1. What is Low Emissivity (Low E) glass?
  2. Types of Low E glass
  3. How does Low E glass work?
  4. Low E Performance considerations
  5. Low E from Metro Performance Glass

What is Low Emissivity (Low E) Glass?

The “E” in Low E is an abbreviation of “Emissivity”, which is a measure of the ability to radiate absorbed energy. Low E glass has a special coating on the glass surface, which reduces the amount of heat transferred through the glass, improving its thermal performance.

Low E coatings are older than you might think. The very first low E coatings were highly reflective silver coatings on "thermos" bottles back in the 1890s.  The energy crisis in the 1970s sparked the development of low-emissivity (low-E) coatings for windows. Pilkington and German firm Flachglas Group created the first commercially viable low-E coatings using thin layers of gold. These coatings produced a green hue, leading German glass manufacturer Interpane to develop the first effectively colorless low-E coating using silver layers in 1981.

Today’s low-E coatings contain one or more silver layers that reflect the sun’s ultraviolet and infrared light to help maintain a comfortable interior temperature while allowing for visible light transmission. In the winter, when a building’s conditioned interior is warmer than the outside, the coating works in reverse, reflecting the heat back inside.

To understand the performance of Low E Glass, it helps to have an understanding of the solar energy spectrum. As you can see in the two graphic below, ultraviolet light (UV), visible light, and infrared light all occupy different parts of the solar spectrum. In the chart you can see how they are delineated according to their wavelengths.

UV light, which contributes to the fading of interior materials such as fgabrics and wall coverings, has wavelengths between 300 to 380 nanometers.

Visible light occupies the part of the spectrum between infrared and UV light wavelengths, measuring from about 380 to 780 nanometers.

Infrared light (heat energy), which is transmitted as heat into a building, begins at wavelengths of approximately 780 nanometers.

Low E coatings have been developed to minimize the amount of ultraviolet and infrared light that can pass through glass without compromising the amount of visible light that is transmitted.

Types of Low E

The Low E coatings are applied either in the Sputter Coating process, also known as Magnetron Sputter Vacuum Deposition (MSVD), which is applied to pre-cut glass in a room-temperature vacuum chamber, or the Pyrolytic coating process, which applies the Low E coating to Float Glass as it is being produced on the float line.  While the Pyrolytic process is a newer method for the application of Low E coatings, continued development of the sputter coating process has meant that the vast majority of commercially available Low E coated glass used in IGU’s is applied using the sputtering process.  These advancements in coating technology have created more resilient “post temperable” sputter-coated glasses, which means they can be tempered (toughened) after they have been coated.  As the coatings are designed to reflect heat they can be difficult to toughen, so with the advent of these new coatings, it has also required glass processors to continue to upgrade their toughening lines to accommodate the processing of these new generation Low E coated glasses.

What this has meant is an explosion of Low E options that can be tailored to meet various building requirements across the world.  The options will vary from tuning for reducing heat loss in cold climates to solar control to stop buildings overheating in warmer climates.  Some climates require both performance attributes, for example, Central Otago, which has some of the coldest temperatures in New Zealand in the winter, is also one of the areas in New Zealand that gets the most sunshine hours.

Low E coatings can also be coloured, reflective, and or applied to Tinted Float glass to further enhance the performance and or the look of a building.

Any coating applied to the surface of the glass will have some effect on the overall clarity and this has been an important consideration in the development of modern Low E products – obtaining maximum performance with minimal optical impact.

How Does Low E Glass Work?

With advances in technology, sputter coating applies microscopically thin chemical layers in an organised stack.  Within these layers are microscopic layers of silver and dielectric (ceramic) materials, which contribute significantly to the performance properties of the coating.  This also allows the stacks to be layered for specific performance targeting specific parts of the solar energy spectrum, while also allowing the visible light parts of the spectrum to pass through. 

Low E Performance Considerations

For New Zealand, typical geography is “subtropical in the North to cool and temperate in the South” (NIWA).  As an indicative comparison, the map below illustrates where New Zealand would sit next to Europe, at the same latitude (similar to the wine belt, Bordeaux to southern Spain, grapes grow from the very South all the way to the to the North of New Zealand). 

Within New Zealand there are 3 zones set out in the building code for insulation performance, however, Metro and the New Zealand Green Building Council (NZGBC) recognize the Central Otago area as a fourth zone for insulation, with temperatures dropping to -10oC in the winter.  Metro has also mapped the areas that receive the highest sunshine hours as a proxy for the need to have solar control.

This means that, while parts of Australia receive much more solar radiation than New Zealand, UV levels in New Zealand are still higher than in Europe and parts of North America. Our most populated centres correspond to latitudes equivalent to the Mediterranean or California.  Overall, New Zealand is sunnier than most of Europe and North America, and its peak UV is higher due to the lack of pollution and closer approach to the sun during summer than in the northern hemisphere.  Thus it is important when considering the likes of Passive House insulation performance levels, that the potential for overheating is also considered, which in most cases means an all-round performance Low E double glazing unit that has solar control and 0.9U value type performance.

The New Zealand Building Code (NZBC) Energy Efficiency Acceptable Solution H1/AS1 requires compliance with NZS 4218 – Thermal Insulation - Housing and Small Buildings. The tables require a minimum Rwindow value of 0.26 and this will normally require IGUs to be used.  The building code has not kept up with what is now considered a minimum standard for livable homes.  The New Zealand Green Building Council (NZGBC) Homestar 6 level is considered the minimum performance level for social housing. 

Building Element Building Element R-Value (m2.k/W)
  Climate Zone 1 Climate Zone 2 Climate Zone 3A Climate Zone 3B
Roof 3.6 3.8 4.1 4.9
Wall 2.1 2.1 2.2 2.9
Floor 1.3 a.b 1.8 b 2.3 3.0
Window and Glazing 0.26 or 2 Star WEERS 0.32 or 3 Star WEERS 0.43 or 4.5 Star WEERS 0.43 or 4.5 Star WEERS
Skylights (max 1.5%) 0.40  0.40 0.50 0.50

Both insulation performance and solar control become more important as the amount of glazed area in a building or home increases.

Low E from Metro

When selecting more specialised glass products such as Low E, it is important to be sure of what you are actually getting. Not all glass is created equally and there can be significant variation in optical clarity – this is not always at the front of a customer’s mind, as many assume that glass clarity is always the same.

Metro Performance Glass can supply a significant range of Low E glass products, selected to suit the specific performance and budget requirements of your particular project. No matter where your project falls within the available range, from more budget-conscious first and volume home builders to high-end architectural designs seeking ultimate performance, Metro Low E glass products have been selected to offer the clearest possible option within each market sector.

Low E Single Glazing

While Low E single glazing was used for a period of time, it has been largely superseded by IGU’s.  As the Low E coating in single glazing cannot be sealed off from the atmosphere it is at an increased risk of diminished performance over time, particularly in high humidity areas.  Single glazed Low E glass does not provide the thermal performance or comfort and condensation control provided by an IGU, and is not recommended in cold climates or high humidity areas. Thus Low E IGU’s, principally double glazing units, with the ability to seal the coated side of the glass within the unit, have become the preferred solution.

Low E IGU’s

Insulated Glass Units (IGU’s) are typically double glazing, occasionally triple glazing, and in more extreme cold climates such as Siberia (-45deg temperatures) quadruple glazing is also used.

The beauty of an IGU is the ability to have different glass types in each pane, to allow more options for achieving desired performance and appearance outcomes.

Initially, Low E coatings were placed on surface 2 or 3 (the inside of the IGU). This was done due to the delicate nature of the original soft coatings so that the coating was protected from potential damage. The Low E coatings were typically used on the clear glass on surface 3 of an IGU to improve the thermal heat loss performance (U-Value). Some were applied to the tinted glass on surface 2 of an IGU and used in hotter climates to reduce heat gain (lowering the shading coefficient).

When included in an IGU, generally the Low E Coated glass is on surface 2 of the double glazing unit, with a Warm Edge 3 spacer and gas fill, all factory sealed ready for site installation.

The latest iterations of high-performance Low E double glazing (Low E Xtreme) have a U value (Insulation performance) of 0.9, similar to typical standard Low E triple-glazed IGU’s used in New Zealand in recent years.

Metro generally recommends Low E on surface 2 for New Zealand conditions, however for specific applications please consult with the technical team technical@metroglass.co.nz, as continuing Low E development is shifting the previous givens in IGU design.

Low E in Residential IGU

The glass you choose can make a huge difference to the comfort of a house and the size of its energy bills. Because windows are a key design factor for the insulation of houses, Metro Performance Glass offers a range of window glass with high-tech coatings that can be specified to assist in creating the optimum internal environment. 

More information on Low E Double Glazing by Metro

The following overview table details the performance attributes of the Residential range with a 12mm spacer.

Outer Glass 4 mm clear 4 mm Low E 4 mm Low E 4 mm Low E 4 mm Low E 4 mm Low E
Space 12 mm air 12 mm air 12 mm argon
+ thermal break
12 mm argon
+ thermal break
12 mm argon
+ thermal break
12 mm krypton
+ thermal break
Inner Glass 4 mm clear 4 mm clear 4 mm clear 4 mm clear 4 mm clear 4 mm clear
Heat Loss & Condensation
U valua (W/m K)
2.9 1.9 1.6 1.3 1.2 0.9
Visibility & Glare 82% 69% 69% 80% 75% 75%
Heat Gain SC 0.89 0.66 0.66 0.69 0.47 0.47
Solar Control SF 77% 57% 57% 60% 41% 41%
Fading Tdw - ISO 0.74 0.64 0.64 0.69 0.48 0.48

For more detailed makeup information click here

Low E in Commercial IGU

Creating a high-performance building envelope can be a challenge – finding the balance between capturing and reflecting solar heat, creating a light work environment, and balancing the running costs of HVAC systems. The Metro Performance Glass “Low E Commercial” range provides a simplified construct for commercial building glass selection. (Specific information on Metro's Low E Commercial offer)

The construct columns separate bands of visible light transmission into groups, from there it lays out what performance characters are possible within that band for the many Low E glass types that Metro Glass has access to.  Thus the tool allows selection to view the tradeoffs between letting in more light versus light reflectance and or shading co-efficient. The range of insulated glazing units is comprised of three separate groupings – Neutrals, Tints & Reflectives. Simply choose your general building look, then flow through to colour, visible light transmission (VLT), shading co-efficient, and U values.

From this simple selection process the team at Metro Performance Glass can help you further refine your requirements, whether they be noise reduction, wind loadings, printed and/or coloured spandrel panels, or a solar reducing screen print.

A neutral optically clear IGU with your choice of performance rating.
Neutral Category 2 3 4 5 6 7
Colour/Colour Code            
Neutral   NGY3 NGY4 NGY5 NGY6 NGY7
Neutral Category Performance Ranges
VLT Range             26 - 35 36 - 45 46 - 55 56 - 65 66 - 75
VLR   16 - 18 11 - 16 11 - 17 11 - 15 11 - 12
SC   0.16 - 0.23 0.16 - 0.29 0.24 - 0.33 0.29 - 0.40 0.41 - 0.62
U Value Air   1.3 - 1.4 1.3 - 1.4 1.3 - 1.4 1.3 1.3 - 1.5
U Value Argon   1.0 - 1.2 1.0 - 1.2 1.0 - 1.2 1.0 1.0 - 1.3
A tinted glass IGU available in a choice of four colours and your choice of performance rating.
Tint Category 2 3 4 5 6 7
Colour/Colour Code            
Grey TGY2 TGY3 TGY4 TGY5 TGY6            
Bronze TBZ2   TBZ4      
Blue   TBL3 TBL4 TBL5    
Green TGN2 TGN3 TGN4 TGN5 TGN6  
Tint Category Performance Ranges
VLT Range 16 - 25 26 - 35 36 - 45 46 - 55 56 - 65  
VLR 7 - 18  6 - 19 6 - 19 7 - 18 8 - 11  
SC  0.22 - 0.24 0.22 - 0.46 0.26 - 0.50 0.31 - 0.53 0.34 - 0.47  
U Value Air  1.3 - 1.7 1.3 - 1.7 1.3 - 1.7 1.3 - 1.7 1.3 - 1.7  
U Value Argon 1.0 - 1.5  1.0 - 1.5 1.0 - 1.5 1.0 - 1.5 1.0 - 1.5  
A reflective glass IGU available in a choice of five colours and your choice of performance rating.
Tint Category 2 3 4 5 6 7
Colour/Colour Code            
Silver RSV2 RSV3 RSV4 RSV5              
Grey   RGY3         
Bronze     RBZ4      
Blue RBL2   RBL4      
Green     RGN4      
Tint Category Performance Ranges
VLT Range 16 - 25 26 - 35 36 - 45 46 - 55    
VLR 20 - 31  22 - 44 23 - 37 23 - 26    
SC  0.22 - 0.26 0.23 - 0.34 0.33 - 0.36 0.36 - 0.44    
U Value Air  1.4 - 1.7 1.3 - 1.7 1.4 - 1.5 1.4 - 1.5    
U Value Argon 1.1 - 1.5  1.0 - 1.5 1.1 - 1.3 1.1 - 1.3              

For more detailed glass specification and performance data, or if you require a glass specification not included in the tables above, please contact your local Metro sales/QS team (contact details available through our website - www.metroglass.co.nz)

Let us help you develop and bring your dream project to life. 

*Ref  https://www.architectmagazine.com/technology/from-energy-sink-to-energy-efficient-a-walk-through-window-technologies_o