Do I Have Poor Crawl Space Ventilation?

A crawl space vent rests behind heaps of fiberglass insulation and a rusty pipe

Issues With Venting Crawl Spaces

Crawl space vents are based on flawed building science that assumes that most of the moisture in a crawl space comes from the dirt floor.

If the crawl space is not flooding, the majority of crawl space moisture actually enters with the humid air that enters through the vents.

More about Crawl Space Ventilation

Almost 60 years ago, contractors began to install crawl space vents in an attempt to keep a crawl space dry and protected from mold and rot damage.

The idea behind the vents was this: creating a system of cross-ventilation in a crawl space would flush out the humid, musty air in the crawl space, keeping it clean, dry, and mold-free.

Even now, general contractors will swear by this method, citing antiquated building codes that remain in place -- but are changing quickly.

Why Are The Building Codes Changing?

Because time and again (and again, and again...) contractors are finding vented crawl spaces that are moldy, rotting, and showing severe damage.

Additionally, building scientists are researching the theory behind crawl space vents, and they're finding that it's fundamentally flawed.

As building codes continue to evolve, there is continued movement in the direction of crawl space encapsulation -- a process that involves sealing the crawl space doors and vents, installing a plastic liner on the walls and floors, and installing a cool-climate dehumidifier in the space.

free quote to seal venting crawl spaces

Sealing crawl spaces is our specialty! It's what we do every day -- and what we've done for hundreds of thousands of homeowners to date. Our warranted system is just what you need to keep your crawl space dry, healthy, and energy efficient!

It costs you nothing to meet with one of our certified experts to receive a a crawl space repair quote! We'll customize a system that will keep your crawl space dry -- all the time!

Major Design Flaws In Ventilated Crawl Spaces

At Basement Systems, not a day goes by when the experts in our network do not see hundreds of moldy, rotting, vented crawl spaces. It's clear they're not working -- but why not, when other forms of home ventilation work so well?

What follows is three false assumptions that support crawl space venting:

A foggy, humid day.

Outside Air To Dries The Crawl Space

It's not often that outside air is appropriate for venting a crawl space!

For this to work, the air would have to be temperate and dry -- ruling out hot summer weather, cold winter weather, and anytime it's humid, rainy, foggy, or damp outside your home.

The Relative Humidity (RH) or air is a measurement of how much water that air can hold. For every degree you cool crawl space air, the RH of the air rises by 2.2%.

Say we have a day with 84°F air and a relative humidity of 75% RH. We vent this air into a crawl space, where the dark, underground environment, which maintains a year-round temperature of about 62°F.

As The Air Enters The Crawl Space...

So... the temperature of the air has dropped 22°F in the crawl space. If the RH rises by 2.2% for every degree drop in temperature, that's a rise in RH of (22 X 2.2) 48.4%!

But wait... if the RH of the outside air was 75%, then the new RH would be (75 + 48.4) 123.4%, and it's impossible to have more than a hundred percent! What happens now?

When the air is at 100% RH, it can't hold any more water -- it's full. Any extra water is deposited on nearby surfaces -- such as your structural wood, fiberglass insulation, iron pipes, and other surfaces in the crawl space.

That's the opposite of what we wanted the vents to do!


several open vents in a crawl space

"Cross-Ventilation" Works

Building code specifies very exacting standards about how many vents are required for venting a crawl space. This is all done in the name of creating cross-ventilation in the space to dry it out.

However, the rules of convection and the Stack Effect show that air moves upwards in a crawl space, and indeed throughout the home.

Hot air rises upwards in the house, exiting through the attic and upper levels. As it does, a vacuum is created in the lower levels.

To fill the vacuum, air is pulled up from the crawl space, basement, and outside. As much as 40% of the air in your home's living space comes from below-grade spaces.

So What Does Venting Do For Your Home?

Open crawl space vents increase the air circulation throughout your home -- maximizing the amount of unconditioned air in your home. Meanwhile, the air in your home is exiting through the roof at an accelerated rate.

If you paid to condition that air, then you're literally throwing money out the window!


An improperly installed crawl space liner

Lining The Floors & Not The Walls Is OK

Far too often, Basement Systems Dealers come across a black plastic crawl space liner installed on the floor -- with none installed on the walls.

The idea here is that most of the moisture in the crawl space will enter through the dirt floor.

While it's true that a great deal of moisture does enter through the floor, water will also pass directly through the concrete in the form of water vapor. Leave the walls bare, and you allow humidity into the space.

To properly seal your crawl space, it's important to cover all wall and floor surfaces with a durable plastic vapor barrier, as well as sealing all vents and doors.

... But What About Pest Inspector Access?

One classic argument against sealing the crawl space walls is that the surfaces need to be visible for pest inspectors. If the entire wall is covered in a plastic vapor barrier, how will inspectors be able to look for mud tubes along the sill plate at the top of the walls?

This is solved easily enough. At Basement Systems, or experts install the WallCap™ System along the top of crawl space block walls. This system seals off the open cavities of the blocks with a clear plastic piece -- keeping humidity back and your pest inspectors happy.

Four Steps To A Dry Crawl Space -- With No Venting!

Sealing and drying a crawl space can be done quickly and permanently. Basement Systems dealers send a crew of experts for each installation, who often can install a complete, warranted system in less than a day.

Our crawl space systems can be customized to accomplish many goals, including waterproofing, sealing, dehumidifying, and insulating. When installing a system in your home, our crews follow these four simple steps:

We Create Healthy Crawl Space Environments!

At Basement Systems, our dealers are dedicated to creating dry, healthy, and energy efficient crawl space environments. We have more than 30 specialized crawl space products, that have been used in hundreds of crawl spaces worldwide!

For a free crawl space quote from your local dealer, contact us by phone or e-mail today! Each free quote includes an on-site professional inspection, a consultation with our experts, and a customized, written cost quote!

We proudly serve throughout Canada, the United States, and the United Kingdom. Our dealers are standing by in your neighborhood to help you!

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