Process whereby a porous material extracts on or more substances from an atmosphere, a mixture of gases, or a mixture of liquids. (from Gatley, “ Understanding Psychrometrics”


Air Conditioning Contractors of America is a group of over 4,000 air conditioning contractors who work together to improve out industry, promote good practices, and keep homes and building safe, clean ad comfortable.


Air changes per hour. A common unit of measure of ventilation rate for a space, or air leakage rate for a building, defined as the volumetric flow rate divided by the volume of the space considered.


A substance or compound used for bonding surfaces together, usually applied in the form of a liquid or paste. An adhesive and sealant or base coat may be the same material.


(1) Process in which fluid molecules are concentrated on a surface by chemical or physical forces, or both; (2) surface adherence of a material in extracting one or ore of the substances present in an atmosphere or mixture of gases and liquids, unaccompanied by physical or chemical change. (from Gatley, “Understanding Psychrometrics”)


(Annual Fuel Utilization Efficiency) - A rating that denotes the efficiency of gas heating equipment. It is the amount of heating your equipment delivers for every dollar spent on fuel. A higher rating indicates more efficient equipment. This rating is calculated in accordance with the Department of Energy test procedures.

Air Barrier

A term often abbreviated from an Air Barrier System or Air Barrier Material. The complete air barrier system is comprised of materials and assemblies, each with their own performance requirements.

Air Barrier Material

A material that has sufficiently low air permeance and adequate strength that it can be part of an air barrier system. Recommended maximum air permeance for a material is 0.02 l/(s-m2)@ 75 Pa (0.004 cfm/sf @ 0.3” WC) when tested according to ASTM E 2178 or E 283.

Air Barrier System

Air barriers are three-dimensional systems of materials designed, constructed, and/or acting to control air flow across a building enclosure, or between a conditioned space and an unconditioned space. In multi-unit/townhouse/apartment construction an air barrier system should also separate the conditioned air from any given unit and adjacent units. The pressure boundary of the enclosure should, by definition, be coincident with the plane of a functional air barrier system. In multi-unit/townhouse/apartment construction the air barrier system may also be the fire barrier and smoke barrier between units. In such assemblies the air barrier system must also meet the specific fire resistance rating requirement for the given separation.

Air Change

Replacing the volume of a space in one hour; space, volume in cubic feet divided by 60 minutes yields 1.0 Air Change in CFM.

Air densities

The density of dry air at sea level is 0.0745 lb/ft2 at 70F or about 1/800th the density of water. But as altitude and temperature (or both) increases, the density drops dramatically. This is because the density of air is proportional to the pressure and inversely proportional to temperature. And the higher you go into the atmosphere, the lower the pressure gets. Pressure is approximately halved for each additional increase of 56 km in altitude.

Air Handler

A steel cabinet containing a blower with cooling and/or heating coils connected to it.

Air Infiltration

Uncontrolled inward leakage of air (that may contain entrained water vapor) through cracks and insterstices in any building element and around windows and doors of a building, caused by the pressure effects of wind or the effect of differences in the indoor and outdoor air density. (from Gatley, "Understanding Psychrometrics")

Air Pressure Boundary

The air pressure boundary is the boundary (comprised of a series of planes to form a three dimensional boundary) that generates the largest pressure drop (usually much more than half the total) when the enclosure is subjected to a pressure difference.

Air Transported Moisture

Moisture movement associated with the movement of air from one place to another by means of a pressure differential.


(American Society of Heating, Refrigeration, and Air-Conditioning Engineers) Published standards for introducing Outside Air for occupant health and comfort; 62.1 for commercial, 62.2 for residential; now adopted into law as part of most building codes. The residential requirement, “Ventilation and Acceptable Air Quality in Low-Rise Residential Buildings” has been in existence since 2003. The standard describes three primary sets of requirements for acceptable indoor air quality in homes; they are whole house ventilation, local exhaust and pollutant control. The most common elements are the following: • Whole house mechanical ventilation using the formula ; Ventilation Rate= 7.5cfm ( number of bedrooms + 1) x (0.01 x Conditioned floor area or home) • Bathroom exhaust specified at low sone, 50 cfm intermittent or 20cfm continuous • Kithchens vented to the exterior at 100 cfm intermittent or equivalent to 5 air changes per hour if continuous. • Clothes dryers vented to the outdoors • Minimum filtration of ducted supply air systems of MERV 6