Q. Do tight houses have higher levels of humidity, radon, and indoor air pollution than older, leakier houses?

A.Terry Brennan responds: With a strong enough contaminant source, any house can have an indoor air quality problem. If contaminants are properly controlled, a tight house can have as clean or cleaner air than a leaky house.

For example, some of the houses with radon problems I’ve worked on would need 20 to 30 air changes per hour to get them below the recommended levels for indoor radon. This could be done by adding big enough fans, but the house would cost a fortune to heat and you couldn’t keep candles lit from the breeze. So the first rule is "No strong sources of air contaminants in the house."

Some sources, however, like moisture and odors from kitchens, laundries, and bathrooms, are unavoidable. These are actually pretty easy to control by locally exhausting these rooms. Local exhaust is better than a general increase in ventilation because it not only brings in outside air to dilute the contaminant, it also keeps it from spreading to the rest of the house. ASHRAE recommends 100 cfm of intermittent exhaust in the kitchen and 50 cfm in each bathroom.

Other unavoidable contaminant sources are scattered throughout the building—notably people. People give off bioeffluent (body odor) as well as carbon dioxide and water vapor from breathing. (With too much CO2 in the air, you feel drowsy and overheated.) There are also fungi, bacteria, mites, insects, rodents, dogs, and cats — who all give off odors — even in the cleanest of houses. ASHRAE recommends (Standard 62-1989) that residences have .35 air changes per hour of general ventilation air to supply oxygen for breathing and to control contaminants from these sources.

The ventilation system should be designed to control the last category of unavoidable contaminants — soil gases, which include water vapor and occasionally radon and methane. Use the air handling equipment to slightly pressurize the basement or use an exhaust fan to depressurize the subslab drainage layer. Similarly you should design the ventilation system to slightly depressurize the upper parts of the house (in northern climates) to protect the walls and ceilings from moisture condensation.

Terry Brennan is a building researcher in Oriskany, N.Y.