LOW EMISSIVE GLASS COATINGS - SURFACE 2 OR 3 – WHAT SHOULD IT BE?
Low E coating were initially used in cold climates to keep heat in however it was soon recognised that they could help reduce solar heat gain as well. In this article Allan Sage, Technical Manager of Metro Performance Glass, discusses the history, difference and usage of Surface 2 and Surface 3 coatings.
Low Emissivity (Low E) glass was first developed around 1983 and used to improve the insulation performance of window glazing in UK, European and USA markets. Prior to this the markets needed triple glazing and these were hard to manufacture, thick, heavy, and difficult to fit into frame profiles at the time. It was common to have single pane glass with double pane IGUs behind in a secondary sash or double IGUs with single panes behind as a secondary sash window. During 1990 to 2000 the USA, UK and European building regulations pushed for higher window performance via lower U values of the Insulating Glass Units (IGUs), and the Low E IGU market was born.
Low E - IGUs
Based on research from the aerospace industry, transparent low emissivity coatings were developed that could be applied to glass, first via sputter coating technology and then via pyrolytic (hard) coating technology. The result was double pane Insulating Glass Units, that had the performance of triple glazing with U values around 2 W/m2K, which was significantly better than 3 W/m2 K for double IGUs, and much easier to glaze. Further developments in coating technology lowered the U values to 1.5 W/m2K and then down toward U1.1 which are current European norms. Some Euro codes are moving below U1 which requires very high performance windows.
Clear Low E
Early Low E coatings were first used in cold climates to keep heat in the building and reduce the need for triple glazing. They do this by reflecting long wave radiation back into the building, and are best understood as being “transparent tin foil”.
So early Low E coatings were relatively clear and on clear glass to allow as much of the suns short wave energy into the building and trap it there. These coatings therefore were best used on surface 3 of the IGU and with clear glass outer.
It is important to note the term “clear Low E” refers to low E coatings on clear glass, not the fact that they are clear – like clear glass.
Solar Low E
It was soon recognised that Low E coatings could help reduce solar heat gain as well as insulate so they were first combined with tinted glass so the tint was the outer glass and the Low E the inner glass with the coating still one surface 3. The tinted glass absorbed heat and the Low E surface help to push it away from the building hence lowering solar heat gain and the shading coefficient (SC).
The next step was to coat the tinted glass with a Low E coating and use clear glass as an inner, and this further improved the solar performance of the IGU and lowered the shading coefficient.
Further development in coating technology meant that the coating could be colour neutral or a range of colours and applied to clear glass to reflect and absorb heat and provide a Solar Low E glass. These coatings are normally surface 2 coatings and are the most common form of high performance Low E coatings used in IGUs.
The thermal insulation provided by these coating varies depending on the emissivity, and the U value can vary from 2 down to 1 W/m2K for IGUs. The lower the emissivity of the coating the lower the U value and hence you need high performance sputter coated glass with emissivity below 0.05 to achieve these low U values. Float glass has an emissivity of 0.84, pyrolytic coatings range from 0.15 to 0.3, and sputter coatings from 0.15 to 0.01 depending on their silver layers.
The U value is same with the coating on surface 2 or 3 but the optical and solar properties can change.
It should be noted the U values can vary by the calculation method, the spacer width, and the use of argon gas to replace air, so these should be reviewed if comparing products.
Recent advancements in coating technology have created “post temperable” sputter coated glasses, which means they can be tempered (toughened) after they have been coated. Early sputter coating had to be applied to the glass after toughening, and this is why pyrolytic coated glasses have dominated the NZ market.
The sputter coated glasses, have advanced from single silver, to double silver to new triple silver multiple stack coatings which have very low emissivity and are very hard to process without advanced machinery and technology.
SO – what’s it to be - Surface 2 or 3
The common saying “horses for courses” is very applicable when considering the use of Low E glasses.
If you want to control some solar heat gain, glare and UV in summer but keep warm at night and in winter then Solar Low E coatings on Surface 2 are the best for temperate and warmer climates (like most of New Zealand). If you want high performance for large glazed areas, such as commercial projects, then high performance Low E coatings on Surface 2 of clear or tinted glass are the best.
If you want a lot of light and be very warm then you are best to use clear glass and clear Low E Surface 3 products, to allow the heat in and trap it – like a glasshouse. Surface 3 Low E products keep the inner panes warmer reducing the onset of condensation. They can also be used where solar gain is not an issue such as south facing or shaded facades of houses or commercial projects. If you want to combine tint and Low E for aesthetics and better solar control then it is easier to achieve this buy using clear Low E on Surface 3.
Warning: Solar Low E glass can get very hot if used on Surface 3 so you need to be careful if considering this.
Masterspec product partner Metro Performance Glass have a residential range of Low E double glazing, known as Low E Plus.
You can check out their Low E Plus, Max and Xcel products at this link.
For further technical advice contact Metro Performance Glass in your area here.