A homeowner laying on his side inspects his crawl space with a flashlight

There’s a lot of confusion about vapor barriers. How do they work? Do you need one in your basement? How do you install them? And so on. 

Today, we’re going to clear the air (pun intended) and talk about vapor barriers once and for all.

This article will explore why you might need a vapor barrier, how they work, and the available types. So let’s get into it!

Do You Need a Vapor Barrier in Your Crawl Space?

You need a vapor barrier in your crawl space, especially in colder climates or climate zones with high humidity. A vapor barrier is essential to keep moisture away from your floor structure and prevent mold and mildew. However, a barrier may not be as necessary for hot climates like Arizona.  

Moisture and dampness in a crawlspace present dangers to you and your family’s health—and a vapor barrier is critical in preventing the spread of these insidious fungi. 

Why You Need a Water Vapor Barrier in Your Crawl Space

You will benefit from a vapor barrier in your crawl space for several reasons. First, that dark and creepy underground lair may cause a host of problems in your home. Dampness and moisture are your enemies—and your crawlspace is a perfect place for moisture – and deterioration – to invade your home.

However, moisture is not the only problem that a vapor barrier can help prevent; let’s explore these vapor barrier advantages below!

Crawl Space Vapor Barriers Help Prevent Mold

The most important reason is to prevent mold and mildew from growing. These fungi love damp, dark, and humid environments, and crawl spaces are the perfect place for them to grow. Mold can release spores that present health issues, including allergies and auto-immune disorders.

These insidious spores can travel in the air and invade your living space. When mold accumulates in your lungs, your body releases chemicals to fight the spores, which can worsen asthma and compromise your body’s natural immunity to other infections.

Crawl Space Vapor Barriers Deter Pests

Termites are another reason to install a vapor barrier. These pesky critters attract moist environments and can damage your home if you don’t control them. They bore into wood joists and consume the cellulose in your wood, weakening your home’s foundation.

Crawlspaces aren’t limited to these wood-loving pests—they can attract a host of other critters, such as:

  • Silverfish
  • Cockroaches 
  • Spiders
  • Rats and mice

Barriers Help Prevent Moisture Damage to Your Floors and Foundation

Let’s say you don’t have a problem with mold, mildew, or termites. And you might be thinking that you don’t need a vapor barrier. But the ugly truth is that you require a vapor barrier because you don’t want to push your luck.

Crawl spaces are often damp and humid when it rains and during winter. This moisture can cause your floors to warp and your foundation to crack. A vapor barrier will help prevent these problems by keeping your crawl space dry.

a rotten subfloor in the hallway bathroom with a hole in the floor
A rotten subfloor in one of our Net-Zero homes needing removal and replacement

Vapor Barriers Can Reduce Utility Bills

Your unsealed crawl space allows conditioned air in your home to travel outside and vice versa. Unfortunately, this air leakage causes your home cooling or heating systems to work harder and increases energy bills. 

In a study conducted on ten homes in cold and hot climates, researchers found that sealed crawl spaces saved an average of 15% more annual energy consumption than traditionally vented crawl spaces. 

Regarding costs, a vapor barrier is not the cheapest item on the shelf, but it’s cheaper to install than to repair damaged floors or a cracked foundation. So, installing a vapor barrier is still a good idea, even if you don’t have a problem with mold, mildew, or termites.

When Don’t You Need a Vapor Barrier in Your Crawl Space?

As a vapor barrier functions primarily to control the movement of moisture, but despite its name, it is not a barrier but a moisture retarder

As the barrier functions to retard vapor diffusion, it might not be necessary if your area is dry and hot. According to Residential Energy Smart Library, vapor barriers may cause issues in these hot and arid climates.

Vapor barriers are also unnecessary if your crawl space materials are impervious to damage by freezing conditions or ambient moisture. 

What Is a Vapor Barrier and What Does It Do?

A vapor barrier is a material, usually plastic or foil, that helps block moisture, which you lay on the ground in your crawl space. The vapor barrier prevents condensation from reaching your joists and subfloor, keeping your home dry and free of mold and mildew.

The most popular vapor barrier boasts 6-mil polyethylene. This thick, heavy-duty plastic is very effective at blocking moisture. However, it can be challenging to install and isn’t always necessary.

An illuminated crawl space with a vapor barrier installed on the ground

There are other types of vapor barriers available. Some are made of foil, while others are paper or fiberglass. These vapor barriers are usually less effective than polyethylene, but they’re also much easier to install.

The most important thing to remember about vapor barriers is that they’re not 100% effective. They will help to reduce the amount of moisture in your crawl space, but they won’t eliminate it. So you will still have some humidity that needs to be vented out.

How To Install a Vapor Barrier in Your Crawl Space

Now that you know why you need a vapor barrier let’s talk about how to install one. The first step is to choose the right type. We recommend using 6-mil polyethylene, which is very effective at blocking moisture.

A vapor barrier roll, staple gun, and some staples resting atop a piece of plywood

Once you’ve selected the barrier, it’s time to install it. The easiest way to do this is to hire a professional. They will have the experience and tools necessary to complete the job.

Still, you can install a vapor barrier yourself if you’re up for the challenge by following these steps:

  • Clean your crawl space – remove any debris, dirt, or dust that might be present. Then, sweep and mop the floor to ensure it’s clean and dry.
  • You’ll need to lay down the vapor barrier – it’s essential to lay it smoothly so it is free of wrinkles. Otherwise, it won’t be effective at blocking moisture. Once you’ve laid down the barrier, use tape or staples to secure it.
  • Ensure the barrier is tight against the walls and floor – you don’t want any gaps or holes where moisture can get through. Once in place, your crawl space should be dry and free of mold and mildew.

Your Vapor Barrier Options

Now that you know why you need a vapor barrier and how to install one let’s talk about your options. As mentioned earlier, we recommend using 6-mil polyethylene. 

However, your other options include:

Elastomeric Coatings

These are rubber-like coatings sprayed on the walls and floor of your crawl space. They’re very effective at blocking moisture, but they’re also costly. 

You can think of them as the “Rolls-Royce” of vapor barriers. However, you don’t need them unless you have a serious moisture problem.

Aluminum Foil

This material is a less expensive option that is also effective at blocking moisture. But it’s not as durable as polyethylene and can be challenging to install.

Aluminum foil works best for small spaces or areas that are difficult to access. But for most people, we recommend using 6-mil polyethylene—it’s more affordable and easier to install.

Paper-Backed Aluminum

This material is another excellent option that is also very affordable. Paper-backed aluminum is much like regular aluminum foil but is thicker and more durable. It’s also easier to install.

We like this option because it’s lightweight and easy to install—and it won’t tear like regular aluminum foil.

Polyethylene Plastic Sheet

A moisture barrier installed on a basement floor in a home

This sheeting is an excellent option if you want something durable and affordable. Polythene plastic is similar to polyethylene, but it’s thinner and less expensive.

It’s also easy to install, which makes it a good option for do-it-yourselfers. Just make sure that you get the right thickness. We recommend using 6-mil polyethylene.

Asphalt-Saturated Felt Paper

This material is a heavy-duty paper saturated with asphalt. It’s very effective at blocking moisture, but it’s also costly. And it can be challenging to install.

Asphalt-saturated felt paper is best for people who have a serious moisture problem. We’re talking about coastal areas with humidity levels of over 70% most of the time. If you don’t have a serious moisture problem, there is no need to splurge on this barrier.

Metallized Film

This barrier is a thin plastic film that reflects heat and blocks moisture. It’s very affordable and easy to install.

A homeowner applies duct tape to a seam on a radiant barrier beneath a ceiling

The metallized film is a good option for people who live in warm climates—the reflective properties of the film will help keep your crawl space cool in the summer.

Vapor Retarder Paints

These are special paints designed to block moisture. They’re very affordable and easy to apply.

Vapor retarder paints are a good option for people who want an inexpensive way to block moisture. Just make sure you choose a paint specifically designed for crawl spaces.

Vapor Barrier Installation Tips

Installing a vapor barrier is a pretty straightforward process. But there are a few things you need to keep in mind to make sure you install it correctly. 

Some tips include:

Clean the Crawl Space

Many people overlook this step, but it’s essential. Before installing the vapor barrier, you must ensure that the crawl space is clean. If there’s dirt, dust, or debris on the floor, it will be harder for the barrier to adhere. So take some time to sweep and vacuum the area before you start.

An immaculate crawl space in a home

Meet the ASHRAE Standards

The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) has established standards for crawl space vapor barriers. Make sure that your selection meets or exceeds these standards.

A professional can help you choose a suitable vapor barrier and install it correctly. But if you do it yourself, make sure you do your research and choose a high-quality product.

Avoid Fully Impermeable Barriers

Some vapor barriers are impermeable, so they don’t allow air to pass through. This lack of airflow can worsen your moisture problem because it will trap humid air between the barrier and the ground. Instead, choose a semi-permeable barrier that will allow some air to pass through while still blocking moisture.

Seal All Wall Cracks and Gaps

If you have any cracks or gaps in your walls, you need to seal them before you install the vapor barrier. Otherwise, moisture will find a way through. Use caulk or expanding spray foam to fill any cracks or gaps.

If you want further information on sealing air gaps, please check out our article, Air Sealing Your Home For Efficiency: Your Ultimate Guide.

Install a Dehumidifier

A dehumidifier is a great way to keep the air in your crawl space dry. It will remove moisture from the air, which will help prevent mold and mildew growth

You can install a whole-house dehumidifier if you live in an area with high humidity. Or you could get a smaller unit for the crawl space if the moisture is not a big concern in your area.

Do You Need a Water Barrier on a Concrete Floor?

If you have a concrete floor, you will need a water barrier. Concrete is very porous, which means it can absorb moisture from the ground, leading to mold and mildew growth.

Installing a water barrier will help prevent this problem by keeping moisture out of the concrete. We recommend using an asphalt-saturated felt paper or a polyethylene vapor barrier.

Does a Vapor Barrier Work for Dirt Floors?

Yes, a vapor barrier will work for dirt floors. Actually, using one is even more critical if your crawl space has a dirt floor. That’s because dirt is very absorbent and can hold much moisture.

A picture of an unfinished crawl space of a home, with spray foam insulation on the underside of the floor.

A sound vapor barrier will keep the moisture in the ground from coming up through the floor and into your home.

A vapor barrier is a good idea, no matter what floor you have in your crawl space. It will help prevent moisture problems and keep your home healthy and comfortable.

Final Thoughts

Adding a vapor barrier to your crawl space is a great way to prevent moisture problems. It will also help keep your home more comfortable and energy-efficient. First, however, make sure you choose the barrier type for your specific needs.

Don’t hesitate to contact a professional if you have questions about vapor barriers or need help choosing the right one. They will be able to help you figure out which type is best for your home, saving you thousands in home repairs.


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