R 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/U and is expressed as m2 °C/W.
Window 5.2 is a software modeling program used to determine the thermal and optical properties of glass types and windows. It was developed by Lawrence Berkeley National Laboratory (LBNL) and contains an international glass data base.
Window 5.2 supersedes the earlier Window 4.1 version which was used to calculate performance data in the 5th Edition Catalogue and Reference Guide.
As a result some performance data may vary from that listed in the 5th Edition.
The National Fenestration Rating Council (NFRC) sets out the environmental conditions and procedures used to determine the performance characteristics of glass.
Environmental conditions include wind speed, internal and external temperatures, solar radiation levels and heat transfer coefficients.
The following are the standard environmental conditions.
It is possible to calculate glass properties using other standard conditions or user defined conditions
Note: The performance data listed in this publication was calculated from Window 5.2 using NFRC 100-2001 environmental conditions, where possible. In some situations the glass properties have been estimated based on similar data. Published data for some glass types may vary slightly due to the manufacturer’s spectral transmission data selected for calculation.
Other Performance Data Terminology
The following data is used in other glass publications such as those based on European Standard EN 410, and Window 5.2.
DET Direct Energy Transmission (Tsol)
Percentage of solar radiation transmitted directly through the glass. DET + EA + ER = 100% of sun’s energy
|Tuv||Transmission of Ultraviolet Light|
|Tdw||Damage Weighted Transmission|
|Tdw-K||Damage Weighted Transmission NFRC|
|Tdw-ISO||Damage Weighted Transmission ISO|
EA Energy Absorption (Asol)
Percentage of solar radiation absorbed by the glass.
ER Energy Reflection (Rsol)
Percentage of solar radiation reflected by the glass.
SF Solar Factor (g)
Total energy transmitted through the glass.
SF = DET + part of EA radiated to the
Same as SHGC.
swSC Short Wave Shading
swSC = DET/87
lwSC Long Wave Shading
lwSC = SC - swSC
RHG Relative Heat Gain.
RHG = 630(W/m2) x SC + 7.8 (°C) x U Value (summer)
Note: American Units: 1W/m2 = 0.317 BTU/(ft2.h)
RD65 Colour Rendering Index (Ra)
Ucog U Value centre of glass (cog)
U-factor – See U Value.
E – Emissivity of the surface
CI – Condensation Index
Cog – Centre of glass
Sound Transmission Loss
The average Sound Transmission Loss is useful for determining the effectiveness of glazed panels to isolate exterior noise (such as 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. The average STL is measured in decibels (dB), the higher the average STL figure, the more effective the glazing will be in reducing sound transmission.
Sound Transmission Class
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.