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 U.S. They are a go-to for most households because they are cheaper than full basements but more functional than slab foundations. These spaces offer a convenient location for plumbing, electrical lines and ductwork for heating and cooling systems. However, they also pose a major problem, and 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 the Southeast. Many unpleasant symptoms are associated with wall-vented crawl spaces, and although they are most often noticed in the humid spring or summer, they can occur any time of year:

  • 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 installing either additional ventilation openings to the outside or fans to 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 studies demonstrating that closed crawl spaces, insulated spaces without vents to the outside, can significantly improve moisture control and provide energy savings.

The Ventilation Myth

So why doesn’t ventilation with outside air simply dry out a crawl space like conventional wisdom would have us believe? Research from Advanced Energy has shown that three main properties of air contribute to the failures of wall-vented crawl spaces: temperature, relative humidity and dew point temperature.

Temperature is the measure of heat in the air. Relative humidity is the ratio of water vapor in the air to the maximum amount of water vapor the air can hold, and there is a limit to how much water vapor can be held before it condenses into condensation or rain. Dew point temperature is a direct indication of how much water vapor is in the air. Putting it together, dew point measurement states the temperature at which the water vapor in the air will condense into liquid water.

“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 state it simply, 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 enhancing the quality of life of the household. Without moisture and mold building in the foundation, air quality in the home improves, which can reduce the health effects of air pollution. Research by Advanced Energy has also found that insulating crawl spaces can reduce heating and cooling costs by up to 18 percent. Furthermore, closed crawl spaces create warmer floors during the winter 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 planning a proper crawl space that will meet and surpass the minimum requirements for the state’s residential codes.

The main aspects of 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 and is the most important requirement. Two basic strategies are used to manage liquid water and water vapor: facilitating removal and blocking sources. Facilitating removal outlines the ways to mitigate the moisture that will inevitably get into even a closed crawl space. Blocking sources describe the methods for 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 taken by wood-destroying insects to get from the ground to the structure, so designers must pay extra attention to the materials applied to these walls.

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

The Installation

Finding a qualified installer can be a 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 collaborations with professional installers and consultants across the country. After an installer has been contacted and the design has been defined, the homeowner and installer will need to work with local code officials to ensure the design is acceptable and meets all code requirements for the state — section R409 of the North Carolina Building Code outlines the requirements of a closed crawl space.

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

Quality installers know the importance of controlling moisture and other elements during construction. Likewise, most installers care about long-term performance and typically offer quality assurance options, such as a monitoring system to inform the homeowner of relative humidity levels or 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 setup, there are many methods to either improve it or convert it to a closed space. However, there is much more to converting a crawl space than just closing the vents. An experienced installer will know the appropriate steps to take to update a conventional wall-vented crawl space.

Basic Crawl Space Maintenance

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

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

Bringing Crawl Spaces to a Close

Thanks to the help of Advanced Energy’s research and investigations, closed crawl spaces are now the standard for LEED homes and easier to install through the North Carolina Building Code. With proper installation and maintenance, closed crawl spaces provide benefits that 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 crawl space webpage for additional resources.