Low-E or Solar-Control Glass? A New-Build Guide

If you are building a new house, you will choose your glass once and then live with it for the next thirty years. It is one of the few decisions that is very cheap to get right on the architect’s drawings and very expensive to fix afterwards.

Most people never think about it. The glass all looks the same. Yet the wrong specification is exactly why so many beautiful new homes are too hot in July and cost a fortune to heat in January. Here is the physics, in plain terms, so you can ask the right questions before the walls go up.

This is the companion to our piece on why glazed rooms overheat. That one is about the air and getting the heat out. This one is about the glass and stopping the heat getting in.

Two different problems, two different properties

A window has two jobs that pull in opposite directions.

In winter you want to keep the heat you have paid for inside the room. In summer you want to keep the sun’s free heat out. These are measured by two completely separate numbers, and confusing them is where nearly everyone comes unstuck.

The first is the U-value. It measures how quickly heat escapes through the glass. Lower is better. This is your winter number.

The second is the g-value (sometimes called the solar factor). It measures how much of the sun’s heat passes through the glass into the room. A g-value of 0.6 means 60 per cent of the sun’s energy gets in. Lower is better in summer. This is your summer number.

They are independent. A window can be excellent at one and poor at the other. That is the whole story.

Low-E glass: brilliant in winter, quietly guilty in summer

Almost every new build in Britain now comes with low-emissivity, or low-E, glass as standard. The building regulations more or less demand it, because it gives a low U-value. In Scotland the standard requires windows to average no worse than 1.6 W/m²K across the whole element, and a modern low-E double-glazed unit has a centre-pane U-value around 1.2 W/m²K.

Low-E works by a microscopically thin metallic coating that reflects the long-wave heat from your radiators and wood burner back into the room instead of letting it drift out through the glass. In winter it is doing exactly what you want.

Here is the catch nobody mentions. Standard low-E glass has a high g-value, around 0.59. It lets most of the summer sun straight in. Worse, once that solar energy has been soaked up by your floor and furniture and re-radiated as heat, the same coating that keeps your radiator heat in also keeps that solar heat in. On a south-facing room in July, standard low-E glass can actually make overheating worse.

So low-E is a winter product. It is not summer solar control. Those are two different coatings.

Solar-control glass: the summer answer, specified early

Solar-control glass is a different coating with a low g-value, typically around 0.32 or lower, and on the best products down to roughly 0.19. It reflects a large part of the sun’s heat away before it ever enters the room.

The clever part is that it does this without sacrificing your winter insulation. Solar-control units still carry a low-E layer, so the U-value stays around 1.1 W/m²K, marginally better than standard low-E. You keep your winter insulation and gain summer solar control at the same time.

Glass type	U-value (winter)	g-value (summer)
Clear single glazing	5.8	0.85
Clear double glazing	2.9	0.76
Standard low-E double	1.2	0.59
Solar-control double	1.1	0.32

The bottom two rows are the decision. For rooms facing anywhere from east through south to west, and especially for large or tall glazing like gables and roof lanterns, solar-control glass is worth specifying. The important word is specifying. You cannot easily add it later, which is the whole point of raising it at the design stage.

There is one honest trade-off. Because a solar-control coating is fixed, it turns away some of the free winter sunshine you might otherwise have enjoyed. Glass cannot tell the seasons apart. That is exactly where internal blinds earn their place, which we come to below.

What about tinted glass?

Tinted glass is often suggested as a cheaper fix. Be careful. A body tint mainly cuts glare and daylight, and it does reduce solar heat, but only modestly and inefficiently. It works by absorbing the sun’s energy, which then makes the glass itself hot, and a fair part of that heat re-radiates into the room. It also does nothing for your winter U-value.

Tint has its place for glare, but on its own it is not a serious answer to overheating.

The retrofit film trap

If you get the glass wrong, the tempting fix is a solar-control film stuck on afterwards. We would urge real caution here.

Film increases how much heat the glass absorbs. That can create a temperature difference between the sun-warmed centre of the pane and its cool, frame-shaded edges, and that stress can crack the glass. It can also overheat the sealed edge of a double-glazed unit and cause the seal to fail, leaving you with permanent misting between the panes.

Just as importantly, applying aftermarket film usually voids the original glazing manufacturer’s warranty. Film makers offer their own cover, but it typically only applies while the window’s own warranty is still valid, and it does not extend it. Modern reflective films are lower risk than the old dyed types, but the warranty problem remains. Far better to specify the right glass once than to patch it later.

The combination that actually works

No single measure does everything. The physics points to a layered approach, and the order matters.

External shading first. The most effective place to stop the summer sun is outside the glass, before the heat ever arrives. External shutters, blinds or a brise soleil intercept the radiation and shed the heat to the outside air.

Solar-control glazing at the glass. For the rooms and elevations that need it, low g-value glass does the heavy lifting all day, every day, with nothing to operate.

Internal cellular blinds for winter and comfort. Honeycomb blinds trap a layer of still air against the glass and can cut heat loss through the window by a useful margin on a cold night. They also handle glare, and unlike fixed glass they can be raised on a bright winter day to let that free solar warmth back in. Cellular blinds are our usual choice for the tall, shaped glazing that new builds love. You can read more about cellular blinds for gable windows and about the honest difference between room darkening and blackout.

It is also worth knowing that both the Scottish overheating standard, in force since February 2023, and the equivalent rules in England now require overheating to be assessed in new homes. Internal blinds, helpful as they are for comfort, are not counted towards that compliance. The glass and the external shading are what the regulations credit, another reason to settle them early.

The one question to ask

When you sit down with your architect or builder, ask this: which of my windows face east through south to west, and what is the g-value of the glass going into them?

If the answer is a blank look, or “standard low-E”, you have found the room that will overheat. That is the conversation worth having now, while it costs nothing but a line on a drawing.

Key things to remember

Low-E glass is a winter product. It keeps your heat in but lets the summer sun through.
Solar-control glass has a low g-value. It rejects summer heat and still insulates well in winter.
Tinted glass helps glare, not much else. Retrofit film risks cracking your glass and voiding the warranty.
The best result is layered: external shading, solar-control glazing where needed, and internal cellular blinds for winter warmth and glare.
Decide it at the design stage. It is the cheapest it will ever be to fix.

Planning a new build?

If you are designing or building a home and want the glazing and shading thought through room by room, we are always happy to help at the drawing stage. Get in touch for a straightforward conversation, with no pressure and no hard sell.

Frequently asked questions

What is the difference between low-E glass and solar-control glass? Low-E glass has a coating that keeps your heating inside in winter, giving a low U-value, but it still lets most of the summer sun through because it has a high g-value. Solar-control glass has a low g-value, so it reflects a large part of the summer heat away while still insulating well in winter. They are different coatings for different jobs.

Does low-E glass stop a room overheating in summer? No. Standard low-E glass is designed for winter insulation and has a high g-value, so it admits most of the sun’s heat. It can even make summer overheating slightly worse, because the same coating traps re-radiated heat inside the room. For summer control you need solar-control glazing, external shading, or both.

What is a g-value? The g-value, or solar factor, is the fraction of the sun’s heat energy that passes through the glass into the room. A g-value of 0.6 means 60 per cent gets through. A lower g-value means less summer heat gain, which is what you want on sunny elevations.

Is it safe to put solar-control film on my existing windows? It carries real risks. Film increases heat absorption in the glass, which can cause the pane to crack through thermal stress or the sealed unit to fail and mist up. It also usually voids the glazing manufacturer’s warranty. Specifying the right glass from the start is a safer and more durable solution.

Which windows should have solar-control glass? The ones that catch the most sun: those facing east through south to west, and any large or tall glazing such as gables and roof lanterns. North-facing windows rarely need it. Decide this at the design stage, because solar-control glass cannot easily be added later.

Do internal blinds help with summer overheating? They help with glare and give some reduction in heat, but they work on heat that is already through the glass, so they are less effective in summer than external shading or solar-control glazing. Their real strength is winter insulation and comfort, and they can be raised to let in free winter sun.