Taking the Vents Out of Vented Crawl Spaces

By Lindsay Brecheisen | June 13, 2017

Crawl space foundations are a popular feature in building construction throughout the United States. They are a go-to for most households because they are cheaper than full basements but more functional than slab foundations. These spaces offer convenient locations for plumbing, electrical lines, and ductwork for heat and air conditioning systems.

Unfortunately, crawl spaces pose a major problem—for several decades they have been built and maintained incorrectly. Building codes and conventional wisdom have insisted that crawl spaces be ventilated with outside air to control moisture and improve damage within the space.

However, wall-vented crawl spaces can actually be the cause of moisture buildup and other complications in households, especially in southeastern United States. Many unpleasant symptoms are associated with wall-vented crawl spaces. These are most often noticed in the humid spring or summer seasons, but can occur in the home anytime of the year. Common symptoms experienced with a conventional wall-vented crawl space are:

  • Mold or moisture damage in the crawl space or living area
  • Musty odors in the living area
  • Condensation (“sweating”) on air conditioning ductwork or equipment
  • Condensation on insulation, water pipes or truss plates in the crawl space
  • Buckled hardwood floors
  • High humidity in the living area
  • Insect infestations
  • Rot in wooden framing members

Before Advanced Energy’s research found the failures of wall-vented crawl spaces, the most common treatment for dealing with these symptoms was to add even more ventilation, by either installing additional ventilation openings to the outside or installing fans to deliberately move more outside air through the crawl space. With high costs and time-consuming repairs to resolve the moisture concerns and other difficulties, homeowners, property managers, tenants, and the construction industry became more aware of the importance of finding ways to stop these symptoms from the start. This awareness led to research on closed crawl spaces. Research conducted by Advanced Energy found that closed crawl spaces, insulated spaces without vents to the outside, can significantly improve moisture control and have major energy savings with proper installation.

The Ventilation Myth

So why doesn’t ventilation with outside air simply dry out a crawl space like conventional wisdom would have us believe? There are three main properties of air that disprove that notion: temperature, relative humidity and dew point temperature. To begin, let’s define these properties. Temperature is the measure of heat in the air and relative humidity is a ratio of water vapor in the air to the maximum amount of water vapor the air can hold. This is important because there is a limit to how much water vapor can be held in air before the water vapor condenses into condensation or rain. Finally, dew point temperature is a direct indication of how much water vapor is in the air. To put it all together, dew point measurement states the temperature at which the water vapor in the air will condense into liquid water. Research from Advanced Energy has confirmed that these three components contribute to the failures of wall-vented crawl spaces:

“The outside air contains more water vapor than the air in the crawl space during the warm seasons, and has no potential to dry the crawl space. Instead, the outside air ends up contributing water vapor to the crawl space.”

To simply state it, using outside air to ventilate a crawl space only adds more moisture to the area—it does not dry it out. Furthermore, wall-vented crawl spaces allow condensation, surface mold growth, high wood moisture content and rotting wood to increase.

The Outstanding Benefits

The most significant benefit of closing a crawl space is improving the quality of life of a household. Without moisture and mold building in the foundation of a home, air quality will easily improve in the home, which can greatly reduce heath side effects caused by air pollution. Research by Advanced Energy has also found that by insulating the crawl space, heating and cooling costs can be reduced by up to 18 percent. Furthermore, closed crawl spaces create warmer floors during the winter seasons and minimize damage to hardwood.

A Proper Design

Now that a solution has been discovered, researchers and installers have been testing and improving closed crawl space designs. There are many components to consider when designing a proper crawl space that will meet and surpass the minimum requirements for the state’s residential codes.

The main components of a good crawl space design are:

  • Moisture management
  • Pest control
  • Combustion safety
  • Fire safety
  • Thermal insulation
  • Radon control

Moisture management is the primary goal for any closed crawl space system, therefore, it is the most important requirement. Two basic strategies are used to manage both liquid water and water vapor: blocking sources and facilitating removal. Facilitating removal outlines the various ways to mitigate the moisture that will inevitably get into even a closed crawl space, whereas blocking sources describe the methods in handling factors such as roof runoff, exterior ground and surface water, humid air and evaporation from the ground and perimeter walls.

In addition, pest control is a priority throughout every step of installing and maintaining a closed crawl space. Perimeter walls are the most common path wood-destroying insects take to get from the ground to the structure so designers must pay extra attention to the materials applied to these walls.

Furthermore, combustion and fire safety, thermal insulation and radon control are additional requirements to keep a crawl space safe and running well without posing any danger to the home.

The Installation

Finding a qualified installer is a large challenge to getting a properly closed crawl space. To help with the process, Advanced Energy offers materials and information generated from research projects, diagnostic investigations and collaboration with a variety of professional installers and consultants across the country. After a qualified installer has been contacted and the design has been defined, both the homeowner and the installer will need to work with the local code official to ensure that the closed crawl space design is acceptable and meets all code requirements for the state—section R409 in the North Carolina Building Code specifically outlines the requirements of a closed crawl space.

Proper scheduling and logistics is another important concern, since many strategies to manage the components of a crawl space are time-sensitive. For instance, as soon as the sub-flooring is installed, all crawl spaces can become ideal environments for growing mold. Consequently, managing the construction schedule is critical.

Quality installers of closed crawl spaces know the importance of managing moisture and other components during the construction process. Likewise, most experienced installers care about the long-term performance and typically offer quality assurance options, such as a monitoring system to inform the homeowner of relative humidity levels or a water alarm to inform the homeowner of a buildup of liquid water.

For those who currently have a wall-vented crawl space—don’t lose hope! If you are unsatisfied with your existing crawl space, there are many methods offered to either improve it or convert it into a closed space. However, there is much more to converting a crawl space than just closing the existing vents. An experienced installer will know the appropriate steps to take in order to update a conventional wall-vented crawl space.

Basic Crawl Space Maintenance

The crawl space may not be the most popular place in the house to explore, but a periodic inspection is imperative to ensuring that potential problems are caught early on. Even though basic maintenance inspections can be done by any homeowner, some may choose to hire a private home inspector or other contractor to do it for them.

Property owners should frequently replace batteries in sensors or alarms as needed, check to ensure that access doors are closed, especially in warmer weather, and ensure that there are no potentially hazardous materials in the crawl space. Most importantly, a homeowner needs to keep an eye on any water intrusion to make sure it is quickly drained or pumped out.

Bringing Crawl Spaces to a Close

With the help of Advanced Energy’s research and investigations, closed crawl spaces are now the standard for LEED homes. This research also had a major impact on the North Carolina building code, making it easier to install closed crawl spaces. With the proper maintenance and installation, the benefits of having a closed crawl space certainly outweigh traditionally vented spaces.

Whether you are interested in updating your existing crawl space or in the process of designing and installing a new one, visit our crawlspace web page for helpful resources.