Technical Information

Notes from Benefits of Double Glazing

1.  “Heat Loss Reduction” is a measure of the percentage reduction in heat loss through the middle of the glass, as compared with 4mm single glazing.  Measurements are made according to the European Glass Standard, EN410:2011.  In New Zealand, BRANZ has assessed EN410:2011 as the best fit as a calulation for U-Values for New Zealand conditions.  This measure does not account for double glazing spacer material so care should be taken to consider the insulating performance of the spacer.

U-Value’ is a measure of the thermal leakage through a building material, measured in Watts per square metre per temperature difference between the outside and inside (W.m^-2.^oC^-1).

2.  “Solar Heat Reduction” is a measure of the reduction in the amount of heat from the Sun passing through the glass relative to 4mm clear single glass.  The reference calculation is SHGC, or Solar Heat Gain Coefficient (calculated as per NRFC 200/300 2010).

3.  “Relative Fading Reduction” is based on a balanced calculation of %UV transmission, %visible light transmission and %heat transmission (from SHGC).  Lower numbers indicate an improvement on 4mm clear glass.  UV contributes to 40%-60% of fading.  Typical calculations use 45% weighting.  Here in NZ UV is more significant, so a 55% weighting has been used.  Solar heat has been weighted at 30% and visible light the remaining 15%.  This is a relative calulation that should not be compared with a “Fading Reduction Coefficient” which is more relevant for assessment of fading in museums and galleries and evidence suggests that it is not relevant for domestic situations.

4.  “Perceived Sound Reduction” is a measure of the percentage by which the human ear detects a lessening in sound pressure or noise.  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 glass is calculated as a perceived sound reduction percentage.

Rw, or the Weighted Sound Reduction Index is a single-number quantity which characterises the airborne sound insulation of a material or building element over a range of frequencies.

Perceived noise reduction is subjective to the individual, who may be influenced differently by different frequencies of sound. Further, there is variation in the measured test data from laboratory to laboratory around the world.  In presenting project specific data, The Double Glazing Company will provide a performance range that can be expected.  

Source of data:  

Saint Gobain Glass, Culmen II database

Glass Association of New Zealand, GANZ data sheets