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THE MEASUREMENTS

Ventilation brings with it a few units of measure that may need explanation. The explanations and definitions in this section are used throughout this paper. Cubic feet per minute (CFM) is the common measure of airflow. It reflects the number of cubic feet of air flowing in one minute. The Standard International (SI) system uses liters per second (l/s). One CFM is about one-half l/s. Static pressure, usually measured in inches of water, is a measure of duct resistance. When used as part of a fan rating, it is the duct resistance anticipated when rating the airflow of the fan; if the actual duct system in an installation provides more resistance, the fan will produce less (CFM of) airflow. The SI system uses Pascals (Pa); 0.1 inches of water is about 25 Pascals. Air changes per hour (ACH) is the number of times in one hour that a volume of air equal to the volume of a room, or a building, is exchanged. Note that one air change does not remove all the air in the room; because of mixing during the exchange, about one-half of the air is new after one air change. Sones are a common unit of measurement that translates decibel readings into numbers that correspond to the way people sense loudness. The average quiet refrigerator operates at one Sone. Two Sones sounds twice as loud as one Sone.

How much air do we need?

Air for breathing is such a basic human need that it is surprising it was not the first requirement written into all building codes. The human body can go for days, even weeks without food, a few days without water, but only minutes without air. And the air must be of a certain quality.

Research done in the ’70s and ’80s measured the condition of indoor air, the relationship between the condition of the air and various indoor and outdoor influences, and the amount of outdoor air required for human comfort and health.

Today’s standards reflect the consensus of various experts in industry and science. Although variously stated, there is agreement that we need about 15 cubic feet per minute (CFM) of fresh air, per person. Living spaces of average size will furnish that quantity of air when ventilated at roughly 0.35 air changes per hour (ACH). In general, older homes may have one to two ACH just through natural leakage. Tightly sealed new homes cannot provide the required air through accidental ventilation.

Mechanical ventilation is the only predictable method of furnishing air for breathing and comfort.