Glossary of Terms
Building Envelope - The elements of a building which enclose conditioned spaces through which thermal energy is capable of being transferred.
Conditioned Space - A heated or cooled space, or both, within a building and, where required, provided with humidification or dehumidification means so as to be capable of maintaining a space condition falling within the comfort envelope per ASHRAE 55.
Conduction - The passing of heat through a roof material into the layer in contact directly beneath the surface.
Convection - The heating of the air that passes over a hot surface.
Cooling Degree Day (CDD) - A unit measuring the severity of summer or cooling energy conditions. Using a reference temperature of 65° F, degree days are determined by the difference in the mean daily temperature and the reference temperature. For example, a day with 85° F mean temperature has 20 CDD (85-65=20). The annual Cooling Degree Days are the sum of the degree days over a calendar year.
Energy Cost - The total estimated annual cost for purchased energy for the building, including any demand charges, fuel adjustment factors and delivery charges applicable to the building.
Heating Degree Day (HDD) - A unit measuring the severity of winter or heating energy conditions. Using a reference temperature of 65° F, degree days are determined by the difference in the mean daily temperature and the reference temperature. For example, a day with 50° F mean temperature has 15 HDD (65-50=15). The annual Heating Degree Days are the sum of the degree days over a calendar year.
Infrared Emittance - The ability of a material to radiate the heat that builds up in the material from absorbed or non-reflected radiation. The emitted energy is in the far infrared part of the spectrum. Solar radiation that is absorbed heats a roof surface. The absorbed energy is no longer solar energy – it is characterized by a surface material’s temperature which is lower than the equivalent solar temperature. A roof surface continuously exchanges radiation in the infrared part of the spectrum with the sky at its equivalent temperature. On clear nights, the sky temperature is lower than the air temperature. The amount emitted is in direct proportion to a roof surface’s infrared emittance.
R-Value - The resistance to heat flow. This unit characterizes the insulating value of materials that are used to minimize heat flow. Higher R-values indicate a material’s ability to resist more heat flow.
Roof Assembly - All roof/ceiling components of the building envelope that are horizontal or sloped at an angle less than 60 degrees from the horizontal through which heat flows, thus creating a building transmission heat loss or gain.
Solar Reflectance - The solar radiation in the near ultraviolet to near infrared spectrum that is reflected away from a surface expressed as a percent or within the range of 0.00 to 1.00.
Solar Spectrum - Radiation originating from the sun, including ultraviolet, visible, and near-infrared radiation. Approximately 99% of solar energy lies between wavelengths of 0.3 to 3.5 micrometers.
Sustainability - “Meeting the needs of present generations without compromising the ability of future generations to meet their needs.” – The World Commission on Environment and Development, 1987 (U.N. Brundtland Report)
Thermal Conductance - Time rate of heat flow through a body from one of its bounding surfaces to the other for a unit temperature difference between the two surfaces, under steady conditions (Btu/h*ft2* F) [W/9m2*K].
Thermal Resistance - The reciprocal of thermal conductance.
U-Value - The overall rate of heat flow. It is the coefficient of heat transfer from outside air to inside air. It is expressed in BTU/h*ft2*F. The lower the U-value, the less heat flow that occurs through a roof assembly from the warm side to the cooler side.
Urban Heat Island - A built environment wherein the large proportion of dark absorbing surfaces (such as asphalt paving and dark roofs) trap sunshine and radiate it back into the atmosphere, causing higher ambient temperatures and higher pollution levels.
Variegated Roof Surface - A surface marked with patches, spots or areas of different colors. In contrast to a surface having either one color or a regular pattern or texture, a variegated surface has a varied design of several colors and/or textures.
Ventilation - The process of supplying or removing air by natural or mechanical means to or from any space. Such air shall be permitted to be conditioned or unconditioned.