Main Media

An Easy Guide to Energy Efficient Windows


In this guide, we’ll explain the ins and outs of energy-efficient windows, from how they use double and triple glazing and special coatings to reduce heat loss to how much they could lower your annual energy bills.

Jump to a section

Why does a window’s energy performance matter?

What is energy-efficient glazing?

Energy-efficiency ratings explained

How much do energy-efficient windows save?

Are energy-efficient windows worth it?

Our most energy-efficient window

Why does a window’s energy performance matter?

The benefits of eco-friendly and energy-efficient wooden windows include:

  • Reducing your energy bills by keeping heat inside
  • Helping the planet by reducing carbon emissions via lower ‘embodied energy’
  • Making your home more comfortable by preventing draughts and cold spots
  • Benefiting the environment by using responsible sourced timber instead of hazardous materials
  • Saving money in the long run by lasting for 60-80 years instead of around 35 years for uPVC.

What is energy-efficient glazing?

Energy-efficient glazing looks like any other window but uses special design features such as Low-E glass to keep heat trapped in your home. Saving energy thanks to eco-friendly windows means smaller household bills and a lower impact on the environment. 

How standard windows lose heat

  1. Through the glass. Known as ‘radiation through glazing’. This accounts for around two-thirds of energy lost through a standard window.

  2. Through the window frame. Heat is conducted through the material of the window frame, including the spacers that separate the panes of glass.

  3. Through cavities and gaps. Heat is also lost due to conduction through cavities and gaps, which are commonly found in older or poorly installed windows.

  4. Through air leaks. Even closed windows can leak air and lose heat through small gaps in and around the frame.


While minimising the four types of heat loss above, energy-efficient windows can sometimes also absorb heat from the outside and transfer it into your home. They do this via energy absorbed through radiation and convection, plus the solar gain transmitted through the glass.

How energy-efficient windows work

Three main window features are used in combination to combat heat loss.

1. Window frame materials. Metal is a good conductor of heat and therefore typically a bad choice for energy-efficient windows. uPVC windows usually offer better energy performance than metal-framed windows, but the best frames for energy-efficient windows are made from timber. Learn more about uPVC vs. timber windows.

According to thee British Woodworking Federation Group, ‘wood is a natural insulator’. Timber window frames absorb and retain heat thanks to wood’s inherent thermal insulation properties. 

2. Glazing type. Single-glazed windows offer the worst energy efficiency, as they lose heat around twice as fast as standard double-glazing. Double-glazing prevents heat loss by using air trapped between the panes to form an insulating barrier between the window and the outside world.


Triple-glazing offers even more thermal insulation by adding an additional pane between the inner and outer panes of double-glazing. This creates two airlocks and further improves the thermal performance of the window. 

The energy-efficiency of double and triple-glazing depends on the size of the gap between the panes of glass, the conductivity of the air or gas in those gaps, and other factors. To learn more, read our guide to glazing types.

3. Coatings. Older types of double-glazing relied on just the gap between the panes to keep heat in. Modern double and triple-glazing incorporates special coatings to further prevent heat loss. 

According to leading UK-based glass manufacturer Pilkington, these invisible coatings can make your home more than twice as energy-efficient in comparison to older double-glazing that lacks low-emissivity glass. 

Energy efficiency ratings explained

There are two main ways to measure the energy efficiency of a window: U-values and Window Energy Ratings.


The U-value represents the amount of heat lost through the window and the rate at which it’s lost. U-values are measured in watts per square metre-kelvin, or W/m2K. Put simply, the lower the U-value number, the better the thermal performance of the window. 


Our most energy- efficient window is the Stormsure Energy+ triple- glazed casement window (above), which has a superb U-value of 0.8 W/m2K and an A+ Window Energy Rating.

Window Energy Ratings

To easily compare windows’ thermal performance, there are several window energy rating schemes. The most widespread are operated by the British Fenestration Rating Council, the British Standards Institute, and CERTASS


How much do money energy-efficient windows save?

Lots of factors influence how much money you could save by choosing energy efficient windows, including:

  • The amount of shade near your home
  • The direction of the windows (in the UK, south-facing windows absorb the most heat from sunlight)
  • Window frame materials
  • Glazing type
  • Glass coatings.

Here’s an idea of how much is saved by choosing double-glazing over single glazing. 

The potential annual saving of replacing single glazing with double-glazing


Estimates by Energy Saving Trust based on a typical gas-heated home, ranging between a small mid-floor flat and a large detached home. Installation costs will vary depending on the size of the property and windows, material used and choice of installer. Savings are rounded to the nearest £5.

Here’s another example, using figures from the Glass and Glazing Federation:

How much could A+ energy-rated windows save per year?

A detached house with electric heating, double-glazing, and typical window sizes could save £242.72 per year on energy costs by upgrading to windows with an A+ energy rating. 

That’s the equivalent of 0.80 tonnes of CO2 emissions and 0.22 tonnes of carbon usage.

Based on calculations by the Glass and Glazing Federation. How much you could save will depend on your personal situation, including your current windows, heating type, house type, and the type and installation of your new windows. 

It’s worth considering how long it could take for your investment in energy efficient windows to pay for itself through the savings you’ll make on your energy bills. 

Using the example above, the cost of our most energy-efficient window (from £899 +VAT for two medium windows without bars), would be covered by energy savings in just under four years. 

Are energy-efficient windows worth it?

When you choose new windows, you’re also deciding how much you’ll spend on energy for at least 35 years, or up to 60 if you choose the longest-lasting timber windows. So selecting windows that offer the best energy performance while ticking the right style, colour and cost boxes is really important. 

While triple-glazed windows typically offer the best energy performance and sound insulation, you might find there are more styles and options available for double-glazed windows. In either case, look for a minimum window energy rating of B to get better energy efficiency, lower your bills and help the environment. 

Our most energy efficient window


The Stormsure Energy+ is JELD-WEN’s most energy-efficient window thanks to triple-glazing, naturally-insulating, responsibly sourced softwood, and an A+ window energy rating.

  • A variety of traditional and contemporary bar design options
  • Available in any RAL colour
  • Standard and made-to-measure sizes available
  • Long-lasting guarantees (40 years against rot and fungal attack)

Find out more about Stormsure Energy+ or get a quick cost estimate to see if our most energy-efficient window is the right choice for you.