Window Glossary
When it comes to Vinyl Replacement Windows or Installing Windows, understanding the terms will help you buy smarter.
Air Leakage Rating: A measure of the rate of infiltration
around a window or skylight in the presence of a strong wind.
It is expressed in units of cubic feet per minute per square
foot (cfm/ft2) of window area or cubic feet per minute per foot
(cfm/ft) of window perimeter length. The lower a window's air
leakage rating, the better its airtightness.
Conduction: The flow of heat through a solid material,
such as glass or wood, and from one material to another in an
assembly, such as a window, through direct con-tact.
Convection: The flow of heat through a circulating
gas or liquid, such as the air in a room or the air or gas between
window-panes.
CRF Rating: The Condensation Resistance Factor is measured
in a standard test and reflects the total capability of the window
to resist the conditions that support condensation due to conduction
and convection of cold, and air infiltration. Industry standards
call for a minimum CRF of 35. The higher the CRF rating, the
better the window's resistance to condensation producing conditions.
Fenestration: A window or skylight and its associated
interior or exterior elements, such as shades or blinds. The
placement of window openings in a building wall is one of the
important elements in determining the exterior appearance of
a building.
Gas Fill: A gas other than air placed between window
or skylight glazing panes to reduce the U-factor by suppressing
conduction and convection.
Glazing: The glass or plastic panes in a window or
skylight.
Infiltration: The inadvertent flow of air into a building
through breaks in the exterior surfaces of the building. It can
occur through joints and cracks around window and skylight frames,
sash, and glazings.
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 through the window
or skylight.
Radiation: The transfer of heat in the form of electromagnetic
waves from one separate surface to another. Energy from the sun
reaches the earth by radiation, and a person's body can lose
heat to a cold window or skylight surface in a similar way.
R-Value: A measure of the resistance of a material
or assembly to heat flow. It is the inverse of the U-factor (R
= 1/U) and is expressed in units of hr-ft 2 -°F/Btu. A high
window R-value, has a greater resistance to heat flow and a higher
insulating value.
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 equal to the Solar Heat Gain Coefficient multiplied by 1.15
and is expressed as a number without units between 0 and 1. A
window with a lower Shading Coefficient transmits less solar
heat, and provides better shading.
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 has replaced the Shading Coefficient
as the standard indicator of a window's shading ability. It is
expressed as a number without units between 0 and 1. A window
with a lower Solar Heat Gain Coefficient transmits less solar
heat, and provides better shading. Spectrally Selective Glazing:
A specially engineered low-E coated or tinted glazing that blocks
out much of the sun's heat while transmitting substantial daylight.
U-Factor (U-Value): A measure of the rate of heat flow
through a material or assembly. It is expressed in units of Btu/hr-ft
2 -°F or W/m 2 -°C. Window manufacturers and engineers
commonly use the U-factor to describe the rate of non-solar heat
loss or gain through a window or skylight. Lower window U-factors
have greater resistance to heat flow and better insulating value.
Visible Transmittance: The percentage or fraction of
visible light transmitted by a window or skylight.
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