Why Crawl Space Vents Exist; And Why They're Outdated

If you live in an older home with a crawl space, chances are you've seen small metal grates or openings in the foundation walls. These are crawl space vents—and they were once considered essential. But today, we know they can do more harm than good.

Here’s why crawl space vents exist—and why it’s time to seal them up.

A Bit of History: Why Vents Were Added

Crawl space vents became common in home construction in the mid-20th century. Builders thought that by allowing outdoor air to circulate through the crawl space, they could reduce moisture and prevent mold or rot.

The idea made sense—at the time. Ventilation was supposed to let moisture escape.

But here’s the problem: It doesn’t work in real life.

What Actually Happens With Vents

Instead of drying the space out, vents often let moist, humid air in—especially in spring and summer. When that warm, damp air hits cooler surfaces inside your crawl space (like ducts, pipes, or insulation), it condenses into water.

This creates the perfect environment for:

• Mold and mildew
• Rotting wood and weakened beams
• Insects and pests
• Poor indoor air quality

In the winter, vents allow cold air in, which can freeze pipes and make your floors cold and drafty.

Why Open Crawl Spaces = Energy Loss

The stack effect pulls air from the crawl space up into your living areas. That means any damp, moldy, or unconditioned air down there gets circulated throughout your home.
You lose heat in the winter, and your AC works harder in the summer—all thanks to open vents.

What’s the Solution? Seal and Encapsulate

Modern building science is clear: Crawl spaces should be sealed and encapsulated.
That means:

• Sealing or covering all vents
• Installing a vapor barrier on the floor and walls
• Adding insulation and a dehumidifier if needed

At Systèmes Sous-sol Québec, we use CleanSpace® vent covers as part of our full encapsulation system. These durable, airtight covers not only block outdoor air and moisture—they also improve energy efficiency and boost your home’s value.

The result? A healthier, drier, and more energy-efficient home.

📞 If your crawl space still has open vents, it’s time to bring your home into the 21st century.
Contact us today for a free inspection and personalized solution.