Put simply, what is Low-E glass comes down to one invisible layer. Low-E glass is ordinary glass carrying a microscopically thin coating of metal oxide that reflects radiant heat while still letting visible light through. The E stands for emissivity, a measure of how readily a surface radiates heat energy. Lower the emissivity of the glass and you lower how much heat that surface gives up to whatever is on the other side. The result behaves far less like a radiator and far more like a thermal boundary, which is why the coating is a standard part of specifying high performance double glazing rather than an optional extra.
The coating works without you noticing it, which is the source of most of the confusion around it. People expect a visible film, a tint or a reflective sheen. What they get is glass that looks very nearly clear but performs quite differently.
Emissivity, in plain terms
Every surface above absolute zero radiates heat as infrared energy, and emissivity describes how efficiently it does so. Uncoated glass has a relatively high emissivity, so it absorbs infrared energy and re-radiates it from its other face. In winter the warmth your heating put into the room is absorbed by the inner glass and radiated outward. In summer the glass absorbs heat from outside and radiates it inward.
A low emissivity coating interrupts that exchange. Because the coated surface radiates very little infrared, heat striking it is largely reflected back towards where it came from rather than passed along. The coating is engineered to be selective: it reflects the long wavelength infrared that carries heat while staying largely transparent to visible light. That selectivity is the whole point. You keep the daylight and reject much of the heat transfer.
Where the coating sits and why that matters
In an insulated glass unit the coating is applied to a surface facing the sealed cavity, which protects it from abrasion, cleaning and weather. Which surface carries it changes what the glass does.
A coating on the inner pane facing the cavity keeps interior heat inside, which suits cool-climate situations where retaining winter warmth is the priority. A coating on the outer pane facing the cavity rejects incoming solar heat before it reaches the room, which suits hot and heavily sun-exposed situations. In much of New South Wales you are managing both cold mornings and hot afternoons, so the answer depends on orientation, shading and which rooms overheat. Northern and western exposures behave very differently from southern ones.
The two main types you will encounter
Hard coat, also described as pyrolytic, is applied while the glass is still hot during manufacture, which fuses it into the surface. It is durable, can be processed like ordinary glass and can be used as a single pane. Its selectivity is less sharp, so it allows more solar heat through, which can help where passive winter heat gain is wanted.
Soft coat, also described as sputtered, is applied in a vacuum chamber after manufacture and uses multiple ultra-thin layers, often including silver. It achieves better thermal performance and far more precise control over how much solar heat is admitted relative to light. The trade-off is that the coating is delicate and must be sealed inside an insulated glass unit, where the cavity protects it.
Working out what a rating is telling you
Low-E glass is described by a few published measures. U value describes how readily heat passes through the whole window assembly, so a lower figure means less heat transfer. Solar Heat Gain Coefficient, or SHGC, is the fraction of incident solar energy that ends up as heat in the room, so a lower figure means more solar rejection. Visible Light Transmittance describes how much daylight gets through.
The authoritative sources for these figures in Australia are the window energy rating scheme known as WERS, which publishes rated performance for whole window systems rather than glass alone, the Australian Glass and Window Association, which publishes technical guidance on glazing performance, and the National Construction Code, which sets the energy efficiency provisions your glazing must satisfy. I am deliberately not quoting U value or SHGC numbers here, because a figure only means something attached to a specific tested window system and attributed to one of those sources.
One trap worth naming: a glass-only performance figure and a whole-window figure are not the same thing, because the frame and the edge of the glass unit both participate in heat transfer. WERS rates the complete window, which is the number that reflects what you will experience.
Why the frame has to be part of the conversation
You can specify excellent glass and still end up disappointed, because heat takes the path of least resistance. A frame that conducts heat quickly creates a thermal bridge through the wall, and the glass cannot compensate. Multi-chambered uPVC profiles contain sealed air pockets within the frame section, which slows conduction and brings the frame’s behaviour closer to that of the glazing it carries. That pairing matters, and the uPVC window and door systems we work with are specified as frame and glass together for that reason.
How it changes the way a room feels
The difference people report first is not a temperature reading, it is radiant comfort. Standing near a large uncoated window on a cold night, you feel cold on the side facing the glass because your body is radiating heat to that cold surface. With a low emissivity coating the inner pane runs warmer, so that chill is reduced and the usable floor area near the window increases. In summer the equivalent is the absence of that hot glow off a sun-struck pane.
Two side benefits follow. The warmer inner surface is less likely to drop below the dew point of the indoor air, which reduces condensation. And because the coating reflects some ultraviolet, fabric, flooring and artwork fade more slowly, though no glazing stops fading altogether.
Low-E in more demanding glazing build-ups
Coated glass rarely works alone in a demanding specification. It gets combined with laminated panes for acoustic damping and safety, with toughened glass where impact requirements apply, and with specific thicknesses and cavity dimensions to meet a target. In bushfire-prone areas the build-up is driven by tested compliance at the relevant Bushfire Attack Level, so the glazing make-up for bushfire rated windows and doors has to satisfy that test regime first, with thermal performance specified within those constraints. The coating is a variable in a system, not a standalone product.
Putting it together
A low emissivity coating only delivers what it promises when the frame, the cavity and the orientation are considered alongside it. Ask for the whole-window rating, ask which surface the coating sits on and why, and judge the specification against the aspect it will face.
If you would like help working out what glazing specification suits your orientation and your rooms, the team at uPVC Windows Expert can walk you through it. Get in touch with your questions.
Frequently Asked Questions
What are the disadvantages of Low-E glass?
A few are worth knowing. A coating that rejects solar heat also reduces useful winter heat gain, which can work against you on a southern aspect or in a cooler part of the state. Soft coat types reduce visible light transmittance slightly, so rooms can feel marginally less bright, and some carry a faint tint at certain angles. Soft coat also cannot be used as an exposed single pane. And because performance is directional, a coating applied for the wrong priority can underperform in the aspect it faces.
How can I tell if I have Low-E glass?
The simplest field test uses a flame. Hold a lit match near the glass at an angle and count the reflections. Clear double glazing gives four reflections of the same colour, while a low emissivity coating produces one with a noticeably different tint, often a faint violet or pink. Failing that, check the spacer bar for an etched product code or ask for the original window documentation.
What is the difference between Low-E glass and normal glass?
Physically, only the coating. Functionally, the difference is how each handles radiant heat. Normal clear glass has a high emissivity, so it absorbs infrared energy and re-radiates it readily in either direction. Low-E glass reflects most of that long wavelength infrared while still transmitting visible light, so you keep daylight and lose far less heat. They look almost identical and behave quite differently.
Is Low-E glass better than double glazing?
They are not alternatives, which is the key point. Double glazing describes the construction of the unit, meaning two panes with a sealed cavity. Low-E describes a coating applied to a glass surface. Most high performing units are both: double glazed for the insulating cavity and coated for radiant heat control. A coated double glazed unit outperforms an uncoated one, so the two work together rather than competing.