Articles on mechanical ventilation commonly warn builders that exhaust-only ventilation systems can pull radon into a house through foundation cracks. The warning makes intuitive sense: after all, an exhaust-only ventilation system works by depressurizing a house with respect to the outdoors, and it seems obvious that depressurization could pull soil gases into a basement.
One thing I’ve learned over the years, however, is that just because an idea is intuitively obvious, doesn’t mean it’s true. Throughout history, many observers have speculated; far fewer have actually made measurements.
Defining an exhaust-only ventilation system
An exhaust-only ventilation system depends on one or more exhaust fans (usually bathroom exhaust fans) that either operate continuously or intermittently. This type of ventilation system is usually set up to ventilate a house at the rate recommended by ASHRAE 62.2 — somewhere in the range of 50 to 100 cfm for most homes.
The air that is removed from a home by an exhaust fan is replaced by outdoor air entering the home through random cracks in the building envelope.
Examples of warnings
It isn’t hard to find authors who warn that exhaust-only ventilation systems can be dangerous. Here are some examples from the Web:
A radon primer
Radon is a naturally occurring radioactive gas that is present in some soils. It can enter a house through cracks in the building’s foundation.
Indoor radon levels can be tested; if long-term testing shows radon levels above 4 picocuries per liter, it’s a good idea to install a radon mitigation system. (For more information, see All About Radon.)
Most experts explain that radon gas is drawn into a house by the stack effect. The stack effect is a function of delta T — the difference between the indoor and outdoor temperature — so the effect is strongest during the winter. Indoor air escapes through ceiling cracks; as a result, the lower sections of a home become depressurized, and soil gases are pulled into the house.
Authors who warn about possible dangers from exhaust-only ventilation systems assume that, since exhaust fans depressurize a house, the fans probably increase the radon risk.
Are there any studies out there?
It turns out that a few researchers have measured radon levels in homes with exhaust-only ventilation systems. The most relevant research paper that I have found is a 2002 paper authored by two Alaskan researchers, Richard Seifert and Jack Schmid: “Exhaust Ventilation and Radon Induction in a Subarctic Climate.”
The researchers gathered data at eight Alaskan homes over a three-year period from 1998 to 2000. The range in airtightness for the eight homes was from 2.39 to 8.75 ach50. One of the homes, called the Goldstream house, had an exhaust-only ventilation system. The exhaust system depressurized the home to -7 Pascals, which is a significant level of depressurization.
To find out how the home’s exhaust ventilation system affected indoor radon levels, the researchers turned off the ventilation fan for five days. The researchers wrote, “Clearly the radon level goes up steadily, nearly doubling in the five days it is turned off. Although counterintuitive, it nonetheless appears that radon induction is reduced by the ventilation fan. One hypothesis is that … the net effect of the exhaust ventilation may also be dilution of the radon.”
The data refute intuitive assumptions
Intrigued by these researchers’ findings, I contacted Max Sherman, a senior scientist and ventilation expert at Lawrence Berkeley National Laboratory. Responding by e-mail, Sherman wrote, “I did do a modeling study some years ago: “Simplified Modeling for Infiltration and Radon Entry.” This study and the data you found [from the paper by Seifert and Schmid] refute the ‘common sense’ notion that radon will increase with exhaust ventilation. Usually radon concentrations go down with ventilation.
“The physics is not complicated. If you take a given house and add an exhaust fan, you will decrease the internal pressure. This will cause an increase in infiltration and it will also likely increase radon entry. So you get more entry but also more dilution. While one can construct pathological cases where radon concentrations could increase, it will almost always decrease as a result of this. It may not decrease as fast as other pollutants, but the concentration will decrease. All data I have seen bears this out, in general.
“To me this is not an interesting case. The interesting case is what happens when one tightens the envelope and adds exhaust ventilation as one might do with weatherization or other building retrofits. Here the outcome space is much richer because it depends on the starting conditions, whether the tightening changes the radon entry paths, etc.
“Scientifically the question is rich, but from a practical standpoint maybe one should not tighten and add exhaust ventilation when radon is a serious issue. Tightening and adding supply ventilation, for example, is essentially a built-in radon mitigation system because you are reducing radon entry while providing ventilation. We know there are other practical issues with supply ventilation depending on climate, but those may be worth tackling when radon is a consideration.”
Anecdotal evidence tends to support Sherman’s generalizations. One such anecdote was recounted to me by Dick Kornbluth, a radon mitigation contractor in Syracuse, New York. “I visited a retired engineer who had a house with a really high radon level,” Kornbluth told me. “He decided to do his own radon mitigation system. He installed ductwork in the basement that he connected to a fan blowing out through the band sill. The fan was depressurizing the basement — that was his system. I thought that would have raised his radon levels, but it lowered them. It turns out that he was was diluting the basement air, either with air from upstairs or with outdoor air entering the basement through cracks near the sill.”
How many Pascals of depressurization are created by the average exhaust fan?
The Alaskan researchers (Seifert and Schmid) reported that the exhaust ventilation system at the Goldstream house depressurized the house to -7 Pascals. This level of depressurization is higher than levels measured by other reseachers.
Relevant data were reported by Andy Shapiro, David Cawley, and Jeremy King, in a 2000 paper titled “A Field Study of Exhaust Only Ventilation System Performance In Residential New Construction in Vermont.” The researchers studied 43 homes with exhaust-only ventilation systems; the homes had an average airtightness of 4.45 ach50.
The Vermont researchers reported, “The pressures induced by fans in these tests, averaging in the range of -1 Pa, were low relative to pressures induced on a house by natural forces including wind and temperature driven stack effect. As a comparison, calculations of natural infiltration often use a seasonal average of 4 Pascals as the magnitude of these forces. These forces are about 4 times greater than the pressure effects created by the EOV [exhaust-only ventilation systems].”
Radon entry is all about the stack effect
The main driver of radon entry into a home is the stack effect, which is why radon concentrations tend to be higher in winter than in summer. (Another reason: occupants are more likely to open windows during the summer.)
In many homes, the stack effect has more of a depressurizing effect than the typical exhaust fan. Moreover, even when the depressurizing effect of a fan is strong, some of the air pulled into the house is fresh outdoor air that enters from above-grade cracks rather than soil gas entering through below-grade cracks. According to the best available data, the net effect of operating an exhaust-only ventilation system is, in most cases, a reduction rather than an increase in indoor radon levels.
That said, exceptions to this generalization undoubtedly exist. Our knowledge about the effects of exhaust-only ventilation systems on indoor radon levels would benefit from more research.
The bottom line: if you are worried about radon, perform a test. “If you want to install a continuous exhaust-only ventilation system, I would just say, monitor the radon levels,” Kornbluth said. “Radon may go up, or it may not go up. And I would also say, ‘Don’t do short-term testing.’ The problem with short-term testing is you might see a short-term spike and think you are going to die. Install an Alpha Track test kit for 6 months and don’t worry about it.”
Finally, if you are sealing a home’s air leakage paths as part of a weatherization job, it’s important to seal below-grade cracks as well as above-grade cracks. Recommended measures include caulking cracks in the slab; caulking penetrations through the slab; caulking the crack at the slab perimeter; and installing an airtight sump lid if the basement has a sump.
Martin Holladay’s previous blog: “What’s the Definition of an ‘R-20 Wall’?”