guide

Energy efficient windows: the complete guide

05 November 2021 | Matt Mostyn

The average home loses 10% of its heat through its windows1. So when you’re thinking about making your home more energy-efficient, your windows should be one of the first places you look. 

Although double-glazing can be expensive, draught-proofing can be comparatively cheap and easy, and often gives immediate results.

Read on for answers to all your main questions about energy-efficient windows, and how to draught-proof them. 

Draught proofing windows: how to do it

draught proofing window

There are several draught-proofing products available that work with windows. Each product can have several different names, depending on the manufacturer – so it’s best to chat to a sales assistant to make sure you’re getting what you want.

  • Wiper seals (also known as dirt excluders) are metal or plastic strips that have brushes or wipers attached. The brush compresses slightly when the window is closed, creating a seal. These come in various sizes, and work well on sliding sash windows, or anywhere there’s an uneven gap or a warped surface.
  • Compression strips, draught excluder strips, or tapes are self-adhesive foam strips that have a little “give”. These fill any gaps between your window and its frame. It’s important to choose the right size strip, as if it’s too large it can actually stop the window closing properly – the opposite of what you want!
  • Silicone sealant can be used for windows that aren’t designed to open (but are still a bit gappy). It’s also good for metal windows that can sometimes develop small gaps. 
  • Heavy thermal curtains can make a big difference, or even adding thermal linings to your existing curtains. These can trap a lot of heat indoors.

How to choose energy-efficient windows

If you’ve decided to go the whole hog and install some new windows, read on for some advice on what to look for. 

The energy performance of a window is affected by a range of factors, so it’s not always easy to choose the right window based only on how it’s made. Luckily, there’s a rating scheme to help you find the most efficient one for you.

Energy-efficient windows use an energy-rating scale from A++ (most efficient) to E (least efficient). Both the frame and the glass are assessed and given a rating that shows the overall impact of fitting that window in your home.

To choose the most energy-efficient window, look for the BFRC rating – and remember, an A-rated window is more energy-efficient than one that’s C-rated. 

Windows with an energy rating will also have a U value. While it doesn’t tell you everything about how efficient your window is, the U value shows how easily heat passes through the material. If you combine this with the energy-efficiency rating, you’ll have a good idea of the window’s energy-saving potential.

Some installers also offer a recycling scheme, where they’ll take away your old windows and recycle the materials. They’ll then reduce the cost of installing your new window in return. How about that for a clever bit of sustainable initiative-taking?!

Find out more about choosing the most energy-efficient windows with our ultimate guide to double-glazed windows.

What are the most energy-efficient windows?

baby looking out of the window

The easy, but expensive, answer if you’re looking for energy-efficient windows is to install double glazing. It creates highly energy-efficient window insulation, and you can choose from a range of materials, all equally effective. The main contenders are uPVC, wood, aluminium, steel, or composite.

It can also be less expensive to install secondary double glazing. It’s equally effective at stopping draughts and soundproofing, but can be quite fiddly to clean.

How does double glazing reduce heat loss?

Double glazing uses 2 panes of glass, and triple glazing uses 3. Here’s how they keep the cold out, and the heat in.

Energy-efficient window materials

  • Energy-efficient windows are made of 2 or 3 glass panes sealed in a single unit, and surrounded by a frame made from uPVC, wood, or another material.
  • Double-glazed windows are made of 2 sheets of glass with a gap between – usually about 16mm.
  • Triple-glazed windows have 3 sheets of glass, and 2 gaps. This can make them better at insulating than many double-glazed windows (but not always). 
  • The gaps between the glass panes are filled with air, or an inert gas like argon, which is completely sealed in.

How well the window performs depends on a few different factors, including:

Energy-saving glass

The effectiveness of your double glazing will depend on what material is used. Low emissivity (low-E) glass is the most energy-efficient type for double and triple glazing, thanks to a microscopic coating of metal oxide. This coating reflects heat back into the home, but still lets in the light from outside.

The easy, but expensive, answer if you’re looking for energy-efficient windows is to install double glazing. It creates highly energy-efficient window insulation, and you can choose from a range of materials, all equally effective. The main contenders are uPVC, wood, aluminium, steel, or composite.

It can also be less expensive to install secondary double glazing. It’s equally effective at stopping draughts and soundproofing, but can be quite fiddly to clean.

Pane gaps

The size of the gap between the 2 panes can affect performance – but bigger isn’t always better! 16mm is often considered optimal, but smaller gaps might actually be better in some cases. 

You could also improve the performance by filling the gap with an inert gas such as argon, xenon, or krypton. 

Pane spacers

These keep the panes of glass apart. More efficient windows generally have pane spacers containing little or no metal.

Frame materials

  • uPVC frames need no regular maintenance and can be recycled
  • Wooden frames can have a lower environmental impact, but need more maintenance
  • Aluminium or steel frames are slim and long-lasting, and can be recycled
  • Composite frames have an inner timber frame covered with aluminium or plastic, reducing maintenance and making them more weatherproof

How much do energy-efficient windows save?

Double glazing can take a while to pay back your investment. But, on the bright side, it could last 20 years or more, and make your home warmer and more comfortable by reducing draughts and cold spots. 

Fully insulated windows can also cut down your carbon footprint, shut out noise (thanks to double glazing’s soundproofing qualities), and also cut down on condensation.

These figures from the Energy Saving Trust show how much you could save each year if you replace single-glazed windows with double-glazed ones.

Energy ratingDetachedSemi detachedMid terraceBungalowFlat
A rated£120 - £160£85 - £110£65 - £90£55 - £75£40 - £60
B rated£110 - £145£75 - £110£60 - £80£50 - £70£40 - £55
C rated£110 - £135£75 - £95£60 - £75£50 - £65£40 - £50

For more ideas to help you save energy in your home, check out our energy saving tips article, which has loads of advice to help you cut your electricity bills and reduce your impact on the planet.

How do I find an installer to make my windows more efficient?

First, check out the Glass and Glazing Federation to find a member who works in your area. 

In England and Wales, choose an installer who’s registered with one of the official Competent Person schemes. These installers will give you a certificate that states your new windows have been fitted in compliance with regulations. 

In Scotland, each local authority has a building standards office. Check with them first to see whether you need a building warrant for your new windows.

More smart ideas to help insulate your home

There are all kinds of ways you can improve your home’s insulation levels when it comes to your roof, loft, walls, windows, and doors. Browse our handy user guides to learn more!

Plus: don't forget your light bulbs! Switching out old incandescents for LEDs can really make the difference. Find out more in our complete guide to energy-saving light bulbs. 

Looking for other ways to save money on your energy? Check out our guide on how to reduce your electric and gas bills. 

Sources and references:

1 https://www.thisismoney.co.uk/money/experts/article-2156438/How-I-house-warm-costs-down.html

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