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Skylight & Daylighting Glossary

As the daylighting industry continues to grow, so too, will its terminology and definitions. We've compiled a list of common terms you may encounter along the way.

A light transmitting, easily fabricated thermoplastic material that is available in molding resin and sheet form for use in skylights. Part of a class of thermoplastics characterized by light weight and flexibility for use as a substitute for glass. Special acrylic grades are available modified for improved impact resistance.

ClearArmour® Coated Prismatic
Hail-Resistant, double-glazed polycarbonate prismatic material available on Sunoptics Factory Mutual (FM) Approved, Signature skylights. Select FMRTG option.

Clerestory Lighting
Clerestory lighting are rows of windows above eye level that allow daylight into a space.

Condensation Gutter
A trough for carrying off condensed or infiltrated water; this may be drained to the exterior or allowed to evaporate.

A ridged roof attachment that is designed to divert water on a roof around a penetration.

Cross Rafter (or Purlin)
In a skylight system, a structural framing member between rafters; generally at or near horizontal.

Curb or Base Flashing
Used for waterproofing at a curb or base.

A skylight frame design that enables installation and anchoring of a skylight to a typically 1 ½” wide flashed curb, which is a permanent part of the roof structure. (This type of installation is typical on a low sloped roof.) Generally, curb mounted skylights are installed after roof construction.

Daylight Harvesting
The effective use of natural light from both the sun and the sky for meeting at least part of the lighting needs within an occupied space. Associated with this is an expectation that all or part of the installed energy consuming lighting system responds to some type of lighting control strategy driven by the available daylight.

Daylighting is the controlled admission of natural light, direct sunlight, and diffused-skylight into a building to reduce electric lighting and saving energy. Daylighting helps create a visually stimulating and productive environment for building occupants, while reducing as much as one-third of total building energy costs.

Dome Rise
The height of the dome above the perimeter flange of the dome. (Maximum strength for a free-blown dome is achieved with a rise of 20% to 25%.)

Any glazing consisting of two panes of glazing separated by an air space.

Effective Skylight-to-Floor Ratio (ESFR)
Effective Skylight-to-Floor Ratio (ESFR) is the percentage of the total square footage of floor area covered by the total square footage of skylights in the roof. High VT skylights can lower ESFR. Should not exceed 5% of the roof. Code may limit cap at 3% of the roof.

Corrosion resistant material used for controlling, redirecting or preventing water from entering a structure.

Foot Candle (fc)
A unit of illuminance (amount of luminous flux) incident on a surface, equal to one lumen per square foot (1 lm/ft2).

Haze Factor
The percentage of light through a glazing material that is diffused. (A 100% haze factor would equate to 100% of the light being diffused. Codes require skylights to have a minimum 90% haze factor.)

High-Impact Acrylic
Glazing material which has an impact modifier blended with the acrylic resin to meet specific impact requirements.

Light Diffusion
The scattering of light caused by passing through a non-transparent material or by bouncing off semi-reflective surfaces. Diffusivity of a glazing material is represented by measurement of its “haze factor.”

Lumen (lm)
The unit of luminous flux in the International System, equal to the amount of light given out through a solid angle (steradian) by a source of one candela (cd) intensity radiating equally in all directions. The lumen (lm) is a measure of the total "amount" of visible light emitted by a source.

Lux (lx)
The SI unit of illuminance, equal to one lumen per square meter (1 lm/m2).

Model Building Code
A construction code developed from input from industry, building officials, and others for use as a guide for the development of state and local building codes. Model building codes have not legislative or jurisdictional power.

Multi-Wall Sheet
A light transmitting thermoplastic glazing material with a cellular internal structure for increased thermal insulation.

A light transmitting, impact resistant thermoplastic material that is available in molding resin and sheet form for use in skylights. Part of a class of thermoplastics characterized by high-impact strength, light weight, and flexibility, and used as shatter-resistant substitutes for glass.

Prismatic Sheet
Thermoplastic sheets with embossed pyramidal patterns that refract and diffuse light. Commonly used in skylights and other light diffusing applications.

See ("Cross Rafter")

Roof Monitor
Another form of top-lighting; a projecting portion of the roof with a vertical glazing component, sometimes referred to as clerestory windows.

Self Flashing
A skylight base that mounts directly to the roof without requiring a curb.

Side Lighting
Side-lighting from windows and doors provides daylight and solar energy along the perimeter of a building.

A glazing and framing assembly consisting of sloped and (sometimes) vertical surfaces; the assembly is generally inserted into the roof of a building to admit daylight.

Snow Load
Load imposed on a building wall, roof, or skylight by the accumulation of snow; generally a long-term load.

Solar Heat Gain Coefficient (SHGC)
The Solar Heat Gain Coefficient (SHGC) measures how well a skylight blocks heat from sunlight. It is the fraction of the heat from the sun that enters through a window. SHGC is expressed as a number between 0 and 1. The lower a skylight's SHGC, the less solar heat it transmits.

Top Lighting

Top-lighting from roof-top fenestration systems can provide daylight and solar energy throughout the interior of a low rise building, on the top floor of a building or in an atrium.

Transmission Haze or Diffusion
Transmission haze (T), also known as diffusion, is the scattering of light as it passes through a transparent or semi-transparent material. ASHRAE 90.1-2010 and IECC 2012 require ≥90% haze.

Tubular Daylighting Device (TDD)
A non-operable fenestration unit designed to transmit daylight from a roof surface to an interior space via a tubular conduit. It typically consists of a skylight, light-transmitting tube with reflective interior surface, and an interior-sealing device such as a diffusing panel.

Tubular Daylighting System
A tubular daylighting device with integrated and controlled elements including louvers, daylighting sensor, optional secondary light source that works with electric lighting for a holistic daylighting solution and increased energy savings. A TDD product line can be tested and rated differently, based upon insulation location, and product configuration (closed or open ceiling). (Based on 2012 IBC)

U-Factor (U-Value)
 measures the rate of heat transfer and tells you how well the skylight insulates. The lower the U-Factor, the better the skylight insulates. 1 / U-Factor = R-Value

Unit Skylights

A factory-assembled, glazed fenestration unit, containing one panel of glazing material that allows for natural lighting through an opening in the roof assembly while preserving the weather-resistant barrier of the roof. Unit skylights are either fixed (non-operable) or venting (operable). (Per 2012 IBC)

Visible Light Transmittance (TVIS or VT)
Visible Light Transmittance (VT)
 measures how much light comes through a product. The visible transmittance is an optical property that indicates the amount of visible light transmitted.

Weep Hole

A hole placed in a framing member to allow condensed or infiltrated water to drain to the exterior of the structure.

Wind Load
Short duration load on a structure and its components due to the effects of the wind.