When you turn on an exhaust fan, it can cause an imbalance in the air pressure in your house, thereby depressurizing it. The air in the house naturally wants to resume balance, and the pressure difference draws outdoor air in through vents or random holes in the outer shell of the home, or what we call the “building envelope”. The air drawn in is called “make-up air”.
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Pollutants will only be pulled into the living space if there are holes in the structure and air pressure to push the pollutants through the holes. An exhaust fan will remove the stale, stagnant air, but homeowners need to pay attention to the make-up air that replaces it. If the incoming air draws in pollutants like radon, mold, debris, or other contaminants, the indoor air quality will suffer. It would certainly be desirable if all the make-up air were pollutant-free. However, this is only possible when a house is very tightly constructed, and all the air is filtered as it passes through deliberate inlets. In the real world, all houses have some uncontrolled air movement through random holes in the structure (infiltration), which brings some pollutants along with it.
A whole-house ventilation system can be used to pressurize or depressurize a house by controlling the amount of air entering and exiting the house.
The direction of the air flow is determined by the air pressure inside the house relative to the outdoor air pressure. When a house is pressurized (e.g., with a whole-house supply ventilation system), air will be continually exfiltrating through the random holes in the structure, so soil gases and contaminants from within the building cavities will be prevented from entering the living space. If the make-up air is adequately filtered before it passes through the ventilation system’s inlet, outdoor airborne pollutants will be prevented from entering. Some airborne pollutants like larger particles can be filtered out and some cannot. Work with your ventilation contractor to ensure the system will do what you want.
Tightening a house is important for comfort and energy-efficiency, and it controls how the make-up air enters the living space.* In a well-designed mechanical ventilation system, which can be a supply-only, an exhaust-only or a balanced supply and exhaust system, fresh ventilation air will be provided to the occupants when desired, in order to help ensure a good indoor air quality and a comfortable indoor environment.
The Home Ventilating Institute (HVI) recommends that you work with a licensed ventilation contractor to get the mechanical ventilation system that is right for your region and for your particular home. Products that are HVI-Certified and installed professionally will help ensure that your indoor air is fresh, clean, and healthy.
Adapted from: Understanding Ventilation: How to design, select, and install residential ventilation systems by John Bower © 2010 The Home Ventilating Institute (HVI).
* Though uncommon elsewhere, the state of Washington still requires inlets if there is no forced air system or distributed mechanical supply. In this case, even though air will enter via both through-the-wall vents and random holes in the structure, in a well-designed mechanical ventilation system, the total area of the through-the-wall vents should be greater than the total area of the random holes. This way, more air will enter the through-the-wall vents to provide fresh, filtered air to the occupants.
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