Low-E glass is one of the most useful upgrades you can specify in a double glazed unit — and one of the most misunderstood. People often mix it up with low-iron glass, assume it will darken a room, or think it is only worth it in a cold climate.
At The Glass Shop Online we specify Low-E coatings every day in replacement sealed units and new double glazing. This guide explains what it actually does, when it is worth paying for, and which coatings we recommend.
What Low-E glass actually is
Low-E stands for low emissivity. It is not a different type of glass in the way toughened or laminated is. It is ordinary float glass with a thin, almost invisible coating applied to one face. That coating changes how heat and light pass through the pane.
Standard clear glass has an emissivity of about 0.84, which means most heat passes straight through. Low-E coatings can bring that figure down to as little as 0.02. In practice that means the glass reflects infrared heat instead of radiating it.
- Reflects infrared heat — keeps warmth inside in winter and reduces solar heat in summer
- Blocks UV — typically 90–99% of UV radiation, which helps stop furniture, flooring and fabrics from fading
- Still lets daylight through — unlike many films or heavily tinted glasses
How Low-E glass works — video from Vitro Architectural Glass.
What a Low-E unit looks like
From the face of the window you should still see a clear, bright room. The coating sits on one face inside the sealed unit. What you can see on a cut-to-size unit is the spacer bar, the two panes and the sealed edge — that is the product we manufacture and deliver.
A typical sealed unit: two panes, spacer bar and Low-E coating on the inner pane. We cut units to your exact size.
Low-E is not tinted glass. Soft coat typically lets through around 95% of visible light.What it does for comfort and bills
The coating works year-round. In winter it holds heat inside the room. In summer it reduces the solar heat that would otherwise build up. That makes the house more comfortable and cuts heating and cooling costs — typically by around 20–30% compared with standard clear glass in the same unit.
Other benefits customers actually feel:
- Better year-round comfort without needing heavy tint
- Lower carbon footprint — a typical house can save around 1,500–2,000 kWh a year, which is in the same ballpark as planting 10+ trees
- Can support property value when windows are upgraded properly
- UV protection for interiors
How it compares on U-value
U-value is the figure that matters for heat loss. Lower is better.
| Typical unit make-up | Indicative U-value |
|---|---|
| Clear / clear with an 18 mm spacer | Around 2.0 |
| 4 mm clear / 4 mm soft-coat Low-E with the same spacer | Around 1.4 |
That is a large step for what is usually a small extra on the unit price. Lead time is the same as a standard unit. Most customers recoup the difference through energy savings within 3–7 years.
Soft coat vs hard coat
There are two main families of Low-E coating.
- Soft coat — higher performance and the better route for energy saving. This is what we specify as standard.
- Hard coat — more durable and a little cheaper, but not as efficient. We can supply it if the job specifically needs it.
Our standard Low-E glass is Planitherm Total Plus. Where a different performance target is needed we can also offer coatings such as Planitherm 1 and Cool-Lite SKN (solar control). Tell us the U-value, g-value or solar-control target and we will match the coating to the unit.
Browse our cut-to-size double glazed units — Low-E and soft-coat options are available on the unit specification.
Common myths
- “It will make the room dark.” It does not. Soft-coat Low-E typically lets through around 95% of visible light. It is not the same as tinted glass or privacy film.
- “It only helps in a cold climate.” It works in both directions — holding heat in during winter and reducing solar gain in summer.
- “It is very expensive.” On a standard double glazed unit, swapping clear glass for Low-E usually adds about 5–10%, depending on the exact coating. Same lead time.
- “You can’t upgrade existing windows.” You can. Old sealed units can be taken out and replaced with new units that include a Low-E coating, provided the frame will take the new unit.
Low-E is also not the same as low-iron glass. Low-iron removes the green tint for a clearer look. Low-E is a thermal coating. You can specify both on the same unit if appearance and performance both matter.
Honest trade-offs
It is not a free upgrade. The unit costs a little more. There can be a very slight tint depending on the coating. External condensation is possible on high-performance units because the outer pane stays colder — that is a sign the unit is holding heat inside, not a fault. Those are minor compared with the comfort and energy benefits for most jobs.
Our recommendation at The Glass Shop Online
For most replacement sealed units and new double glazing we recommend a soft-coat Low-E pane — Planitherm Total Plus as standard — unless the project needs a specific solar-control or heritage make-up. We will check the spacer, gas fill and coating against the performance you actually need, rather than adding every upgrade by default.
Tell us the sizes, whether it is a replacement unit or new glazing, and any U-value or solar-control target. We will specify the right Low-E coating and unit make-up before anything is made.
Same lead time as a standard unit. Fast quote, cut to your exact sizes.
Questions you might still have
Is Low-E the same as low-iron glass?
No. Low-iron reduces the green tint for a clearer look. Low-E is a thermal coating that reflects infrared heat. They solve different problems and can be combined in one unit.
Will Low-E make my rooms darker?
No. Soft-coat Low-E typically allows around 95% of visible light through. It is not the same as tinted glass.
How much extra does it cost?
On a standard double glazed unit, specifying Low-E instead of clear glass usually adds about 5–10%, depending on the coating. Lead time is the same.
What coating do you supply as standard?
Planitherm Total Plus soft coat. We can also supply Planitherm 1, Cool-Lite SKN and other coatings where a specific thermal or solar-control performance is required. Hard coat is available if the job needs it.
Can I upgrade existing windows to Low-E?
Yes. The old sealed units can be removed and replaced with new units that include a Low-E coating, provided the existing frames will accept the new unit size and thickness.