Get to know your performance data terms.


European Standard(2011) for glass in buildings that specifies the calculation of performance factors including luminius and solar charateristics of glazing.
In New Zealand, BRANZ has assessed EN410:2011 as the best fit as a calculation for U-Values for New Zealand conditions


NRFC 200/300: 2010 – this is the American Standard for glass in buildings.  This standard sets out the calculation method for the solar heat gain coefficient.
This is the BRANZ accepted method for solar heat transfer calculations in New Zealand.

U-Value – centre of glass

The U-Value measures how well your glass insulates your home or how much heat loss occurs through your glass due to differences in temperature between the indoors and the outdoors.
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).
cog means that this measure is taken from the centre of the glass
The lower the U-value, the better the glass insulates your home


Heat loss transmitted through the glass per unit of surface and per degree of temperature  between outside and inside.

The lower the Ug value, the better in terms of thermal insulation


The R-value is the inverse of the U-value.  It measures how well your glass provides thermal resistance or how well it resists the transfer of heat.

Heat Loss Reduction

The 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 calculation 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.

Solar Heat

Solar Heat is the solar radiation or solar light, that is transmitted through your windows in the form of heat gain.  More simply, solar heat is the warmth you feel from the sunlight coming through your windows.
Letting the right amount of solar heat into your home is vital in improving home comfort and reducing energy bills.  If you have large windows with lots of natural sunlight streaming in for large portions of the day, you most likely find the inside of your home far too hot on sunny days and want to reduce the amount of heat entering your home.   If you have windows that don’t get a lot of sunshine, you are more likely wanting to maximise that solar heat gain and keep it in.
Selecting the right glass to suit the positioning and comfort levels of your home is important and in many cases you will need more than one glass solution to achieve optimal comfort.

Solar Factor

This is a way to measure how much solar energy enters a space through the glass.
The percentage of energy entering through the glass into a space as a ratio of the incident solar energy.   This takes into account the direct transmission of the glass and the phenomena of the glass wall’s energy reflectance towards the interior space (glass heated up by the absorption of the sun’s rays)
In other words, the proportion of sunlight that passes through a window into a room, considering both the direct passage of light and the heat the glass absorbs and reflects back inside.

Solar Heat Reduction

The measure used to determine the amount of solar heat a glass type will allow through a window into the interior of your home is the SHGC, or Solar Heat Gain Coefficient (calculated as per NRFC 200/300 2010).
The Solar Heat Gain Coefficient (SHGC) is the total fraction of available solar radiation that is transmitted through the window as heat gain (1).
SHGC value of 0.65 (or 65%) means that 65% of the solar heat (or heat from the Sun) will be transmitted through the glass into your home as heat gain, whilst 35% will be rejected.
A low SHGC value means the glass performs well at reducing the amount of heat from the sun that enters your home.  The higher the value, the more heat energy from the sun that is entering your home.

Visible Light Transmission
Tι or VLT

Visible Light Transmission is the percentage of visible light that is transmitted through the glass into your home.
Visible light, or daylight, keeps the inside of your home lighter and brighter helping to reduce the use of lights and therefore helping to lower electricity costs.  Too much visible light can be a negative as it increases issues with glare and is a contributor to furniture fading within your home.
The higher the VLT percentage the more daylight that is entering your home.
The VLT is measured in the 380-780nm wavelength range perpendicular to the surface.
Also known as Tv, Tvis, LT and VT.

Ultraviolet Elimination

Ultraviolet is part of the solar energy or solar light spectrum.  Though UV is important for our health, it is also one of the main contributors to furniture fading and can cause damage to our skin if overexposed.
UV Elimination is the percentage of ultraviolet radiation eliminated by the glass or the percentage that is rejected back into the external environment.
The higher the percentage the less UV is transmitted.
Measured over the 290-380 nm wavelength range.
This value is calculated from the percentage transmission of ultraviolet (Tuv). Therefore, UV Elimination = 100 – Tuv.

Relative Fading Reduction

Fading is caused by solar radiation/energy entering your home.  Though many believe UV is the only contributor, it is only one part.  Visible light, solar heat and infra-red are also contributors.
Based on a balanced calculation of % UV transmission, % visible light transmission and % heat transmission (from SHGC).
Lower numbers indicate an improvement compared to 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 calculation that should not be compared with a “Fading Reduction Coefficient” which is more relevant for the assessment of fading in museums and galleries and evidence suggests that it is not relevant for domestic situations.

Perceived Sound Reduction

The measure of the percentage by which the human ear detects a lessening in sound pressure or noise or in other words the noticeable decrease in noise levels due to how well the window absorbs or blocks soundwaves.
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.

Weighted Sound Reduction

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.
Weighted sound reduction is a way to measure how well a material, such as a wall or window, can block different types of sounds. It gives a single number that represents the overall sound-blocking performance, making it easier to compare different materials and their ability to reduce noise.
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, RetroGLAZE® will provide a performance range that can be expected.


  1. Passive Design (2023,March 7).