encountered a log you rolled over. No matter how intensely the sun shone that day, the bottom of that log was damp. The top of the log may have been dry, but the bottom was in direct contact with the dirt and never saw the sun.

Your crawl space is similar to the bottom of a log. It doesn't see the light of day and is surrounded by dirt. The ground is naturally cold and wet, so your crawl space is also damp.

Your crawl space, with a dirt floor and porous masonry walls, lets in moisture from the surrounding ground. Without external intervention, it cannot be dry or temperature-controlled.

This is where vapor barriers come in. A vapor barrier can stop moisture from infiltrating your crawl space, bringing you one step closer to a space that is dry.

What Happens In Crawl Spaces Without Vapor Barriers?

But do crawl spaces need to be dry? It's not like you have to live in your crawl space. Why do vapor barriers matter? Let's talk about what happens in crawl spaces with unchecked moisture.

You're likely researching these improvement options because your crawl space smells musty. That odor comes from the excessive moisture in the crawl space.

Musty odors can also indicate mildew and mold growth. With so much moisture, the crawl space is an ideal environment for mold to grow.

In severe cases, excessive moisture can lead to wood rot and structural damage. Since your crawl space sits below some portion of your house, damage to those floor joists can translate to damages in your living space.

When we work with homeowners to fix their crawl spaces, we often find the crawl space looks like this.

Batt insulation, once installed in the crawl space ceiling, has begun to drip and fall from the ceiling. Fiberglass batts are a popular choice for crawl spaces despite their inability to handle excessive moisture.

Crawl spaces without vapor barriers are dirty, damp, and musty. Unchecked moisture can lead to mold and structural damage.

Vapor Barriers Are Necessary For Crawl Spaces.

If you want a dry crawl space, vapor barriers are necessary in the insulation process. Several products can be used to create vapor barriers. You'll likely choose one of the following depending on how you insulate your crawl space.

1. Plastic sheathing
2. Foam board
3. Closed cell spray foam

Let's examine each of these potential vapor barriers in more detail.

1. Some Plastic Sheathing Can Act As A Vapor Barrier.

Not all plastic sheathing can create a vapor barrier. Your crawl space may have a plastic tarp laid on the floor and still face moisture infiltration from the ground. You must choose the right plastic sheathing if you want a vapor barrier for your floor.

When we encapsulate a crawl space, we use plastic sheathing that is at least 12 mil thick. For some crawl spaces, we opt for 20 mil plastic to boost its durability in case of foot traffic. Any plastic thinner than 12 mil cannot reliably act as a vapor barrier long term.

Think of your plastic sheathing like a swimming pool liner. The job of a pool liner is to keep the water in the pool. If the pool liner is too thin, it cannot hold water. The vapor barrier in your crawl space is the reverse. Plastic sheathing cannot stop water vapor if it is too thin.

Even if you install 12 mil plastic on your crawl space floor, it cannot act as an actual vapor barrier until it is sealed. Like a tear in your swimming pool liner will leak water, any tears or gaps in your plastic sheathing will leak vapor.

Plastic sheathing on your crawl space floor can be an excellent vapor barrier. Be sure to seal the edges of the plastic sheathing with crawl space tape, and only use 12 mil thick or thicker plastics.

2. Foam Board Insulation Can Create A Vapor Barrier.

Foam board insulation is generally installed on the walls of a crawl space. A foam board can create an effective vapor barrier in the crawl space if certain conditions are met.

Foam board is best used in crawl spaces with flat masonry walls. Crawl spaces with stone walls tend to have more texture, which is unsuitable for foam boards. Foam board should be installed flush against the crawl space walls.

Just like with plastic sheathing, the edges of foam board have to be sealed. While home improvement stores like Lowe's and Home Depot sell large pieces of foam board, you will inevitably have to cut pieces to size. There will be seams where one piece of foam board meets another.

No matter how tightly you press those seams together, vapor can pass through. It is a crack. To make foam board seamless, you must spray can foam into those seams to create an airtight seal.

Foam board isn't a vapor barrier option for every crawl space, but it can work well for crawl spaces with flat masonry walls.

3. Closed Cell Spray Foam Can Create A Seamless Vapor Barrier.

For crawl spaces where foam board cannot do the job, closed cell spray foam is the option you need. Even for flat masonry walls, closed cell can still be installed.

Of the two types of spray foam, closed cell is the one that can create both an airtight seal and a vapor barrier. At approximately 2 inches depth, closed cell offers both. Less than 2 inches of closed cell cannot create a vapor barrier, so the amount of foam installed is important.

There are two significant differences between foam board and closed cell spray foam: seamlessness and ease of application.

Because spray foam is spray-applied, there is no need to seal joints or cracks in the insulation. Closed cell creates a seamless vapor barrier from start to finish.

Additionally, this spray-applied nature gives closed cell spray foam more flexibility when insulating hard-to-reach areas. Foam board is flat and cannot contour to nooks and crannies. Spray foam is unphased by strange shapes, protrusions, and small spaces.

Closed cell spray foam creates a vapor barrier at approximately 2 inches. It offers a seamless vapor barrier that works for any crawl space.

How Can I Use A Vapor Barrier In My Crawl Space?

Now that you know what products can create vapor barriers, you need to know how to use them. Our recommendation for crawl spaces is almost always encapsulation. While there are other crawl space methods, they do not address the excess moisture and the problems unchecked vapor creates.

What Do Vapor Barriers Look Like With Encapsulation?

Crawl space encapsulation can be completed by professionals or as a DIY. Most qualified professional contractors will install plastic sheathing as a vapor barrier for the floor and closed cell spray foam as a vapor barrier for the walls.

For homeowners who attempt encapsulation on their own, the products of choice are plastic sheathing for the floor and foam board for the walls. We do not recommend using a DIY spray foam kit for your crawl space. DIY kits are difficult to use, offer mixed results, and do not save money on the cost of spray foam.

Encapsulation seals off the dirt floor and porous masonry walls, preventing vapor transfer from the ground. Once the crawl space is dry, you can condition the air inside like you do in the rest of your living space.

If You Don't Recommend Encapsulation, What Do You Recommend?

In the rare instance that we don't recommend the encapsulation method for a crawl space, it is because there are extenuating circumstances in that crawl space.

An example would be standing water in the crawl space, when the source of water cannot be remedied. A vapor barrier is concerned with stopping the water content in the air, not with stopping liquid water.

We recommend sealing the crawl space ceiling with closed cell spray foam in these situations. This method cuts off the crawl space from the home.

This method is not ideal because it leaves the crawl space with excessive moisture. While encapsulation leaves a crawl space clean and dry, ceiling insulation just prevents musty crawl space air from exchanging with the conditioned air in your home.

The Bottom Line About Vapor Barriers In Your Crawl Space

Installing vapor barriers in your crawl space is in your best interest. Crawl spaces are below the ground and deal with a lot of vapor transfer. Even if the vapor in your crawl space does not lead to mold or structural damage, vapor infiltration causes unpleasant odors.

If you install vapor barriers, you can also encapsulate your crawl space. Encapsulation combines vapor barriers with insulation to make a crawl space clean, dry, and accessible.

Now that you understand why crawl spaces need vapor barriers, your next step is to:

• Discover if DIY crawl space encapsulation is right for you
• Estimate the cost of professional encapsulation for your crawl space
• Compare encapsulation to another crawl space method