Windows lose and gain heat by conduction, convection, radiation and air leakage. This heat transfer is expressed with U-values.

As a rating of energy efficiency, U-value can be used to consider performance of windows, exterior doors, skylights and all other exterior building components, including exterior walls.

In windows, the U-value measures how well heat is transferred by the entire window, that is the frame, sash and glass combined.

The lower the U-value, the more insulated the window unit and therefore the better the window will be in retaining the interior’s heat in the winter and keeping heat out during summer.

U-values are important because they form the basis of any energy or carbon reduction standard.

A significant part of the thermal energy transmission can be through the frames, or profiles, depending on how well the profile material conducts heat. Metals are generally good conductors which is why they feel hot to touch when exposed to the sun. This makes them poor thermal insulators.

Timber and plastics such as uPVC are poor conductors which therefore stops the heat transfer from the heat-exposed side to the non-exposed side. PVC and timber window frames are approximately thermally equivalent and provide a high level of energy efficiency compared to a standard aluminium frame.

The above U-values indicate the potential to save valuable heating and cooling energy being lost through your windows.

According to the Australian Windows Energy Rating Scheme , the U-value ratings for Australian products generally fall between 2.0 and 10.0. A typical double glaze uPVC window rates better than this at 1.8.

Assessing the impact of radiation

A second element used to measure the energy efficiency of a window system is the Solar Heat Gain Coefficient (SHGC) – a measure of solar radiation going through the glazing into a building and therefore warming the interior.

The SHGC is therefore an important factor to consider in the hotter climate zones across Australia.
Australians are now using more energy to artificially cool their homes in summer than to heat them in winter. However, the use of energy efficient windows and doors can provide significant energy savings.
In climates where reliance on heating and artificial cooling is significant, choosing a window unit with a lower U-value and a lower SHGC is important.

Excerpt and image from article written by Vinyl Council Chief Executive Sophi MacMillan and published on industry news website Sourceable

We tend to think of energy efficient buildings as being ‘air tight,’ but this is not actually the desired outcome.

There are many factors that affect levels of indoor comfort, including air quality. Air quality can be affected by external air pollutants entering the building, the materials used inside the home, the household products used, as well as activities in the building. Energy efficient buildings need to be well sealed and insulated to conserve energy and minimise thermal conductivity, but activities such as showers, cooking and clothes drying, for example, can increase the humidity in indoor air and lead to possible mould issues.

The key to having an energy efficient home or office and healthy indoor air quality is to ensure effective ventilation.

Today’s window designs, however, can facilitate easy ventilation while maintaining good thermal properties.

Common in Europe are Tilt and Turn windows and doors, which work effectively in a cross ventilation situation: the old, stale ‘hot’ air escapes over the top of the inward tilted window, while the new, fresh air enters the dwelling around the sides of the tilted window sash. This ventilation is ideal for maintaining healthy indoor air quality and a high degree of security. The window or door can also be fully opened (inwards) to provide a strong draft, for example to purge the home with cool evening air on a hot summer day.

All properties lose heat through their windows. But energy-efficient glazing keeps your home warmer and quieter as well as reducing your energy bills. That might mean double or triple-glazing, secondary glazing, or just heavier curtains.

How much you can save

By installing double glazing in an entirely single-glazed house you could save the following each year:

England, Scotland and Wales

These savings are for typical gas-heated homes.

Northern Ireland

These savings are for typical oil heated homes. 

Benefits of energy-efficient windows

• Smaller energy bills.
• Smaller carbon footprint.
• More comfortable home: energy-efficient glazing reduces heat loss through windows and means fewer draughts and cold spots.
• Peace and quiet: as well as keeping the heat in, energy efficient-windows insulate your home against external noise.
• Reduced condensation: energy-efficient glazing reduces condensation build-up on the inside of windows.

The costs and savings for energy-efficient glazing will be different for each home and each window, depending on its size, material and the installer you choose. Double glazing should last for 20 years or more.

To get a better idea of how much you could save by replacing your windows, use the Energy Saving Calculator on the Glass and Glazing Federation’s website, developed in conjuction with the Energy Saving Trust.

How energy-efficient glazing works

Double-glazed windows have two sheets of glass with a gap in between, usually about 16mm, to create an insulating barrier that keeps heat in. This is sometimes filled with gas. Triple-glazed windows have three sheets of glass, but aren’t always better than double-glazed windows. To choose the most energy-efficient window look for the BFRC rating.

Energy-efficient windows come in a range of frame materials and styles. Performance criteria vary according to the following:

• How well they stop heat from passing through the window.
• How much sunlight travels through the glass.
• How little air can leak in or out around the window.

What to look for

• Glass
• The most energy-efficient type for double glazing is low emissivity (Low-E) glass. This often has an invisible coating of metal oxide, normally on one of the internal panes. This lets in light and heat but cuts the amount of heat that can get out.
• Gaps between the glass  
• Very efficient windows might use gases such as argon, xenon or krypton in the gap between the sheets of glass.
• Pane spacers
• These are set around the inside edges to keep the two panes of glass apart. For maximum efficiency, look for pane spacers containing little or no metal – often known as ‘warm edge’ spacers.

Frame materials

For all frame materials there are windows available in all energy ratings.

• uPVC frames last a long time and may be recycled.
• Wooden frames can have a lower environmental impact, but require maintenance. They are often used in conservation areas where the original windows had timber frames.
• Aluminium or steel frames are slim and long-lasting, and may be recycled.
• Composite frames have an inner timber frame covered with aluminium or plastic. This reduces the need for maintenance and keeps the frame weatherproof.

Energy rating

Some window manufacturers show the energy efficiency of their products using an energy-rating scale from A to G. The whole window (the frame and the glass) is assessed on its efficiency at retaining heat. The scheme is run by the British Fenestration Rating Council (BFRC). Visit BFRC for more information.

U-values

Windows that have an energy rating will have the u-value of the window displayed on the energy label. A u-value is a measure of how easily heat can pass through a material. Materials that let out more heat have higher u-values whereas materials that let less heat pass through them have lower u-values.

In some cases, windows with a higher energy performance rating might have a higher u-value than windows with a better energy efficiency rating. This might seem the wrong way round as lower u-values indicate better insulation levels. However, in these cases it will be that there are other aspects of the window that make them better overall such as coating used on the glass and the gap between the glass panes.

You can find more information on the u-values we use in our calculations here

Ventilation

Replacement windows will be more airtight than your original frames, so condensation may build up in your house due to the reduced ventilation. If your house does not have much background ventilation, look for replacement windows with trickle vents incorporated into the frame to let in a controlled amount of ventilation.

If you start to see condensation building up around your windows, there may be a damp problem in your home. As a general rule, damp occurs when there is inadequate ventilation, inadequate heating, inadequate insulation or a combination of these. If you’ve started to notice condensation in between the panes of glass in your double-glazing units then it is likely that the seal is broken, and the unit will need to be replaced.

Conservation areas

These areas are of special architectural or historic interest, meaning that any work you carry out on your home must preserve or enhance the character of the area. This does not necessarily mean you cannot replace your windows, but might mean you will need to get windows that complement the character of the building and area. Double glazing can be made to look like your building’s original windows, but for any changes you do need to contact your local council’s conservation officer for guidance.

Windows in period properties

If you live in a conservation area or in a listed building there may be restrictions on what you can do to your windows. There are a number of non-intrusive window insulation options available for historic homes such as heavy lined curtains, shutters, secondary glazing and sealed blinds. However, each historic building is considered individually so check with your local council to see what options are available to you.

Listed buildings

Listed buildings have tight controls on what you can change on the outside and sometimes the inside as well, depending on their grading. Old sash windows in historic properties can be protected not only for their appearance but also the materials and methods used to make them. But secondary glazing can be a non-intrusive way of insulated historic windows from the inside, and may be granted permission.

There are other ways to make historic buildings more energy efficient but you will need to consult, and apply for permission from, your local planning authority.

Visit Historic Scotland and English Heritage for ways to make a historic home more energy efficient.

Sash windows

Sash window units are common features of period properties and can be a design feature. They consist of two vertically sliding frames, but are often badly fitting and made of single pane glass so have poor insulating properties.

If you want to insulate your sash windows there are a number of alternatives to conventional double glazing. If you want to keep the design and look of the sash windows, there are units available that are in keeping with the original design; these are fitted and sealed to prevent draughts and incorporate double glazing to reduce heat loss. The frames don’t need to be plastic, but can be metal or wood with an insulated core.

An increasing number of double glazing companies offer double glazing in period properties. Replacing sash windows can be expensive, so good-quality secondary glazing may be worth considering.

Alternatives to double glazing

If you can’t install double glazing – for example, if you live in a conservation area, period property, or listed building – you can install secondary glazing, or use heavy curtains, or do both. Find out how to fix the draughts around your windows by visiting our draught-proofing pages.