The Unexpected Energy Benefits of Mechanically Vented Crawl Spaces
The debate on how best to treat crawl spaces has gone on for decades, and the building codes regulating crawl space design have constantly evolved. For a long time the entrenched thinking was to rely on foundation vents to properly condition crawl spaces. After all, that seems to be the most logical way to let moist, contaminated air exit the crawl. However, numerous studies have proven that theory wrong. Allowing warm, humid air to enter an unconditioned environment that contains cold surfaces only exacerbates the problem. The information gleaned from those studies has shifted the thinking to sealing the crawl space and installing mechanical ventilation. What I find most interesting is the results those studies revealed about energy consumption, and about how that information is being used today.
Mechanical Ventilation—Energy Efficiency and Improved Indoor Air Quality
Before discussing why mechanical ventilation improves energy efficiency, and the scientific studies that prove this to be true, I want to highlight some of the ways in which mechanical ventilation improves indoor air quality (IAQ). Because the only thing better than building a more energy-efficient home is building a more energy-efficient and healthier home.
Crawl spaces are notorious moisture traps. They are typically unconditioned spaces with numerous infiltration points yet few exit points. The moisture can lead to mold growth and deterioration of building materials. It can also provide an ideal environment for dust mite proliferation, asthmatic triggers, and insect infestation. Research has shown that moisture indoors also fosters the growth and transmission of viruses and bacteria. Controlling moisture indoors can thus limit the spread of infectious diseases. It is also imperative to maintain continuous ventilation, because airflow can affect how diseases spread indoors. The more stagnant the air is, the more likely diseases are to spread.
In addition, gases such as radon, toxic vapors, and other pollutants can infiltrate into crawl spaces and basements through the ground and through foundation walls. From there, they can infiltrate into the home via stack effect, and leakage through cracks in the ceiling and the HVAC ductwork. In a 2008 study published by ASHRAE, researchers tested 45 crawl spaces that were vented using foundation vents and found transmission of fungal species from the crawl into the living space in 21 cases. They also found that high moisture levels in vented crawl spaces were linked to high mold counts in the living space.
All of this research has shifted the thinking away from incorporating foundation vents in crawl spaces, and toward recommending instead that the crawl be sealed and that mechanical ventilation be installed. As researchers Bruce Davis and Cyrus Dastur of Advanced Energy, a Raleigh, North Carolina, building science consulting company put it, closed and conditioned crawl spaces do double duty (see “Closed Crawlspaces Do Double Duty,” HE 2005 special Issue: “The Best of Building Performance,” p. 32). Not only do they improve IAQ, but they also improve energy efficiency. Through a controlled study done on 12 identical homes in North Carolina, Davis and Dastur showed that sealed crawl spaces that had a continuous supply of conditioned air introduced into the crawl reduced moisture levels, maintained relative humidity (RH) at 60% or below, and reduced HVAC energy consumption for the entire home by 15–18% annually (see Figure 1).
Does That Make Scientific Sense?
But why is this the case? It seems counterintuitive that taking air you just spent money to heat or cool and blowing it into the crawl, and then exhausting it out as per the EPA guidelines, would lead to energy savings. The scientific answer is that it takes less energy to heat or cool a dry environment than to heat or cool a moist one. If you can reduce the RH in the crawl space or basement and reverse stack effect, you reduce RH in the house overall. And a drier house diminishes the load on the HVAC system.
This leads us to the next question. What is the best way to condition the crawl so that you derive the most benefit? Some people like to choose the simplest, least comprehensive approach; they believe that a dehumidifier will do the job. Yes, a dehumidifier will help draw out moisture, and assuming that you create a way of draining the moisture from the dehumidifier, will get rid of it. However, installing a dehumidifier is far from the best solution. It does nothing to improve the air quality of the crawl, because it is recycling the same air mass. It will not remove gases, radon, toxic vapors, and pollutants, nor will it reverse stack effect to prevent the pollutants from penetrating the living area. Dehumidifiers are also energy guzzlers and cost hundreds of dollars a year to operate.
So what is the best solution?
Based on the information presented by the aforementioned studies and the disadvantages of dehumidifiers, the answer is to create a way to expel any moisture- and pollutant-laden air that infiltrates the crawl space, and to replenish that air with conditioned air from the living space (see Figure 2). Reversing stack effect this way improves the IAQ of the entire home, as it expels gases, radon, toxic vapors, and other pollutants at the point of entry and prevents them from entering the living area. Continuous mechanical ventilation will also help to remove any moisture that is saturated within the walls or ceiling of the crawl. The continual movement of air over these surfaces helps draw out trapped moisture and wick it away.
EPA Weighs In
The EPA Indoor airPLUS technical guidelines for basements and crawl spaces require installing a ventilation unit to constantly exhaust moisture and pollutants, circulate air through the crawl, and draw in conditioned replenishment air. The EPA indoor airPLUS program builds upon the Energy Star program, and its specifications were developed to recognize homes with comprehensive IAQ features. A home, though, can’t earn the indoor airPLUS label unless it is also Energy Star certified. Energy efficiency and a focus on improving IAQ can go hand in hand.
Learn more about the Humidex system.
To learn more about research into mechanically ventilated crawl spaces, see the following studies:
Davis, Bruce, and Cyrus Dastur. “Closed Crawl Spaces: An Introduction to Design, Construction and Performance.” Advanced Energy, Raleigh (2005): 54. Download a copy of this report
Institute of Medicine, Division of Health Promotion. Damp Indoor Spaces and Health. Washington, DC: National Academies Press, 2004.
Li, Y, et al. “Role of Ventilation in Airborne Transmission of Infectious Agents in the Built Environment: A Multidisciplinary Systematic Review.” Indoor Air 17, no. 1 (2007): 2–18.
Malkin-Weber, Melissa, et al. Vented Crawlspaces as Mold Amplification and Delivery Systems? Measured Performance and a Dry Alternative. ASHRAE, 2008. See an abstract of this article.
“Technical Guidance to the Indoor airPLUS Construction Specifications: 1.4 Basements and Crawl Spaces,” EPA, accessed January 19, 2005.
It goes without saying that all this helps only if you first do your utmost to prevent water, moisture, and harmful pollutants from entering the crawl space. That is why the EPA guidelines state that you must first place a vapor barrier on the ground, seal all exterior entry points, and make sure that all piping ductwork is properly sealed. The guidelines also recommend that you insulate foundation walls, but “if insulation is applied to the interior side of a basement wall, materials that can act as vapor barriers (for example, polyethylene film, foil-faced paper or vinyl wall covering) should not be used. The vapor barrier can trap any moisture present in the foundation wall, creating moisture and mold problems on the wall's interior surface.”
And as you’d expect, based on all that we’ve discussed, EPA recommends installing an exhaust unit that expels air from inside the crawl space to the outside and replenishes it with conditioned air from the living space. It is important to exhaust and replenish at the same rate, so that the air pressure will remain in equilibrium. Negative pressure in the crawl space could draw unconditioned air from the outside, and positive pressure could force moisture into the crawl space walls or into the living area through cracks in the ceiling. A system that incorporates a unit that exhausts, and that works in tandem with a replenishment booster fan, takes all the guesswork out of creating an ideal solution. All that is required is proper installation (see Figure 3).
But (you say) won’t all this be prohibitively expensive? No. There are systems that cost much less to operate than a dehumidifier or even a lightbulb. For instance, Humidex offers a crawl space system that conforms to the EPA guidelines and costs pennies a day to run. It consists of a unit that expels the moist, polluted air outside and works in tandem with a booster fan that actively draws conditioned air down from the living space so that they expel and replenish at the exact same rate. At the maximum ventilation rate, this system uses only 76 watts of energy. Most of the time, the system won’t have to operate at the maximum rate, so energy consumption will be even less.
We have all been accustomed to focusing on building homes tighter, and improving energy efficiency wherever possible. All of that focus comes at a cost, for recent studies have shown that it has a negative effect on IAQ and occupant health. But there is no reason why the two can’t coexist. When it comes to addressing the crawl space, incorporating the solutions promoted by EPA and backed by scientific studies will accomplish both goals. You can have your cake and eat it too!
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