Cap Flashing

A flashing overlapping the vertical leg of base flashing to prevent water from migrating behind the base flashing.

Capillary action

(1) The ability of water to move through materials, even upward against gravity, through small tubes or spaces. (2) The movement of water within the confined spaces of a porous material or between two adjoining hydrophilic materials due to the attractive force of surface tension. Only significant in gaps of less than about 1/8” (3 mm).

Capillary Break

A hydrophobic material or non porous material (such as glass, plastic, or metal) or gap between parallel layers of material (often less than 1/16” or 1.5 mm) sufficient to stop capillary action.

CFM-cubic foot per minute

Cubic foot/feet per Minute; measure of Airflow. A unit of volumetric flow rate, often used as a metric of ventilation, airflow, or air leakage.


The number of cubic feet per minute of air flowing through the fan housing of a blower door when the house pressure is 50 pascals (0.2 inches of water). This figure is the most common and accurate way of comparing the air-tightness of buildings that are tested using a blower door.

Coefficient of Performance (COP)

A ratio calculated by dividing the total heating capacity provided by the heat pump, including circulating fan heat but excluding supplementary resistance heat (Btu's per hour), by the total electrical input (watts) x 3.412. (See Heating Seasonal Performance Factor, below.)


A snakelike piece of copper tubing surrounded by rows of aluminum fin that clamp tightly to the tubing in order to aid in heat transfer.


The change of state from vapor to liquid. A common factor in moisture damage. Occurs on surfaces, which must be cooler than the air containing vapor next to it. Vapor supply to the condensation surface is usually by airflow but can be by diffusion.


The coil in an air conditioning system where the refrigerant condenses and releases heat


Transfer of heat, but not mass, from warmer to colder regions of a substance, or adjacent substances in contact.


Circulation caused by the buoyancy of heated or cooled fluids; more likely a reference to air than to water in HVAC.

Cooling Degree Day

The number of degrees that a day's average temperature is above 65 Fahrenheit, the temperature at which people will start to use air conditioning to cool their buildings.


Color Rendering Index. A metric of the ability of a lighting source to render colors correctly relative to ideal light. A CRI of 100 is the maximum value. For reference incandescent lights are in the 95-100 range and compact fluorescents are in the mid-80’s.