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.
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.
It isn’t hard to find authors who warn that exhaust-only ventilation systems can be dangerous. Here are some examples from the Web:
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,…