Albedo
Reflective power; the percentage of incident light or electromagnetic radiation that is reflected by a surface. High albedo roofs reflect most of the solar energy away from their surface before it is absorbed and converted into heat energy. A High Albedo roof system reduces cooling energy costs by reflecting away most of the solar energy that strikes its surface. Georgia statutes, for example, define a high albedo roof as meeting two requirements: 1) a minimum total solar reflectance of 0.75 when tested in accordance with ASTM E903; 2) a minimum infrared emittance of 0.75 when tested in accordance with ASTM E 408.
Building Envelope
The elements of a building that enclose conditioned spaces through which thermal energy may be transferred to or from the exterior. Typically includes the roof, walls, windows and slab.
CDD Cooling Degree Day
A unit based upon temperature difference and time, used in estimating cooling
energy consumption. It measures the severity of summer conditions. For any one
day when the mean temperature is more than a reference temperature (65ºF for
example), there are as many degree-days as degrees Fahrenheit temperature
difference between the mean temperature for the day and the reference
temperature. For example, a day with 75º F mean temperature has 10 CDD; 75 - 65
= 10.
The annual Cooling Degree Days are the sum of the degree-days over a calendar
year.
Emissivity
The rate at which absorbed energy is radiated away from an object. In roofing it describes the ability of the roof covering to quickly release absorbed heat energy. A desirable roofing membrane will easily release its absorbed heat energy, keeping the roof cooler than those that get hotter by slowly radiating their stored energy.
HDD Heating Degree Day
A unit based upon temperature difference and time, used in estimating heating
energy consumption. It measures the severity of winter conditions. For any one
day when the mean temperature is less than a reference temperature (65º F for
example), there are as many degree-days as degrees Fahrenheit temperature
difference between the mean temperature for the day and the reference
temperature. For example, a day with 55º F mean temperature has 10 CDD; 65 - 55
= 10.
The annual Heating Degree Days are the sum of the degree-days over a calendar
year.
The resistance to heat flow, this is the value most often used to indicate the insulating value of materials used to conserve energy. The higher the R-value the more resistance to heat-flow.
The overall rate of heat flow; the coefficient of heat transfer from outside
air to inside air. In roofing it is the time rate of heat flow per square foot
from the warm side of the roof to the cooler side, per unit temperature
difference between the two sides.
Btu / h · ft2 · F. The lower the U-value, the less heat flow occurs through a
roof assembly.
Urban Heat Island
A condition whereby highly developed urban areas are noted to be warmer than the surrounding country-side. The temperature rise is caused by a combination of reducing natural tree canopy and the abundance of heat absorptive dark materials on roofs, walls, streets, parking lots, etc. Atlanta's heat island has been measured up to 12º F warmer than the surrounding countryside.