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Glossary of Terms H – M

Head track

The track provided at the head of a sliding glass door. Also, the head member incorporating the track.


The upper horizontal member of a window frame. Also called head.

Heat-absorbing glass

Window glass containing chemicals (with gray, bronze, or blue-green tint) which absorb light and heat radiation, and reduce glare and brightness. See also Tinted glass.

Heat gain

The transfer of heat from outside to inside by means of conduction, convection, and radiation through all surfaces of a house.

Heating degree day

Term used by heating and cooling engineers to relate the typical climate conditions of different areas to the amount of energy needed to heat and cool a building. The base temperature is 65 degrees Fahrenheit. A heating degree day is counted for each degree below 65 degrees reached by the average daily outside temperatures in the winter. For example, if on a given winter day, the daily average temperature outdoors is 30 degrees, then there are 35 degrees below the base temperature of 65 degrees. Thus, there are 35 heating degree days for that day.

Heat loss

The transfer of heat from inside to outside by means of conduction, convection, and radiation through all surfaces of a house.

Heat-strengthened glass

Glass that is reheated, after forming, to just below melting point, and then cooled, forming a compressed surface that increases its strength beyond that of typical annealed glass.

Hinged windows

Windows (casement, awning, and hopper) with an operating sash that has hinges on one side. See also Projected window.


Window with sash hinged at the bottom.

Horizontal slider

A window with a movable panel that slides horizontally.


International Code Council. A national organization that publishes model codes for adoption by states and other agencies. Codes include the International Building Code (IBC) and the International Energy Conservation Code (IECC).


International Energy Conservation Code published by the ICC. The successor to the Model Energy Policy Act (EPAct) as the baseline for residential Energy Codes in the United States.


See air leakage.

Infrared radiation

Invisible, electromagnetic radiation beyond red light on the spectrum, with wavelengths greater than 0.7 microns

Insulated shutters

Insulating panels that cover a window opening to reduce heat loss.

Insulating glass

Tow or more pieces of glass spaced apart and hermetically sealed to form a sing glazed unit with one or more air spaces in between. Also called double glazing.

Insulating value

See U-factor.


Construction materials used for protection from noise, heat, cold or fire.


An upright frame member of a panel in a sliding glass door which engages with a corresponding member in a adjacent panel when the door is closed. Also called interlocking stile.


Window made up of horizontally-mounted louvered glass slats that abut each other tightly when closed and rotate outward when cranked open.


A vertical member at the side of a window frame, or the horizontal member at the top of the window frame, as in head jamb.


An inert, nontoxic gas used in insulating windows to reduce heat transfer.


Kilo Watt Hour. Unit of energy or work equal to one thousand watt-hours.

Laminated glass

Two or more sheets of glass with an inner layer of transparent plastic to which the glass adheres if broken. Used for safety glazing and sound reduction.


Lift. Handle

For raising the lower sash in a double-hung window. Also called sash lift.


A window; a pane of glass within a window. Double-hung windows are designated by the number of lights in upper and lower sash, as in six-over-six. Also spelled informally lite.

Light-to-solar-gain ratio

A measure of the ability of a glazing to provide light without excessive solar heat gain. It is the ratio between the visible transmittance of a glazing and its solar heat gain coefficient. Abbreviated LSG.


A horizontal member above a window or door opening that supports the structure above.

Liquid crystal glazing

Glass in which the optical properties of a thin layer of liquid crystals are controlled by a en electrical current, changing from a clear to a diffusing state.

Long-wave infrared radiation

Invisible radiation, beyond red light on the electromagnetic spectrum (above 3.5 micro meters), emitted by warm surfaces such as a body oat room temperature radiating to a cold window surface.

Low-conductance spacers

An assembly of materials designed to reduce heat transfer at the edge of an insulating window. Spacers are placed between the panes of glass in a double-or triple-glazed window.

Low-emittance (low-E) coating

Microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight glazing surface primarily to reduce the U-factor by suppressing radiative heat flow. A typical type of low-E coating is transparent to the solar spectrum (visible light and short-wave infrared radiation) and reflective of long-wave infrared radiation.

Meeting rail

The part of a sliding glass door, a sling window, or a hung window where two panels meet and create a weather barrier.

Metal-clad windows

Exterior wood parts covered with extruded aluminum or other metal, with a factory-applied finish to deter the elements.


One millionth (10-6) of of a metric meter.


One thousandth of an inch, or 0.0254 millimeter.

Model Energy Code (MEC)

The Model Energy Code is cited in the 1992 U.S. Energy Policy Act (EPAct) as the baseline for residential Energy Codes in the United States. It has been succeeded by the International Energy Conservation Code (IECC) published by the International Code Council (ICC).


A major structural vertical or horizontal member between window units or sliding glass doors.


A secondary framing member (horizontal, vertical, or diagonal) to hold the window


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