Two panes of glass separated by a spacer around the perimeter and filled with argon gas, know as an insulating glass unit (IGU)
A coating applied to one pane to reflect heat. Included in a IGU, this gives greater control over heat loss and gain
For when your building conditions call for maximum heat retention
When choosing glass, there are 5 key elements to consider. Choosing what is important to you and your home can have a big impact on comfort and functionality
Ug = U-value of the glass, this is how much the heat the glass lets through, the lower the number the better.
SINGLE GLAZING
Ug 5.7
DOUBLE GLAZING
Ug 2.9
DOUBLE GLAZING
WITH LOW-E
Ug 1.1
TRIPLE GLAZING
WITH LOW-E
Ug 0.5
With glazing, one of the primary factors to consider is heat loss, or insulation performance. Heat loss is most commonly referred to in NZ by the R-value for the whole window – this is what the building code references. Here we refer to heat loss of the glass in Ug values, this is the heat loss value of the glazing alone. U-value measures heat lost and R-value measures heat not lost (or resisted, hence the ‘R’ in R-value), they are the inverse of each other. All you need to know for now is: the lower the U-value the less heat lost.
Single glazing is by far the worst type of glass you can put in windows for heat loss. It is no surprise then that it has been non-compliant for a while now. Single glazing has a Ug 5.7 – a lot of heat transfer!
Double glazing improves on this with a Ug 2.9, however this is still a long way from an IGU (insulated glazing unit – also called a DGU, Double Glazing unit) which is designed to minimise heat loss even further.
Double glazing with Low-E coatings and argon gas infil is where it’s at. Low Emissivity glass is a coating applied to one of the panes in the IGU which is designed to reflect heat from inside back inside, and argon gas between the panes to further prevent heat transfer. A good Low-E double glazed unit will have an Ug 1.1 – much better. This is the glazing we supply as standard with our Ambiance system.
The next step up is triple glazing. This is a bespoke offering and comes with incredible performance; but as building performance is about balance, triple glazing can lead to negative building outcomes also – feel free to reach out to us to discuss triple glazing. Triple glazed Low-E can achieve numbers as low Ug 0.5 – just wow.
g-value = a measure, as a percentage, of the amount of heat from direct sunlight that can pass through the glass into the room.
CLEAR DOUBLE
GLAZING
77%
STANDARD LOW-E
56%
SOLAR CONTROLLED
LOW-E
35%
Standard clear double glazing has a very high g-value, that is, almost all the heat from the direct sunlight hitting the glass will pass through and begin heating up the room. Standard clear DGUs have a g-value of around 0.77, this means 77% of the heat from solar light will pass through the glass.
Standard Low-E glass, such as LightBridge 1.1, which is designed to keep heat inside with less focus on blocking solar heat gain has a g-value of 0.56, or 56% solar heat gain.
Solar controlled Low-E glass, such as PerformaTech from Viridian, is designed to further reduce the gain from solar heat into the room while still minimising heat loss in the same way as LightBridge mentioned above. PerformaTech has a g-value of 0.35.
It is important to consider heat loss and heat gain elements together, keeping all seasons in mind too. While including a high solar controlled glass may be good for summer, it won’t help in letting the suns heat through in winter. The most clever designers will use external shading elements to shade the windows from the summer sun, and allow the lower winter sun to shine in allowing a glass makeup that lets more solar heat through to be used.
Twd-ISO is the measure of damaging light wavelengths that can pass through the glass, and cause fading to your furniture and flooring.
CLEAR DOUBLE
GLAZING
0.74
LAMINATED CLEAR
DOUBLE GLAZING
0.57
SOLAR CONTROLLED
LOW-E WITH TINT
0.43
All wavelengths on the spectrum contribute to fading, although some do more damage than others, such as UV light. Twd-ISO is a weighted measure to account for this and provide an accurate result. Standard clear double glazing lets most of these light waves through, and has a Twd-ISO of 0.74 – single glazing is even worse, and significantly contributes to the experiences many have of the harsh NZ sun fading flooring and couches.
The best answer to dramatically reduce UV light penetration is to make one pane into a laminate (this also upgrades the acoustic performance). Adding a laminated pane improves the Twd-ISO to 0.57, and for very little cost will make your furnishings, carpet, couches and flooring all last significantly longer without fading – and don’t forget the acoustic advantages also!
STC (Sound Transmission Class) is the measure of the glass ability to reduce sound penetrating through it.
CLEAR SINGLE
GLAZING
STC 25
CLEAR DOUBLE
GLAZED
STC 33-35
LAMINATED DOUBLE
GLAZING
STC 38
A simplified method of thinking about STC rating is that is close to a “decibels reduced” rating. So if there is a noise of 75dB, on the outside, glazing with an STC (or dB reduced) value of 30, you would expect the dB rating inside to be 40dB. Because dB ratings are a logarithmic scale, STC also is a logarithmic scale. The higher the rating, the exponentially better the glass is at preventing sound passing through it. The STC Rating ranges from 25 (no soundproofing) to 65 (completely soundproof). A clear single glazed window has an STC of around 25, and a standard clear double glazed window will be around 33-35.
Laminated glass is the best option to increase the STC rating of windows and doors, with values up to STC37 readily available, and up to STC45 available as bespoke options.
Visible Light Transmission (VLT) is a measure, as a percentage, of how much visible light passes through the glass into the room
CLEAR DOUBLE
GLAZING
82%
STANDARD CLEAR
LOW-E GLASS
79%
GREY TINTED
DOUBLE GLAZING
42%
With no glass, there will be a VLT of 100%, therefore an IGU with a VLT of 80% will let through 80% of the outside light.
Generally speaking, as the performance of the glazing improves (high performance low-e, laminate, triple glazing etc) the number that suffers is the VLT; anything over 70% is considered acceptable. There are many high performance products on the market with VLT numbers of less than 70%, and this begins to have a negative impact on the occupants.
Grey tint is also a popular option to include for aesthetics, note that it significantly reduces the amount of light able to enter a room. Tinted glass, contrary to some thoughts, doesn’t do much to reduce fading – for this, consider Low-E or laminated options. Grey tint can be fantastic for improving privacy from the outside.
Tinting the glass also reduces the solar heat gain (g value), however, this can be an artificial benefit as the glass itself heats up and then radiates this heat into the room.
