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Understanding Crawl Space Ventilation

An open crawl space vent letting humidity into a dirt crawl space

Crawl Space Vents

Crawl space vents are designed to circulate outside air into your crawl space in an attempt to dry the space.

This building technique has proven to be ineffective, and is slowly being phased out of local building codes.

Home ventilation has long been a time-honored way to dry out attics and cool the house during the hot summer months.

Based on this success, many building codes quickly adopted codes in the 1950's to also require crawl space vents to be installed.

These were adopted under the assumption that a crawl space would benefit from circulating air from the outside in the same ways that the rest of the house has been improved.

However, crawl spaces (and basements) are different from other parts of the home. Most particularly, they are located underground, where the earth maintains a cool environment year-round. What has worked for the rest of the house is simply not effective here!

External Reading:

USDA Forest Service: Crawl Space Ventilation [PDF]

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Condensation on a soda bottle


When warm, humid air touches a cold surface, it will drop some of its moisture on that surface as condensation.

Located underground, a crawl space is naturally cooler than outside during the summer. What will happen when hot, humid summer air is vented in?

Venting A Crawl Space In The Winter

Crawl spaces often house many pipes and utilities that need to be hot. This includes your furnace or HVAC system, your water heater, air ducts, and hot water pipes.

When cold winter air enters a crawl space, it will cool these utilities, ducts, and pipes, forcing them to work harder than ever just to do their daily work. This means they're using more energy, and costing you more on your monthly bills.

Snow trapped in the wells around crawl space vents, as well as puddles formed by melting snow, can also lead to water leaking through.

Other Limitations Of Crawl Space Vents

While it is possible to seal the crawl space vents during the winter, there are other times that crawl space ventilation simply makes no sense at all.

For example, venting a crawl space with air during rainy weather is not likely to keep the space dry. And what will happen during foggy, damp, or humid weather?

What Happens To A Vented Crawl Space During Hot Weather?

Sometimes the solution is worse than the problem. And sometimes the solution is not a solution and is worse than the problem it's failing to solve!

When it comes to venting a crawl space during the summertime, that is exactly the scenario.

A house on a hot, hazy, humid day.

While The Air Is Outside...

During the summertime, the air outside is hot! It isn't uncommon for the temperature to reach 85°F or higher during the day. Let's set our temperature for a hypothetical summer day:

Current Temperature: 85°F

Hot air is able to hold more moisture than dry air. We measure how "full" of moisture the air is by its Relative Humidity (RH) Level. When the air reaches 100% relative humidity, it can hold no more moisture.

Summer weather is often hot and humid. Let's set our hypothetical RH level:

Current RH Level: 75%

A vented crawl space opening, shown from the inside.

As Outside Air Enters The Crawl Space...

A portion of this outside air now enters the crawl space through the vents. Despite the hot summer weather, the crawl space is located underground and remains cool throughout the year.

Current Temperature: 65°F

Remember that hot air can hold a lot of moisture? As this hot air is cooled in the crawl space, its ability to hold moisture diminishes. For every one degree that it's cooled, its RH level rises by 2.2%. If you drop the temperature by 20F, then the RH rises by (20 x 2.2) 44%.

Current RH Level: 119%

But wait -- that's impossible! Instead, excess moisture condensates on surfaces, maintaining the RH at 100%.

Rotting structural wood in a crawl space

Mold & Rot Develop

On an especially humid day, with all that condensation everywhere, the crawl space looks like it's raining!

Moisture will soak everything -- including wood, fiberglass insulation, and any other porous materials. Nonporous materials will be dripping with water.

Mold needs dark spaces with temperatures of at least 40°F, with 60% RH. What are those numbers again?

Current Temperature: 65°F
Current RH Level: 100%

Given the right conditions, mold will grow within 48 hours, growing on wood, cardboard, and any other organic surfaces in the space. Over time, it will cause severe damage to the space -- including structural issues!

Bad fiberglass in a wet crawl space

Damaged Fiberglass Insulation

Fiberglass insulation is damaged by a humid environment in two ways:

Mold Damage: While fiberglass itself does not support mold growth, there are many resins and dyes within the material that do (fiberglass in not naturally pink!). Additionally, fiberglass that's been installed in a crawl space often has a paper backing -- which easily supports mold growth.

Water Weight: Fiberglass is like a sponge! As moisture builds up within the material, it will become weighed down. Over time, it will sag and fall, useless, to the floor.

Visit our Moldy Fiberglass Gallery

A dead mouse lying on fiberglass in a crawl space

Rodents & Pests Move In

At this point, we've got a dark space that's cool all year long, that's filled with rot and mold, and that has fiberglass insulation lying everywhere. If you're a rodent, termite, carpenter ant, or other pest, this is heaven!

Rodents will nest in the warm, soft fiberglass -- living, breeding, and dying there. They love that the space is sheltered from predators, and they'll stop by your kitchen anytime for a snack.

Termites and other wood-destroying insects prefer the damp environment and soft, rotting wood.

Eventually, snakes, spiders, and small predators will also move in. The crawl space will begin to stink, and will soon be in need of expensive structural repairs.

More on Pests In The Crawl Space

An airtight crawl space door

Airtight Doors & Vent Covers

Now that it's become clear that crawl space vents actually encourage mold growth, building code is changing. Contractors are now able to seal the crawl space, install airtight doors, and place airtight covers on vents.

We recommend sealing a crawl space to control mold and rot, to deter pests, and to enhance the energy efficiency of the space.

More about our Vent Covers
More about our Airtight Doors

The "Stack Effect"

When the crawl space vents were created, the idea was to cross-ventilate the space. It was supposed that new air would be brought in from one vent, while humid crawl space air was sent out from another.

However, the building contractors who first designed these failed to realize that your crawl space is part of a bigger system: your home.

Air Movement In A Home

Air is constantly moving upwards in a house, with warm air rising in the home and exiting the attic and upper levels.

As this air movement occurs, a vacuum is created in the lower levels of the home. This air is replaced by air from the outside. Additionally, 40% of the air in your home is pulled upwards from the crawl space.

If your crawl space air is filled with musty odors and allergens such as mold spores, dust mite waste, animal dander, and others, these elements will enter your living space and affect your quality of life.

Cold air from your crawl space can cool the floorboards and make the space drafty and uncomfortable. Humidity in the space can add to your air conditioner's workload, costing you money.

More about The Stack Effect

We Can Help You Seal & Dry Your Crawl Space!

At Basement Systems, our warrantied crawl space systems have been installed in hundreds of thousands of homes worldwide! Homeowners love how dry, healthy, and energy efficient their new space becomes.

It costs you nothing to meet with one of our specialists for a free, written crawl space repair estimate! Your free estimate includes an on-site professional inspection and measurement of your crawl space and a written cost estimate. We'll answer any questions you have, and make recommendations for a customized solution.

Our dealer network extends across North American and Europe, including Canada and the United States. Contact us by phone or e-mail today to get started!

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Our innovative and forward-thinking approach to designing products has solved wet basement problems that the basement waterproofing and crawl space industry has faced for decades, and our patented sump pump systems are the smartest and most reliable. Plus we listen to our customers and respond accordingly! Basement Systems Inc. has been awarded 24 Patents and 19 Innovation Prizes on waterproofing products we use every day, including a battery back up sump pump system. This approach ensures the best waterproofing systems and solutions available.