Weatherization and Indoor Air Quality

A version of this article appears in the Spring 2017 issue of Home Energy Magazine.

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Air sealing is one of the best ways to lower heating-and-cooling bills for many home weatherization programs, and especially so for the federal Weatherization Assistance Program (WAP). Most homes receive blower door-guided air sealing to reduce the unwanted infiltration of outdoor air.

But in most older homes, natural infiltration is also what brings fresh air indoors and helps drive out indoor pollutants. Could air sealing degrade indoor air quality (IAQ)? Research that we conducted as part of the recent national evaluation of WAP managed by Oak Ridge National Laboratory suggests that it can indeed do so—but also shows that properly implemented mechanical ventilation can reduce these unwanted effects.

The research that we undertook on behalf of the Department of Energy was one of largest studies of its kind, involving more than 500 homes in 35 states. We were fortunate in being able to randomly assign which homes received weatherization during the study and which remained in a control group of unweatherized homes. So-called randomized control trials like this are generally considered to be the gold standard for research, since they ensure that the experimental treatment (weatherization) is the only systematic difference between the two groups.

Project Design

To implement our project, we used statistical sampling methods to select geographic areas around the country (see Figure 1). Then we contacted weatherization agencies in these areas to get waiting lists of clients scheduled to be weatherized in the near future. We randomly assigned these households to the treatment group or to the control group and then recruited households into the study with the offer of a gift card in exchange for participation. Households in the control group were asked to delay weatherization for a few months and were offered a bigger incentive for agreeing to do so.

Study Home Locations

Figure 1. The study was implemented in 35 states, with an average of 7 homes per location.

For both groups, we visited the home to deploy samplers that measured radon and formaldehyde levels over a one-week period, and to install data loggers to track CO, temperature, and relative humidity (RH). After a few weeks, homes in the treatment group received weatherization following the normal state and weatherization agency protocols, while those in the control group did not. Weatherization consisted of installing insulation, replacing outdated heating systems and implementing other measures typically installed by the program; in addition, every home in the group was air sealed. Air sealing reduced measured leakage levels in the homes by an average of 30%.

The study involved more than 500 single-family homes like these in Wisconsin and Minnesota.

Within a month of weatherization we returned to all homes to deploy new radon and formaldehyde samplers, which we later collected, along with the data loggers. This gave us before- and after-weatherization snapshots of these IAQ parameters for homes that received weatherization, along with comparable data for the same time period for homes in the control group that did not receive weatherization.

A typical radon canister and formaldehyde sampler deployment in the study. A carbon monoxide logger is just visible in the background.

The study focused on homes in heating-dominated climates, and we conducted the research during the heating season, when windows and doors would be kept closed and indoor pollutant concentrations would be higher. Because the study focused in particular on the effect of weatherization on indoor radon levels, our sample was deliberately chosen to include parts of the country known to have a higher incidence of elevated radon levels.

Radon and Formaldehyde

Radon is an odorless radioactive gas that has carcinogenic properties. It is produced deep underground by natural processes, and typically enters the home by migrating through the soil and seeping through cracks and gaps in the foundation. EPA estimates that radon is the second leading cause of lung cancer after smoking, and is responsible for about 21,000 deaths per year in the U.S.

Radon is also a fickle foe. Indoor radon levels vary wildly from one home to another—even homes that are in close proximity—and radon levels in a given home can fluctuate dramatically over time. In the United States, radon levels are generally expressed in units of picocuries per liter (pCi/l). Our one-week tests showed radon levels that ranged from near zero to almost 60 pCi/l on the lowest occupied level of the home. EPA recommends that homeowners take action when radon levels exceed 4 pCi/l, and the data that we collected suggest that nearly a quarter of homes that enter the program in high-radon areas (EPA zone 1 counties) have radon levels that exceed this threshold even before they are weatherized. (At the conclusion of the study, radon mitigation systems were installed in all homes with elevated radon levels. For more on this, see “Radon Mitigation—A New Business Opportunity,” HE Winter ’16, p. 16.)

On average, radon levels increased among treated homes following weatherization and decreased in the control group, with a control-adjusted change of +0.4 pCi/l (see Figure 2). While this is not a large increase in absolute terms, preweatherization radon levels in the study homes averaged only about 2.0 pCi/l, so the observed change represents about a 20% relative increase. We saw the largest impact on radon levels among site-built homes in high-radon counties. These homes had an average control-adjusted increase of 1.1 pCi/l, a nearly 40% increase from the average preweatherization radon level of 2.8 pCi/l.

Changes in Radon Levels Postweatherization

Figure 2. After weatherization, radon levels rose among treated homes, and declined in the control group.

To be sure, our testing measured radon levels over a relatively short period before and after weatherization, so the results for any particular home in the study are subject to considerable uncertainty. But the large number of homes in the study—along with the random assignment to treatment and control groups—makes it unlikely that these results are a statistical fluke or the result some factor other than weatherization. In fact, the statistics suggest that there is less than 1 chance in 1,000 of observing a difference as large as we did between the two groups if weatherization in fact has no effect on radon levels.

The study also found a statistically significant increase in formaldehyde levels, despite that fact that budget constraints limited this testing to about 120 homes. Formaldehyde—a lung irritant—is emitted by many building materials and furnishings, especially those made of composite wood materials, and is also one of the byproducts of cigarette smoke. Formaldehyde levels in the study homes averaged about 15 parts per billion (ppb) prior to weatherization. Levels rose in both groups following weatherization, but rose more among treated homes, for a control-adjusted increase of +1.6 ppb. We saw the largest increase among mobile homes in the study, but only 23 mobile homes received formaldehyde samplers, so these results are uncertain. (Less equivocally, the radon testing in mobile homes showed little detectable radon—and no impact from weatherization. This is probably because most mobile homes sit above ground, allowing soil radon to dissipate before it enters the home.)

Carbon Monoxide and Relative Humidity

The CO loggers revealed a different story. The risks from acute CO poisoning are well known, and WAP goes to great lengths to ensure that treated homes have working CO detectors, and that furnaces, water heaters, and other combustion devices are not emitting high levels of CO into homes. But much less is known about the extent to which households are exposed to low levels of CO from sources such as ranges, or even automobile exhaust that migrates from attached garages into homes. Symptoms of CO poisoning can occur at ambient concentrations of 70 ppm, and become dangerous at ambient concentrations above 150 ppm; health effects at concentrations below 70 ppm are less well known.

Reassuringly, the CO loggers that we deployed showed very low ambient CO levels in the study homes. CO levels in two-thirds of the homes never exceeded 5 ppm over the duration of the study. About one in ten homes showed episodic spikes in CO to between 20 and 90 ppm, but only about one in a hundred had persistent CO levels above 5 ppm.

Overall, these proportions remained substantially unchanged following weatherization. We did, however, document a few cases where we could trace a reduction in indoor CO levels to weatherization activity. In one case, a home with a tuck-under garage and an open doorway between the garage and the living space showed regular indoor CO spikes—probably due to car exhaust entering the home—that ceased after weatherization installed a door between the home and the garage. In a second case, we observed persistent indoor CO of 10–30 ppm that cycled up and down at about the same frequency as a typical furnace cycles. CO levels dropped to near zero when the weatherization program replaced the furnace.

Indoor temperature and RH remained largely unaffected by weatherization, at least in the short term. Average temperatures in the homes ranged from 60 to 80 °F, but changed by an average of only 0.3 °F following weatherization. RH increased by an average of 1%.

Mechanical Ventilation

Our study was conducted in 2010 and 2011, just as WAP was beginning to roll out a requirement that the program comply with mechanical ventilation guidelines specified in ASHRAE Standard 62.2. Only 2% of the study participants received the kind of continuous mechanical ventilation equipment that ASHRAE 62.2 calls for in older homes that have been air tightened; installing this equipment is now standard WAP practice.

To gauge the potential impact of this kind of mechanical ventilation on indoor radon levels, we conducted a small follow-up study involving 18 homes in Colorado, Iowa, Minnesota, and Ohio that formed part of the larger study—and that had been shown to have moderately elevated radon levels. We installed continuously operating exhaust fans in these homes (following standard practice under WAP), and connected the fans to timers that operated the fans at the 62.2 ventilation rate every other week while we tracked radon levels in the homes continuously. This allowed us to compare radon levels in the homes with and without the ventilation system operating (see Figure 3).

Changes in Radon Levels with Whole-House Exhaust

Figure 3. Radon levels in this home dropped considerably whenever the whole-house exhaust fan was operated.

Prior to implementing the study, we wondered: Would the exhaust fans reduce radon levels by increasing the ventilation rate of the home, or might they increase indoor radon by drawing more soil gas into the home? The results clearly showed that the increased ventilation effect dominates. All but one home showed a reduction in average radon levels associated with fan operation, and six of the homes showed reductions of 15% or more; the overall average reduction was 12%. Though by no means definitive, the results of this experiment suggest that WAP is on the right track in complying with ASHRAE 62.2.

Overall, the study showed that air-sealing efforts from weatherization can indeed affect IAQ, and in ways that are not always positive. This sets up potentially difficult policy choices regarding trade-offs between energy savings and health outcomes, not to mention how programs should deal with homes that have preexisting IAQ issues, such as elevated radon. These choices are made even more difficult by a dearth of solid studies relating health outcomes to airtightness and ventilation rates. Debates about the fundamentally apples-and-oranges trade-off between energy and health will no doubt continue in the years to come. In the meantime, what seems clear is that mechanical ventilation is a useful complement to air sealing in existing homes to help mitigate the adverse effects of weatherization.

Scott Pigg is a principal researcher at Seventhwave.Dan Cautley is a senior research engineer at Seventhwave. Paul Francisco is a research engineer at the University of Illinois.

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