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Glossary of Terms R – S


A measure of the resistance of a glazing material or fenestration assembly to heat flow. It is the inverse of the U-factor (R=1/U) and is expressed in units of hr-sq ft-F/Btu. A high –R-value window has a grater resistance of heat flow and a higher insulating value than one with a low R-value.


The transfer of heat in the form of electromagnetic waves from one separate surface to another. Energy from the sum reaches the earth by radiation, and a person’s body can lose heat to a cold window or skylight surface in a similar way.


Horizontal member of a window sash.


The ration of reflected radiant energy to incident radiant energy.

Reflective glass

Window glass coated to reflect radiation striking the surface of the glass.


The deflection of a light ray from a straight path when it passes at an oblique angle from one medium (such as air) to another (such as glass).

Relative humidity

The percentage of moisture in the air in relationship to the amount of moisture the air could hold at that given temperature. At 100 percent relative humidity, moisture condenses and falls as rain.


Adding or replacing items on existing building. Typical retrofit products are replacement doors and windows, insulation, storm windows, caulking, weather stripping, vents, landscaping.


A computer program used to calculate energy use based on window selection in residential buildings.

Roof window

A fixed or operable window similar to a skylight placed in the sloping surface of a roof.

Rough opening

The opening in a wall into which a door or window is to be installed.

Safety glass

A strengthened or reinforced glass that is less subject to breakage or splintering.



The portion of a window that includes the glass and framing sections directly attached to the glass, not to be confused with the complete frame into which the sash sections are fitted.



Woven mesh of metal, plastic, or fiberglass stretched over a window opening to permit air to pass through, but not insects.


A compressible plastic material used to seal any opening or junction of two parts, such as between the glass and a metal sash, commonly made of silicone, butyl tape, or polysulfide.

Shade screen

A specially fabricated screen of sheet material with small narrow louvers formed in place to intercept solar radiation striking a window; the louvers are so small that only extremely small insects can pass through. Also called sun screen. Also, an awning with fixed louvers of metal or wood.

Shading coefficient (SC)

A measure of the ability of a window or skylight to transmit solar heat, relative to that ability for 1/8 inch clear, double-strength, single glass. It is being phased out in favor of the solar heat SHGC multiplied by 1.15. It is expressed as a number without units between 0 and 1. The lower a window’s solar heat gain coefficient or shading coefficient, the less solar heat it transmits, and the greater is its shading ability.

Sheet glass

A transparent, flat glass found in older windows, now largely replaced by float glass.

Short-wave infrared radiation

Invisible radiation, just beyond red light on the electromagnetic spectrum (between 0.7 and 2.5 microns), emitted by hot surfaces and included in solar radiation.


The lowest horizontal member in a door, window, or sash frame.

Sill track

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

Simulated divided lights

A window that has the appearance of a number of smaller panes of glass separated by muntins, but actually is a larger glazing unit with the muntins placed between or on the surfaces of the glass layers.

Single glazing

Single thickness of glass in a window or door.

Single-hung window

A window consisting of two sashes of glass, the top one stationary and the bottom movable.

Single-strength glass

Glass with thickness between 0.085” and 0.100” (2.16-2.57 mm).

Skylight (operable or pivot)

A roof window that gives light and ventilation.

Sliding glass door

A door fitted with one or more panels that move horizontally on a track and/or in grooves. Moving action is usually of rolling type (rather than sliding type). Also called gliding door, rolling glass door, and patio sliding door.

Sliding window

A window fitted with one or more sashes opening by sliding horizontally or vertically in grooves provided by frame members. Vertical sliders may be single- or double-hung.

Smart window

Generic term for windows with switch-able coatings to control solar gain.

Solar control coatings

The film coatings on glass or plastic that absorb or reflect solar energy, thereby reducing solar gain.

Solar heat gain coefficient (SHGC)

The fraction of solar radiation admitted through a window or skylight, both directly transmitted, and absorbed and subsequently released inward. The solar heat gain coefficient indicator of a window’s shading ability. It is expressed as a number between 0 and 1. The lower a window’s solar heat gain coefficient, the less solar heat it transmits, and the greater its shading ability. SHGC can be expressed in terms of the glass alone or can refer to the entire window assembly.

Solar radiation

The total radiant energy from the sun, including ultraviolet and infrared wave lengths as well as visible light.

Solar screen

A sun shading device, such as screens, panels, louvers, or blinds, installed to intercept solar radiation.

Solar spectrum

The intensity variation of sunlight across its spectral range.

Sound Transmission Class (STC)

The sound transmission loss rating of a material over a selected range of sound frequencies. The higher the number, the less sound transmitted.

Spectrally selective glazing

A coated or tinted glazing with optical properties that are transparent to some wavelengths of energy and reflective to others. Typical spectrally selective coatings are transparent to visible light and reflect short-wave and long-wave infrared radiation.


The upright or vertical edges of a door, window, or screen.


The shelf-like board of the interior part of the window sill, against which the bottom rail of the sash closes.


The molding on the inside of a window frame against which the window sash closes; in the case of a double-hung window, the sash slides against the stop. Also called bead, side stop, window stop, and parting stop.

Storm windows

A second set of windows installed on the outside or inside of the primary windows to provide additional insulation and wind protection.

Sun control film

A tinted or reflective film applied to the glazing surface to reduce visible, ultra-violet, or total transmission of solar radiation. Reduces solar heat gain in summer and glare. Some can be removed and reapplied with changing seasons.

Super window

A window with a very low U-factor, typically less than 0.15, achieved through the use of multiple glazing, low-E coatings, and gas fills.

Switch-able glazing

Glazing with optical properties that can be reversibly switched from clear to dark or reflective.


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