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The Truth About Crawl Space Vents: A Tidewater Regional Guide

When it’s pleasant outside, it’s satisfying to throw open the windows and let fresh air into your home. 

This is all well and good for your main living space, but it’s not a good idea for other areas of your house. 

Take your crawl space, for example. “Airing out” or venting your crawl space is a dangerous practice, despite what many builders think. 

As you can see in this crawl space image, vents contribute to a host of problems that lurk unnoticed just beneath your feet. 

In this guide, we’ll cover the reasoning behind crawl space vents, how they impact your crawl space and home, and what you can do to mitigate their effects. 

Crawl Space Vents: Are They Needed?

Virginia itself is home to many old, historical homes. Many older and even newer homes have crawl space features that are detrimental – particularly open crawl space vents. 

Crawl spaces have traditionally been required to have open vents to improve ventilation, let the crawl space “breathe,” and reduce moisture and mildew problems. 

However, foundation specialists have discovered that crawl space vents actually contribute to the very issues they were designed to solve.

How Crawl Space Vents Affect Your Crawl Space

If your home has crawl space vents, you need to know what they’re doing to your home and family. 

Open vents let rain runoff, humid and unconditioned air, and other nuisances into your crawl space. This leads to a variety of problems, including: 

  • Water Infiltration 
  • Dangerous Growths 
  • Structural Instability 
  • Pest Invasion 
  • Damaged Protective Measures
  • Energy Loss 
  • Unpleasant Odors 

1. Water Infiltration

The presence of water is always a problem in crawl spaces, and these two formations are particularly concerning: 

  • Condensation 
  • Flooding 


When warm, humid outside air enters your naturally cooler crawl space, it condenses into water droplets when it encounters cool surfaces like ductwork. 

This water may also soak through other components of your crawl space, such as the wooden supports, fiberglass insulation, and plastic liners, causing further issues. 


Runoff from rain easily gets inside open vents. This can lead to puddles of water on the floor or any liners, which you can see here, as well as widespread flooding. 

2. Dangerous Growths

Your crawl space naturally has plenty of oxygen, moisture, and organic material. When they combine, they form dangerous growths such as: 

  • Mold 
  • Wood Rot 


While numerous kinds of mold exist, they are all born from the combination of air and excess moisture. 

Mold begins to grow when humidity levels are greater than 50 percent. 

Some are not harmful. However, others, like black mold, are particularly toxic to vulnerable people and pets with allergies and compromised immune systems. 

Wood Rot

Similarly, a different kind of mold develops on wooden supports and floor joists. 

This is known as wood rot, and it destroys the structure of your home. 

3. Structural Instability

Fungi like wood rot thrive in dark, damp, wet areas like your crawl space that has a mostly wooden structure. 

This eats away at the wood over time and weakens it. 

Rotten, compromised wood, like in this photo with a failing support jack, cannot properly support your home. You’ll know there’s a problem when you notice the floor is sagging, uneven, soft, bouncy, and buckling. 

4. Pest Invasion

Having open vents is like putting out the welcome mat for pests

Not only is it simple for them to get inside, but bugs, rodents, and small animals are also drawn to your crawl space for its seclusion, food, and nesting and breeding sources. 

5. Damaged Protective Measures

As water, humid air, and pests get into the crawl space through vents, they all wreak havoc on any protective measures installed below your house such as: 

  • Insulation 
  • Vapor Barrier Liners 

In this photo, you can see water has inundated both the insulation and floor liner, making for an ineffective and nasty combination. 


While it is commonly used throughout homes, fiberglass insulation in a crawl space is bad news. 

Pests love to nest in the soft material, plus it easily absorbs moisture and contributes to mold growth and wood rot.

Vapor Barrier Liners

Many crawl spaces have uncovered dirt floors, but some have plastic liners. 

Most of these liners are thin and ineffective on their own, but they are easily damaged by debris, pests, and water intrusion. 

6. Energy Loss

When crawl space insulation is damaged, it loses its function and cannot provide proper temperature control in your crawl space or home. 

This results in problems like: 

  • Decreased Energy Efficiency 
  • Higher Energy Bills 

Decreased Energy Efficiency

Without protection from insulation or vapor barriers, your crawl space will be cold in winter and hot in the summer. The same goes for your above floors. 

Higher Energy Bills

To achieve your preferred interior temperature, your furnace overcompensates in the winter, and your air conditioner is overloaded in the summer. 

When each scenario happens, it drives up your energy costs

7. Unpleasant Odors

Along with the aforementioned problems, water and humid air originating from open vents result in musty, nasty odors. 

Some stinky culprits include: 

  • Wet insulation or liners 
  • Mold growth 
  • Rotting wood 
  • Pest waste and carcasses 
  • Wet dirt floors 

With all the moisture and other problems seen here, you can practically smell this photo. 

Understanding the Stack Effect

Unlike the Las Vegas adage, what happens in your crawl space does not stay in your crawl space. It’s going to impact your home and family. 

That’s because of a phenomenon known as the stack effect, which is aided by open vents. 

The stack effect, also known as the chimney effect, is the natural flow of air in your home from bottom to top. 

Air pressure is also a factor. Air moves from high-pressure areas to low-pressure ones. When there is a considerable difference between indoor and outdoor temperatures, airflow becomes more significant. 

As a result of this airflow, everything in your crawl space, including mold spores, insect feces, and damp air, will become airborne and circulate throughout the rest of your home. 

This not only results in poor air quality but also poses health risks. 

Curious what the stack effect looks like? Check out this video to learn more. 

The Bottom Line: Cover Your Vents

The best way to deal with crawl space vents is to close and seal them. 

But this is not always as simple as it sounds. There’s room for error with DIY installations of big-box store items, which allows crawl space problems to continue. 

Crawl space repair professionals will block off and cover your vents with superior materials, as well as install other crawl space encapsulation and repair solutions

Knowledge About Open Vents Will Empower Your Home Repair Decisions

As you can see, open crawl space vents are a major problem.  

Although the design of crawl spaces has been critically defective, it is important to note that building codes have recently been updated to allow for unvented crawl spaces. 

While unvented crawl spaces still need to be conditioned, homes with these vents also need to be properly sealed. 

Crawl space professionals can help you with this task, as well as create a healthier home with crawl space encapsulation. 

If this sounds like a project your home and family can benefit from, give our experienced team a call! We’d love to meet with you and provide you with a free inspection and estimate

Crawl Space Vents


Some crawl spaces, especially in older homes, are vented because it was required by building code. 

It was thought that open vents would help with the airflow through the crawl space and minimize the presence of moisture and mold. 

But in more recent years, foundation experts have found that vents actually cause those problems and more.

Replacing, removing, or altering crawl space vents in any way is a project best suited for crawl space and foundation repair professionals. This ensures your crawl space has the appropriate repairs it needs to lock out outside air and moisture. 

Professionals will seal off the vents with sturdy foam insulation blocks, as well as place vent covers on the home’s exterior for aesthetic purposes upon your request. 

Call JES for a Free Estimate!

Our team at JES Foundation Repair is proud to serve homeowners in locations throughout Virginia, including:

  • Chesapeake 
  • Newport News 
  • Norfolk 
  • Richmond 
  • Virginia Beach 

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