British Thermal Unit (BTU)
A standard measure of the amount of energy required to raise one pound of water by one degree Fahrenheit.

The accumulation of water vapor resulting from warm, moist air coming in contact with a cold surface and cooling to its dew point temperature. Condensation may occur when cold window glass is exposed to humid indoor air. Highly insulated glass windows and doors reduce condensation.

The transfer of heat through solid materials (such as glass) through direct contact. Heat flows from areas of higher temperature to areas of lower temperature.

The flow of heat that occurs through circulating gases or liquids within a confined space (e.g., the rise of warm air and the fall of cool air within a room).

Curtain Wall
The large seemingly continuous glass walls that clad the exterior of commercial buildings and office towers.

Electrochromic Coating
A very thin metal or metallic oxide layer deposited on a pane surface. An electrochromic coating dynamically manages visible and infrared light. The emissivity of the coating can be configured.

The relative ability of a surface to reflect or transmit radiated heat. The lower the emissivity, the less radiated heat passes through a window.

Energy Star® Program
Energy Star is a joint program of the U.S. Environmental Protection Agency and the U.S. Department of Energy (DOE) to promote energy conservation and environmental protection through the use of energy-efficient products and practices. With the help of the Windows industry, the program promotes sales of energy-efficient windows, doors and skylights. All Energy Star windows must meet standards for both u-factor and solar heat gain coefficient.
Fenestration (Fenestration Products)
Fenestration refers to residential and commercial windows, glass doors and skylights, as well as their associated interior or exterior peripherals (e.g., shades, blinds).

Gas-Fill (Gas-Filled)
A gas such as argon or krypton placed between glass panes in an IGU to improve its insulating value (reduce its U-factor) by suppressing conduction.

Gas tight. A hermetic seal does not allow gas to pass through it.

Insulating Glass Unit (IGU)
An Insulating Glass Unit is the transparent component of a window or door assembly. An IGU comprises two or three panes of glass separated with spacers and sealed at the edges. IGU also includes the insulating air space between the panes. Often this air space is replaced with a

Low-Emissivity (Low-E) Coating
A very thin metal or metallic oxide layer deposited on a pane surface and sealed in an IGU. A low-e coating reduces the IGU’s U-value by suppressing radiated heat flow.
Depending on which pane is treated, a low-e coating will reflect heat either into the room (useful in mostly cold-weather climates) or to the outside (for warm-weather climates).

A unit of energy equal to one quadrillion (1015) BTUs, used for expressing national or global energy use or supply.

Resistance Value (R-Value)
A measure of the capacity of a material, such as insulation, to impede heat flow, with increasing values indicating a greater capacity.

Radiation (Radiated Heat)
The transfer of heat in the form of electromagnetic waves from one surface to another.

A substance that adheres to glass panes and the standoff of an IGU to seal it. A good sealant must provide strong adhesion, preventing gas leakage out of the IGU and moisture seepage into the IGU.
Solar Heat Gain Coefficient (SHGC)
The fraction of solar radiation transmitted through a window, expressed as a percentage. The lower a window’s SHGC, the less solar heat it transmits and the greater its shading ability. Generally, a lower SHGC is desirable in warm climates, and a higher SHGC in cold ones.

Solder Glass
A special glass, with a relatively low softening point, used to join two pieces of glass (with higher melting temperatures) without softening or deforming them. Solder glass is sometimes used to seal the edges of IGUs and VIGUs.

A material placed between the panes of glass in an IGU, at its outer edge, to maintain separation of the panes, and to seal the IGU.

U-Factor (U-Value)
A measure of the rate of non-solar heat flow through a material or assembly. The lower the U-factor, the greater a window’s resistance to heat flow and the better its insulating value. The U-factor is the reciprocal of the R-value (U=1/R).

Vacuum Insulating Glass Unit (VIGU)
A type of IGU in which the air space between panes has been replaced with a vacuum. VIGUs have the highest theoretical thermal insulating value of any IGU. (A perfect vacuum cannot conduct heat.) A typical gas-filled IGU has a U-factor of .35, whereas a similarly constructed VIGU has a potential U-factor of .10 or better.

Zero-Energy-Building (ZEB)
A zero-energy-building is a term applied to a building or home with a net energy consumption of zero – the energy provided by onsite renewable energy sources are equal to the energy used. A ZEB combines energy-efficient construction and appliances with renewable energy systems (e.g. solar power, wind turbines). Like most buildings, it is connected to the utility grid, but with reduced energy needs and renewable energy systems, it gives back as much energy to the utility as it takes.