The percentage of visible light transmitted through the glass. VLT is measured
in the 380-780nm wavelength range perpendicular to the surface. The higher
the percentage the more daylight. Also known as Tv, Tvis, LT and VT.
The percentage of visible light that is reflected by the glass surface,
measured in the 380-780nm wavelength range perpendicular to the surface.
The VLR can be given as the reflection from the external surface 1 or internal
surface 2. The higher the percentage the more reflection. Also known as
LR, VR and Rvis
The percentage of ultraviolet radiation eliminated by the glass, measured
over the 290-380 nm wavelength range. The higher the percentage the less
UV is transmitted. This value is calculated from the percentage transmission
of ultraviolet (Tuv). Therefore UV Elimination = 100 - Tuv
The ratio of fading reduction of a glass type when compared to the fading
protection of 3mm clear float. The FRC of 3mm clear float is by definition
1.0 and represents the minimum fading protection offered by standard glazing.
The lower the fading reduction coefficient, the better the fading protection
Heat Gain Coefficient
The measure of the total solar energy transmittance entering a building
through the glazing as heat gain. It is the total heat transmission of
direct solar transmission and that proportion of absorbed radiation that
is re-radiated into the building from the action of heat absorbing glass.
The lower the SHGC the better the glass restricts heat energy transmission.
Also known as the Solar Factor (SF) or g.
The ratio of the total solar heat gain through a particular glass compared
to the total solar heat gain through 3mm clear float glass. (86%) The shading
coefficient of 3mm clear float is by definition 1.0 and represents a base
glass performance. The lower the shading coefficient the less heat gain
and thus more shading is provided by the glass. The shading coefficient
is calculated as SC = SHGC / 0.86
Efficiency Rating System
The complete WERS rates the performance of windows including frames in
various defined New Zealand climate zones. For this catalogue star ratings
apply to centre of glass only (WERS cog) to compare glass
types and a maximum rating of five stars indicates the best performance
possible. Ratings in star and half star values are given, with five stars
indicating the premium performance. Refer Section 12 (Page 105) for a full
explanation of each star type.
The U Value is the measure of air to air heat transfer through glass due
to the thermal conductance of the glazing and the difference between indoor
and outdoor temperatures. It is expressed as W/m2K (Watts per m2 per 1° Kelvin)
or W/ m2 °C. 1 Kelvin equals 1°C. The U value is a measure of the
rate of heat gain or heat loss through the glazing due to environmental
differences between outdoor and indoor air. It is measured at the centre
of the glass (cog).The lower the U Value the lower the heat transfer, the
better the insulation.
The Coolness Factor (or luminous efficacy) is the visible light transmission
divided by the shading coefficient.
CF = VLT / SC. It is a useful means of comparing different
glass types in terms of the trade-off between light transmission and heat
control in selecting glass. Glass types with a coolness factor of 1 transmit
as much light as heat, those with a coolness factor lower than 1 transmit
more heat than light and those with a coolness factor greater than 1 transmit
more light than heat
Value (Total Thermal Resistance)
The R Value is the value of thermal resistance of a building element which
is the sum of the surface resistances on each side plus each component
of a building element. It is the inverse of the U Value R=1/R and is expressed
as m2 °C/W.
The average Sound Transmission Loss is useful for determining the effectiveness
of glazed panels to isolate exterior noise (e.g. traffic) from a building.
It is derived from the average of the measured transmission loss at eighteen
1/3 octave frequency bands between 100Hz and 5000Hz, or 16 bands from 125
to 4000Hz. Average STL is measured in decibels (dB), the higher the average
STL figure, the more effective glazing will be in reducing sound transmission.
The Sound Transmission Class is useful for determining the noise reduction
offered by internal building elements such as partitions and walls. It
is a measure that relates the sound reduction performance against sounds
which normally occur inside a building (such as voices, telephones, music
etc). STC is a numerical class rating and cannot be compared with the STL.
It is derived from a best fit curve comparison of a reference STC curve
to the insulation curve. The higher the STC rating, the better the overall
Sound Reduction Index
The Weighted Sound Reduction Index incorporates frequency modified correction
for the human ear’s response. The RW is
reported in dB and is a composite rating of sound reduction at frequencies
from 100 - 5000Hz. Numerically, it is comparable to the STC values but
the numbers are in dBA.
The percentage by which the human ear detects a lessening in sound pressure
or noise is known as the Perceived Sound Reduction. A 10dB reduction in
sound pressure level is generally perceived as a halving of the original
noise. Sound reduction values are ‘weighted’ and used to calculate
the perceived sound reduction as a ratio relative to the RW for
3mm clear float. The dB difference in the RW value
for a glass type when compared to 3mm float is calculated as a perceived
sound reduction percentage. For other sound indices see Section
11.4.5 and 11.4.6