Whether you’re building a new home or renovating your current home, the choice of glass for the windows impacts more than just the aesthetics. In addition to choosing the right type of window frames, choosing the right type of glass can significantly help reduce costly energy bills and keep your home at a consistent, comfortable temperature without sacrificing looks or visual clarity. One of the best options is Low-E glass.
It starts with how you lose and gain heat through windows: Conduction, Convection, Radiation and Air Infiltration (see Diagram 1).
While double and single glazing can more or less reduce the loss/gain of heat caused by conduction and convection, it is very inefficient in reducing the loss/gain of radiant heat. This is where Low-E glass can make a substantial difference in the energy efficiency of windows. Low-E glass is a standard clear glass that has a special coating on one of its surfaces. The coating is referred to as a Low-E coating, and its purpose is to reduce the amount of IR (Infrared) light, which results in heat, and UV (ultraviolet) light, which causes colours to fade, while still allowing plenty of visible light to pass through.
The “E” in Low-E glass stands for emissivity, a measurement of how effective a material is at transferring IR and UV light. Emissivity values range between 0 and 1, with lower values providing better performance from a climate control perspective. A surface or material with a low emissivity value will reflect more UV and IR light.
When the surface of a glass window has low emissivity, it is harder for IR and UV light to pass through. This has a two-fold effect: (1) heat has a harder time entering your home through the windows during the hot months, and (2) heat from within your home has a harder time exiting through your windows during the cold months.
Because of its low emissivity, Low-E glass is considered an energy-efficient window glass because it can significantly reduce the amount of heat conducted through the glass, thus lowering your heating and cooling bills.
There are two basic categories of Low-E glass based on the type of coating used: soft coat and hard coat.
Soft coat Low-E glass allows more visible light to pass through the glass and exhibits a much less visible haze and better optical clarity while preventing up to 70% less UV transmission compared to standard windows. Because it is an ultra-low emissivity coating, it also possesses excellent insulating properties.
There are, however, drawbacks to using a soft coat. It will be less durable than a hard coat option, and there may be slight colour variations present. A soft coat is also extremely fragile and tends to cost more because of the number of coatings necessary and the type of material that makes up the coating.
A soft coat is applied using a method called Magnetron Sputter Vacuum Deposition (MSVD), which deposits metal particles (usually titanium or silver atoms) on the surface of the glass while it is inside a vacuum chamber. This process takes place after the glass has been fabricated.
Extensively used, hard coat Low-E glass is much harder to scratch and far more durable than soft-coat Low-E glass. In addition, it can easily be used on single-glazed windows but typically costs more than soft-coat Low-E glass.
Hard coat Low-E glass has drawbacks, including haze levels that are slightly higher and somewhat less optical clarity. While it is still energy efficient, it does not provide quite as much insulation and isn’t quite as good at preventing UV ray transmission as its soft coat counterpart.
The hard coat for Low-E glass is added to the glass while it is still in a semi-molten stage through a process called pyrolysis. The hard coating is chemically bonded to the glass through Chemical Vapor Deposition (CVD) and requires fewer layers than its soft coat counterpart.
The U-value is a measurement of the rate of heat flow by convection, conduction, and radiation. This particular measurement is primarily used by the glass window industry and expressed in Watts per m2 / Kelvin. Windows that have a low U-value provide better insulation, which is what Low-E glass achieves.
The Solar Heat Gain Coefficient (SHGC) is the total amount of solar energy (heat gain) that is being transmitted through glass. Part of this heat gain is absorbed by the glass itself, and part of it is reradiated from the glass to the inside of the building.
Because this refers to heat gain, it is primarily used in reference to the impact of solar energy on cooling. And the lower the SHGC, the less heat gain the building will experience. Note that SHGC can either be expressed as a percentage or in Watts per m2 of surface area.
Low-E glass exhibits low U-values and lower SHGC, indicating that it provides excellent insulation and does a good job of limiting how much heat gain passes through the glass. Also, note that soft coat Low-E glass can have a lower U-value and SHGC than the hard coat option.
Consider these typical values for comparison,
Low-E glass offers several distinct advantages for your Melbourne home. As already mentioned, it is cost-effective and contributes to energy efficiency by reducing heat loss through your windows in the cold months and provides heat blocking (because of its low SHGC value), which prevents heat gain in the hot months. This reduces your dependence on heating and cooling while also lowering the bills associated with keeping your home at a comfortable temperature.
Low-E glass also provides UV protection which is not only beneficial to the health of you and your family but also prevents your furnishings and carpet from fading due to sun exposure–all without compromising the amount of visible light that does pass through. In addition, Low-E glass also reduces glare, meaning that you can enjoy the outdoor views much more comfortably. Finally, Low-E glass windows are known for their durability.
There is an interesting misconception that you can only use Low-E glass or double glazing, but not both. However, they actually perform quite well together–in fact, Low-E glass is often used with uPVC double-glazed windows. And the result is even lower SHGC and U-values than can be attained using Low-E glass or double glazing alone.
The key issue in combining these two approaches lies in determining which of the double panes should have the Low-E coating. In a single pane window, the Low-E coating is placed on the inside surface. In a double-glazed window, where the Low-E coating is placed depends heavily on whether your home is in a hot climate or a cold climate. For hot climates, the Low-E coating should be on the inside facing surface of the outer pane (surface #2 in Diagram 2). For colder climates, it should be on the inside pane facing outwards (surface #3 in Diagram 2).
Low-E glass is an excellent way to improve the energy efficiency of your Melbourne home and is even more effective when combined with our uPVC double-glazed windows.
If you’re in the market for high-quality, energy-efficient windows that complement the beauty of your home, Windows for Life has the solution for you. Our team of window and door experts will work with you all the way from an initial consultation to after-sales support to find the right type of window, including custom-made solutions.