Skip to Content

How to Seal Crawl Space Vents and Make Your Basement a Breath of Fresh Air

How to Seal Crawl Space Vents and Make Your Basement a Breath of Fresh Air

Share this post:

This post may contain affiliate links. If you click an affiliate link and make a purchase, I may earn a commission. Also, as an Amazon Associate I earn from qualifying purchases.

Did you know that the average home has between 200 and 400 cubic feet of indoor air and another 150 to 300 cubic feet of soil-covered crawl space air?

There are two potential problems with these conditions. First, it’s not safe to have that much indoor air filled with carbon dioxide, methane, and other gasses that can be explosive.

Second, it’s also not healthy for anyone to have so much humid air in the home—mainly because that humidity can breed mold spores.

Fortunately, there’s a way to get your basement back under control by sealing your crawl space vents. If you think this sounds like something you need to do, read for details about how to seal crawl space vents.

What Is a Crawl Space?

A crawl space is a storage area underneath the floor of any building with a basement.

The crawl space might be enclosed in a separate room or open to the floor joists of the building itself. The walls of a crawl space aren’t that high, hence the name. Here’s an overview of its uses and properties.


Crawl spaces are often used to store equipment like HVAC systems, water heaters, and furnaces. You can also use them to store extra furniture.

Other items you don’t want to store in your garage can also stay in crawl spaces. That’s if they’re large enough to fit.


Such spaces are often damp, dirty, and prone to pest infestation. They can also be prone to flooding.

Crawl spaces can stay at one consistent humidity all year long. As soon as it rains, the water will drain down through the soil and into the crawl space.

They’re typically cooler than other parts of the house in the summer and warmer in the winter. The ground surrounding the crawl space will also be cooler in the summer and warmer in the winter.

Why Do You Need to Seal Crawl Space Vents?

Humid air inside the home is one of the primary culprits of mold and mildew, which can cause a wide variety of health issues. Moist air can also make your home feel muggy and uncomfortable.

Since crawl spaces are enclosed, any trapped water will stay there until it evaporates. Since water evaporates slowly, the moisture level in the air will be relatively high.

That moisture is then trapped in the crawl space without escaping since crawl spaces don’t have enough airflow.

What Are the Consequences of Not Sealing Your Crawl Space Vents?

Not sealing your crawl space vents means the humidity inside will be 10 to 15 percent higher than the rest of your home. High humidity usually means a suitable environment for mold to grow.

Crawl spaces can also have carbon dioxide levels that are much higher than they should be.

CO2 is a normal part of the air inside your home, and its levels should be around 1000 ppm. However, the CO2 level in your crawl space can be as much as ten times that amount; it can also be high in methane.

Sealing your crawl space vents limits the entrapment of humidity and carbon dioxide, thereby preventing the formation of mold and mildew.

How to Seal Crawl Space Vents

As a DIYer, there are a few different ways to seal your crawl space vents, and the one you choose will depend on the layout of your crawl space.

You can install a barrier or wall inside the crawl space or add an exterior barrier. You can also install a vent cover in the wall of your crawl space.


A barrier is probably the easiest way to seal crawl space vents. It’s basically a wall that separates the inside of your house from the crawl space.

The barrier could be made of any number of different materials, including a thick piece of plastic or a concrete wall.

Exterior Barrier

An exterior barrier is a wall or roof that seals the crawl space vent.

Use an exterior barrier to seal crawl space vents located in less-than-ideal areas, such as the bottom of a basement or against the house.

An exterior barrier can be a simple obstruction, such as a plywood fence. It can also be a more elaborate structure, such as a metal plate suspended from the ceiling.

The idea is to create a physical barrier between the crawl space and the house’s exterior.


Gaskets can be made from metal tape, rubber, or mesh cloth. They can also be designed so that they create an airtight seal without requiring a lot of pressure.

Gaskets are commonly used in crawl spaces with fiberglass or plastic vents because they’re relatively inexpensive.

When you’re sealing crawl space vents with gaskets, be sure to use the appropriate type for your situation. You don’t want to install a metal gasket in a plastic duct, for example.

Slotted Vent Covers

Another option you may opt for is a slotted vent cover. It’ll take some effort on your part, but it’s a generally simple process.

Here’s how to install a slotted vent cover:

  1. First, you need to find a crawl space vent that needs to be sealed.
  2. Then, cut out a hole in the vent cover large enough for the vent pipe to fit through.
  3. Next, place the vent cover over the opening and secure it with an adhesive or tape.
  4. After that, simply slide the vent cover down over the pipe so that it fits snugly around it. Make sure there are no gaps in between.
  5. You can seal any gaps that are left between the vent pipe and the wall by applying an additional piece of tape or caulk. And, you’re done!

Professional Options for Sealing Crawl Space Vents

If you live in a highly humid area and are looking for more professional sealing options, here are some.

Concrete Sealant

A concrete sealant is an impenetrable sealant that can be applied to the surface of a crawl space vent. They’re available in two types; quick-drying and long-lasting.

Quick-drying sealants dry quickly and can be used for interior purposes. Long-lasting sealants are a good choice for both indoor and outdoor applications, including crawl spaces.

Concrete Caulk

Concrete caulk is a special concrete sealant used in places where a liquid sealant can’t be used.

There are two types of concrete caulk with different uses and properties: self-leveling and non-self-leveling.

Self-leveling caulk is meant to flow from the bottom to the top of the vent, while the non-self-leveling caulk is intended to flow from top to bottom.

Non-self-leveling concrete caulk has a wider applicator tip than self-leveling concrete caulk, so it should be applied with a paint roller or putty knife.

Final Thoughts

The foundation of your home is often overlooked, but it’s crucial to the health and happiness of everyone living there.

Not only will a clean and sealed crawl space help keep your home clean, but it can also help prevent pests, improve your home’s energy efficiency, and even help protect it from flooding!

So if you notice a lot of humidity in the air or your floors seem sticky or dirty, check your crawl space vents. You’ll be glad you did.


If you want more backyard tips including recipes, how-tos and more, make sure you subscribe to my youtube channel

Share this post: