Vacuum Evaporation
The deposition of thin films of semiconductor material by the evaporation of elemental sources in a vacuum.

Valence Band

The highest energy band in a semiconductor that can be filled with electrons.

Vapor Retarder

A material that retards the movement of water vapor through a building element (walls, ceilings) and prevents insulation and structural wood from becoming damp and metals from corroding. Often applied to insulation batts or separately in the form of treated papers, plastic sheets, and metallic foils.

Variable-Speed Wind Turbines

Turbines in which the rotor speed increases and decreases with changing wind speed, producing electricity with a variable frequency.


A component of a heating or ventilation appliance used to conduct fresh air into, or waste air or combustion gases out of, an appliance or interior space.

Vent Damper

A device mounted in the vent connector that closes the vent when the heating unit is not firing. This traps heat inside the heating system and house rather than letting it draft up and out the vent system.

Vented Heater

A type of combustion heating appliance in which the combustion gases are vented to the outside, either with a fan (forced) or by natural convection.


The process of moving air (changing) into and out of an interior space either by natural or mechanically induced (forced) means.

Ventilation Air

That portion of supply air that is drawn from outside, plus any recirculated air that has been treated to maintain a desired air quality.

Vent Pipe

A tube in which combustion gases from a combustion appliance are vented out of the appliance to the outdoors.

Vertical-Axis Wind Turbine (VAWT)

A type of wind turbine in which the axis of rotation is perpendicular to the wind stream and the ground.

Vertical Ground Loop

In this type of closed-loop geothermal heat pump installation, the fluid-filled plastic heat exchanger pipes are laid out in a plane perpendicular to the ground surface. For a vertical system, holes (approximately four inches in diameter) are drilled about 20 feet apart and 100 to 400 feet deep. Into these holes go two pipes that are connected at the bottom with a U-bend to form a loop. The vertical loops are connected with horizontal pipe (i.e., manifold), placed in trenches, and connected to the heat pump in the building. Large commercial buildings and schools often use vertical systems because the land area required for horizontal ground loops would be prohibitive. Vertical loops are also used where the soil is too shallow for trenching, or for existing buildings, as they minimize the disturbance to landscaping. Also see closed-loop geothermal heat pump systems.

Visible Light Transmittance

The amount of visible light that passes through the glazing material of a window, expressed as a percentage.

Visible Radiation

The visible portion of the electromagnetic spectrum with wavelengths from 0.4 to 0.76 microns


A unit of electrical force equal to that amount of electromotive force that will cause a steady current of one ampere to flow through a resistance of one ohm.


The amount of electromotive force, measured in volts, that exists between two points.


A unit of electrical measurement equal to the product of a volt and an ampere.