Low-E and solar control glass: the guide to choosing right

When people replace their windows, almost everyone looks at the frame: PVC or aluminium, the colour, the number of chambers. Very few stop to ask “but what exactly is the glass you’re putting in?” That’s a mistake, because in a window the glass covers most of the surface and does most of the thermal work. Two windows with the exact same frame can behave very differently depending on the glass: one retains heat in winter and keeps the sun out in summer, the other turns your living room into a greenhouse in July.

In this article, written by a window manufacturer, I explain the two treatments that matter most on glass: low-emissivity glass (low-E), which works on heat loss in winter, and solar control / selective glass, which works on solar heat gain in summer. I also explain how to read the three values you’ll find in specifications — Ug, g and LT — without being dazzled by an acronym. The goal is for you to choose the right glass for your orientation and your climate zone, not simply the most expensive one regardless.

A scope clarification, so as not to mix up topics: here I’m only covering the thermal and optical treatments of glass. Whether triple glazing is worth it compared with double glazing (i.e. the number of panes) is covered separately in double or triple glazing: when it’s really worth it, and acoustic glass for noise reduction has its own logic, covered in how to soundproof against noise with the right windows. In this guide we stay on thermal-solar performance.

How modern window glass is built

The glass in a window isn’t a single pane. It’s an insulating glass unit (or sealed glazed unit): two panes (double glazing) or three (triple glazing) separated by a sealed cavity filled with gas. Schematically, in the most common double-glazing package:

1. Outer pane — facing outside; this is often where the solar control treatment is applied.
2. Gas-filled cavity — the cavity, usually filled with argon (sometimes krypton), which insulates better than air.
3. Spacer bar — the perimeter profile that keeps the panes apart; the “warm edge” type is made of a low-conductivity material.
4. Inner pane — facing the room, where the low-emissivity treatment is usually applied.

Everything hinges on this “sandwich”. The treatments are invisible micro-coatings applied to the inner faces of the panes (never exposed to air, or they would degrade). Let’s look at them one by one.

Low-emissivity glass (low-E): retaining heat in winter

Low-emissivity, or low-E, glass is a microscopic metallic coating — typically silver-based — deposited on one inner face of the glass. You can’t see it with the naked eye: it’s transparent to light. What it does is act like a mirror for infrared radiant heat: it lets light in, but reflects back into the room the heat that the glass would otherwise radiate outwards.

In practice: in winter, the heat you produce inside the house (radiators, people, appliances) tends to “escape” through the glass. Low-E coating significantly reduces this loss by reflecting the radiation back. That’s why a low-E glass, all else being equal, has a much better Ug (see below) than “bare” glass.

To be honest about it: today, low-E is the standard. Almost all recently manufactured insulating glass units have it, because without it the required insulation levels simply can’t be reached. If a quote offers you double glazing that doesn’t specify low-E, it’s worth asking for clarification: either it’s implied, or it’s dated glass.

Solar control / selective glass: keeping the summer sun out

Here we run into the opposite problem, which in Italy — given our summers — carries real weight. Low-E works on heat loss; solar control works on heat gain, the sun beating down on the glass in July.

Selective glass (also called solar control glass) has a treatment — more sophisticated metallic layers — designed to do two things at once:

• Let plenty of light in (high light transmission, LT), so the room stays bright;
• Block a large share of solar thermal energy (low solar factor, g), so the room doesn’t overheat.

The term “selective” means exactly this: the glass selects, letting through the light-carrying part of the solar spectrum while rejecting most of the heat-carrying part. It’s different from an old-style plain “dark” or reflective glass, which reduced heat by reducing light too, making rooms gloomy.

Solar control glass usually also retains low-E properties: it does both jobs, keeping heat in during winter and the sun out in summer. It’s the interesting solution for the warmer orientations (south, west), for large glazed areas and for conservatories.

The three values to read: Ug, g, LT

Here’s the heart of the choice. When you look at a glass specification, three acronyms appear. Understanding them is enough to stop anyone selling you smoke and mirrors.

Value	What it measures	Expressed as	Better when…
Ug	thermal transmittance of the glass alone (how much heat passes through it)	W/m²K	lower (insulates more)
g	solar factor: share of solar energy that enters	number from 0 to 1 (or %)	depends: low in summer, high where solar gain is wanted
LT	light transmission: how much visible light passes through	%	generally higher (brighter)

Let’s go through them one by one, because the key point is that they shouldn’t all be maximised at once: some of them trade off against each other.

Ug — the insulation of the glass. This measures how much heat passes through the glass due to a temperature difference. The lower it is, the better it insulates. It’s different from Uw, which is the transmittance of the entire window (glass + frame + edge): the value that matters by law and for the real-world performance of the window is Uw, which I cover in Uw, climate zones and thermal insulation explained. Ug is only the “glass” piece of the puzzle. As a purely indicative order of magnitude, low-E double glazing with argon can sit around Ug values of roughly 1.0–1.1 W/m²K; triple glazing goes lower still, with the trade-offs covered in the dedicated guide.

g — the solar factor. This indicates how much solar energy enters. A g of 0.60 (indicative) means that around 60% of incident solar energy enters; selective glass can drop to values of around 0.30–0.40. Here’s the tricky point: a low g is an advantage in summer and a disadvantage in winter. In summer you don’t want the sun heating the room; in winter, on a south-facing orientation, that free solar gain helps with heating. That’s why there’s no single “best” g in absolute terms: it depends on orientation.

LT — light transmission. This is how much visible light passes through. A high LT (e.g. in the order of 70–80%, indicative) keeps rooms bright; too low an LT makes them gloomy and has you switching the lights on during the day. Good selective glass is glass that keeps g low without dropping LT too much — that is precisely its reason for existing.

A word of caution against smoke and mirrors: these values must be taken from the technical data sheet of the glass you are actually buying, not from a generic table. All the figures in this article are indicative orders of magnitude given as examples, not the specification of a particular product.

Cavity gas and warm-edge spacer: the details that matter

Two “invisible” components of the insulating glass unit affect the values above.

The gas in the cavity. Air conducts heat; a noble gas such as argon conducts less, so it improves Ug. It’s now the standard. Krypton insulates a little better still and is used when the cavities are thin (typical in triple glazing), but it costs more. In practice, for double glazing, argon is the sensible and widespread choice.

The “warm edge” spacer bar. This is the perimeter profile that separates the panes. The traditional aluminium type is a thermal bridge at the edge of the glass, where condensation forms more easily. The warm edge spacer, made from low-conductivity materials, reduces this perimeter thermal bridge, marginally improves the values and, above all, limits condensation at the edges of the glass. On a modern window, it’s a detail worth insisting on.

Laminated glass: when the glass is also a safety feature

A brief note, since this often appears in specifications alongside the treatments above. Laminated glass consists of two panes bonded together by a plastic film (PVB): if it breaks, the fragments stay stuck to the film instead of falling away. It’s a safety glass (it reduces the risk of cuts and hinders forced entry) and it also has acoustic benefits.

Laminated glass is not a thermal-solar treatment: it’s a structural feature of the glass. However, it can be combined with low-E and solar control treatments. It’s useful on French windows, full-height glazing, ground-floor openings and any situation where personal safety matters. I cover the acoustic side of laminated glass in more depth in the guide to noise-reducing windows.

How to choose glass by orientation and climate zone

Now for the practical part. Choosing glass isn’t about “the most insulating option for the whole house” — it depends on the orientation of each individual window and on the climate zone. Here’s an indicative table to help you think it through:

Orientation / context	Priority	Indicative glass type
North (little direct sun)	retain heat, maximise light	low-E, high g and LT
South (useful winter sun)	balance winter gain against summer overheating	low-E, consider selective glass for large glazed areas
West and south-west (strong afternoon sun)	reduce summer overheating	selective / solar control glass (low g)
Large glazed areas, conservatories, loft conversions	avoid a greenhouse effect	solar control almost always recommended
Cold climate zones (northern inland Italy)	thermal insulation	priority on low Ug/Uw
Hot / very sunny zones	solar control	priority on low g for exposed orientations

The underlying logic: to the north you want light and insulation — the summer sun isn’t a problem there. To the south, the winter sun is a free ally, so it doesn’t always pay to cut it out entirely; it needs a case-by-case judgement. To the west, the low afternoon summer sun is the most troublesome and penetrating: that’s where selective glass performs best. It’s worth saying that glass is only part of solar protection: an external shading system (roller shutters, shutters, sun louvres) remains the most effective way to stop the summer sun, because it blocks it before it hits the glass.

To place all this within the complete window system — frame, glass, installation — our complete guide to PVC windows may help, where glass is one of the chapters.

Standards: what’s declared and where to find it

On the regulatory side, windows as a product fall under EN 14351-1 and the CPR 305/2011 (Construction Products Regulation): the manufacturer draws up the Declaration of Performance (DoP) and affixes the CE marking, where the window’s performance values are declared. The transmittance of the entire window (Uw) is the reference figure; the values for the glass alone (Ug, g, LT) are found in the technical data sheet of the insulating glass unit supplied by the glass manufacturer.

In practical terms: always ask for the glass data sheet along with the quote. A serious manufacturer will provide it without issue. If you’re only given “low-emissivity glass” as a description, with no figures, you have little basis for comparison.

When it’s NOT worth it

In the spirit of the honesty we like to keep, here are the cases where pushing for “top-tier” glass is a waste.

• Selective glass on a north-facing orientation. No significant direct sun reaches the north side. Paying for solar control there achieves little: you only risk lowering light levels with no real summer benefit.
• Very low g on every window across the board. If you fit aggressive selective glass even on a south-facing window in a cold zone, you lose free solar gain in winter and make the heating system work harder. Glass should be chosen window by window, not as a single blanket package.
• Top-tier glass with poor installation. The best insulating glass unit, badly fitted, with thermal bridges at the joint, performs below expectations and causes condensation at the edges. Correct installation comes first — a subject covered separately — then premium glass. Spending on the glass and saving on the installation is the classic wrong order.
• Chasing the lowest possible Ug while ignoring Uw. An excellent Ug with a mediocre frame gives a mediocre window. What counts is the whole package: a balanced glass-frame combination beats extreme glass on an ordinary profile.
• Loading everything onto the glass for problems the glass can’t solve. If your problem is noise, the solution is acoustic glass (a different logic altogether); if it’s the number of panes, that’s another matter. Thermal-solar glass solves heat and cold, not everything.

FAQ

Can you see low-emissivity glass with the naked eye? No. The low-E treatment is a transparent micro-coating: to the eye, you can’t tell a low-E glass from one without it. At certain angles and in certain light, selective glass can occasionally show a very slight reflected tint, but under normal conditions both are neutral. The reliable way to know what you have is the technical data sheet, not your eyes.

Are low-E and selective glass the same thing? No, but they’re related. Low-E works mainly on heat loss (winter insulation) and is now the standard. Selective / solar control glass adds the ability to block solar heat gain in summer while keeping light levels high. Good selective glass is usually also low-E: it does both jobs.

Does a low g make a house dark? Not necessarily, and that’s precisely the point of “selective” glass: it keeps g low (little solar heat) while keeping LT high (plenty of light). The dark-house problem was a flaw of old-style reflective glass, which cut everything. Modern selective glass is designed to avoid this: always check the LT value on the data sheet.

Is selective glass worth it if I already have roller shutters? External shading (roller shutters, shutters) is the most effective way to stop the sun, because it blocks it before it reaches the glass. If you use them and are happy to lower them, selective glass becomes less essential. It remains useful wherever you can’t or don’t want to keep things shaded — conservatories, large glazed areas, rooms you want bright but cool.

What’s the difference between Ug and Uw? Ug is the transmittance of the glass alone; Uw is that of the entire window (glass + frame + edge). Uw is the value that really counts for real-world performance and is the one declared on the CE marking. An excellent Ug isn’t enough if the frame is mediocre. This is covered in more depth in Uw and climate zones.

Can I trust the Ug, g and LT values a salesperson quotes me? Only if they come from the technical data sheet of the glass you’re actually buying. Always insist on that document. Figures quoted “from memory” or taken from generic tables are useful for understanding orders of magnitude, but not for comparing two quotes: for that you need the data of the actual product, declared in writing.

In summary

Glass is half the window, and its treatments determine how it behaves in heat and cold. Low-E retains heat in winter and is the standard; selective / solar control glass keeps the summer sun out while preserving light, and performs best on west-facing orientations and large glazed areas. The three values to read are Ug (glass insulation), g (solar heat gain) and LT (light): they shouldn’t all be maximised at once, but chosen window by window based on orientation and climate zone. Beneath all this sit the details that matter — argon in the cavity, warm-edge spacer bars — and the golden rule: always ask for the technical data sheet.

If you want to work out which glass best suits your home, window by window, we can think it through together by looking at orientations and climate zone: take a look at our PVC windows and bespoke windows that we manufacture. Once you have an idea in mind, request a quote and we’ll help you choose the right glass without paying for treatments you don’t need.