Skip to Main Content
CFI Trainer in front of class

Open Crawl Space Vents

Open crawl space vents were meant air out the crawl space, but builders are now discouraging them as they cause moisture problems.

Like everyone around your neighborhood, you’ve probably grown up seeing crawl space vents. Almost everyone who built a home before made sure its crawl space had vents. You didn’t think much about them and nobody ever questioned why they exist or what purpose they serve. 

Fast-forward and now you’re a proud homeowner. But you can’t quite figure why vents were installed on your crawl space walls. Here, we will demystify open crawl space vents and show you why their presence isn’t a good idea. 

What Are Crawl Space Vents? 

Crawl space vents are common fixtures on the foundation walls of many homes and a requirement under the building code, which obliges homeowners with below-grade areas like the crawl space to have them for proper ventilation. In theory, having vents is a nice thing. But the problem is vents allow wet air to enter your home unabated. This defeats the purpose of having them. 

Crawl space vents also facilitate the stack effect, the movement of air from the bottom to the top of your home. Whatever is in the crawl space – mold spores, dust particles, allergens – will find its way to the rest of your home. 

Should I Seal the Vents or Leave them Open? 

One of the dilemmas homeowners with open crawl space vents face is whether they should seal the vents or keep them open. 

Problems Associated with Open Crawl Space Vents 

These vents allow outside air to circulate under the floor. It was previously thought that this would prevent moisture buildup, which encourages mildew and rot. However, this has been proven untrue. Crawl space vents can also cause additional issues in winter, like bursting pipes as a result of freezing temperatures. 

Instigates moisture: One of the problems with using open crawl space vents is they allow moisture-laden air into the crawl space. This can make it difficult to control crawl space humidity. Outside air can also bring with it mold spores, allergens, and pollutants that can spoil the quality of indoor air. 

Encourages pests: Critters can also use the vents to enter your home. For example, a mouse only needs a 1/4 inch of space to squeeze into your home. Other crawlies will also come in and get out at their will, which is bad for your crawl space health. 

Lowers your home’s energy efficiency: Leaving crawl space vents open means you’re going to experience temperature fluctuations that will force you to run your heater to warm up your home in the winter and air conditioner to cool it in the summer. This will increase your utility costs. 

Poor indoor air quality: Since outside air flows in unabated, it’s likely to bring with it dust and other harmful particles that will contaminate the indoor air. This can lead to asthma and other respiratory problems. 

Sealing Off Crawl Space Vents 

When winter arrives, you want to make sure your crawl space vents are sealed off so your crawl space stays dry. The simplest way to do that is by placing airtight vent covers over the vents, preventing intrusion from outside air, water, and pests. 

Studies show that homes with closed vents are much drier and healthier than their counterparts. As well as being structurally sound, such homes have fewer mold problems and better energy efficiency. By sealing your vents, you push down relative humidity levels to below 60%. 

Protecting Your Crawl Space 

Sealing off your crawl space vents is a step in the right direction. However, this won’t be enough to stop your moisture problems. Once you seal the vents, you need to encapsulate the crawl space by covering the floor and walls with a 20-mil plastic vapor barrier. What this does is lock out outside air, the #1 source of moisture and the leading cause of crawl space dampness. 

Leaving the crawl space vents open can lead to a myriad of problems, the biggest of which is moisture buildup. If you’d like to encapsulate your crawl space, contact JES Foundation Repair. We can help you seal up the crawl space so moisture and pests will have less opportunities degrade it. 

Publish Date:

Last Modified Date:

Our Locations


8361 Town Center Ct
Nottingham, MD 21236


311 Central Rd.
Suite 2-02
Fredericksburg, VA 22401

Hampton Roads & NE NC

2569 Quality Ct
Virginia Beach, VA 23454

Northern VA & DC

7940 Gainsford Ct.
Bristow, VA 20136


309 Quarles Rd
Ashland, VA 23005

Southwest Virginia / Roanoke

2033 Cook Dr.
Salem, VA 24153

Western Virginia

456 Old Courthouse Rd
Appomattox, VA 24522


45 W Boscawen St,
Winchester, VA 22601