Daylighting Resources

Daylighting Resources and Related Terms


Sponsored by the AAMA Skylight Council

Acrylic – A general purpose glazing material with excellent optical clarity, weatherability, good chemical resistance and thermoformability.

Air Leakage – Air Leakage (AL) is indicated by an air leakage rating expressed as the equivalent cubic feet of air loss through a square foot of product area (cfm/sq ft).

Annealed Glass – Glass that has not been heat-treated and is essentially strain free; often referred to as “ float glass”.

Bent Glass – Glass that has been curved by heating to above its softening point and then bent by gravity or press molds; also termed “curved glass”.

Bite – Depth of glass engagement within the glazing channel; also termed “purchase” and “edge engagement”.

Cap or counter flashing – Used to protect and waterproof (by overlapping) terminated top edges at curb or base flashing.

Cladding – An exterior covering or skin applied to framing or a structure for aesthetic or protective purposes.

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

Condensation Resistance Factor – Condensation Resistance Factor (CRF) measures the ability of a product to resist the formation of condensation on the interior surface of the product. The higher the CRF rating, the better that product is at resisting condensation formation. While this rating cannot predict condensation, it can provide a credible comparison of the potential of condensation formation between products.

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.

Curb Mount – A skylight frame design that enables installation and anchoring of a skylight to a typically 1 ½” wide flashed wood 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.

Dead Load – Load from the permanent parts of a building or structure.

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%.)

Dome Spring Line or Point of Curvature – The transition point at which the horizontal mounting flange of the dome begins to rise. (The spring line must be over the internal skylight gutter for condensation to be captured and eliminated through weep holes.)

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

Edge Block – Blocks of elastomeric material used at the sides of the glass as support and to prevent direct contact with the framing member; mandatory for dry glazing systems.

Fiberglass Reinforced Polyesters (FRP) – Composite material that consists of a plastic matrix containing reinforcing glass fibers to increase strength or stiffness.

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

Fully Tempered Glass – Glass that has been heated and quenched in a controlled operation to provide a high level of surface compression. ASTM Standard C1048-85 specifies that the surface compression be a minimum of 10000 psi. Generally considered to have four times the strength of annealed glass and two times the strength of heat-strengthened glass.

Haze Factor – The percentage of light through a glazing material that is not diffused. (A 100% haze factor would equate to 100% of the light being diffused. Codes require
skylights to have a minimum 90% haze factor.)

Heat-Strengthened Glass – Glass that has been heated and quenched in a controlled operation to provide a degree of surface compression. ASTM Standard C1048-85 specifies that the surface compression be between 3500 and 10000 psi. Generally considered to have two times the strength of annealed glass.

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

Insulating Glass Unit – Factory fabricated double glazing with the periphery of the air space sealed to minimize infiltration of water vapor.

Laminated Glass – A sandwich of two or more glass plies bonded together with resilient plastic interlayers, normally polyvinyl butyral (PVB).

LEED – Leadership in Energy and Environmental Design. An Internationally recognized green building certificaton system.

Light or Lite – A single glazing pane; use of “lite” is preferred to avoid confusion; most often referred to as “pane.”

Live Loads – Loads from people and non-permanent parts of a building; loads from window washing and glazing rigs are live loads.

Low Emittance (Low E) Coating – A coating that has a reduced ability to radiate heat energy; when facing an air space this property reduces the amount of heat transfer across the space.

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.

Muntin – A non-structural dividing member between glass panes.

Pane – (see “Light or Lite”).

Point of Curvature – ( see “Dome Spring Line”).

Purlin – (see “Cross Rafter”).

Rafter – For sloped glazing, a main nominally sloped framing member.

Seismic Load – Building movement and forces caused by earthquake motion.

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

Setting Block – Blocks of elastomeric material used at the bottom edges of the glass as support and to prevent direct contact with the framing member; mandatory for dry glazing systems.

Skylight – 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.

Sloped Glazing – A glazing and framing assembly that is sloped more than 15 degrees from vertical and which forms essentially the entire roof of the structure; generally this is a single slope construction.

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 – A measurement of how well a product blocks heat from the sun. (SHGC) is the fraction of incident solar radiation admitted through the glazing, both admitted through and directly transmitted, and absorbed and subsequently released inward. The lower a product’s SHGC, the less solar heat it transfers.

Stepped Flashing – Sections of overlapping flashing pieces usually woven into tile or shingle roofs at curb or base conditions.

Tempered Glass – (see “Fully Tempered Glass”).

U-Factor – A measurement of how well a product prevents heat from being transmitted. The lower the U-factor, the better the product’s insulating value.

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.


Links to definitions of
Daylighting and related terms:


Daylight Factor
Passive Daylighting

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