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What to look for in energy efficient windows?

Posted by Sharon Russell-Verma on 1 July 2015 at 12:40 pm

If you are in the process of buying new energy efficient windows you could be forgiven for getting in a muddle. With the vast array of window styles, frames, panes and coatings in addition to window rating schemes on offer, it is not an easy task. Let’s begin with what most of us think about when looking for new energy efficient windows - the glass panes and window frames.

Glass Panes

Energy efficient windows help reduce your overall energy consumption by preventing unwanted energy loss, and one of the best ways to achieve this is by installing double glazing. Insulated window glazing (double glazing or triple glazing) is two or three panes of glass spaced evenly apart; the space between the panes is filled with an inert gas, commonly argon, xenon or krypton, then sealed.

Normally, triple glazed windows give lowest U-values (explained below); however it very much depends upon the types of glass used. You might, for example, choose a low-emissivity (low-e) glass. 

Low-e glass is a clear glass that has had a microscopically-thin coating of metal oxide deposited directly on the surface of one or more of the panes. This allows the sun's heat and light to pass through the glass into the building, while simultaneously blocking heat from leaving the room and reducing heat loss considerably. Sunshine coming into the house this way and heating up the internal air is called 'solar gain'. Low-e glass helps to increase solar gain and stops that gain from leaving so easily.

Window Frames

When selecting window frames there are various types to choose from. Some such as uPVC, and hardwood offer greater thermal resistance compared to metal/aluminium frames which conduct heat very quickly.

Nonetheless the insulation of metal frames can be improved by installing a thermal break (an insulating barrier between the inside and outside of the window). The table below compares the different types of window frames.

Window Frame Type Fabric Thermal Properties Additional Information
uPVC Polyvinyl Chloride with UV Very good
  • Hollow cavities of vinyl frames can be filled with insulation
  • Lower cost and comes in a range of colours
Wood Hardwood Good
  • Expands and contracts in response to weather conditions
  • Requires maintenance
Metal Aluminium or Metal Poor
  • Insulation can be improved by installing a thermal break
  • Long lasting and requires little maintenance 

Table 1: A comparison of types of window frames

Evaluation Tools

Once you have selected your windows (frame and pane) there are number of tools and rating schemes that can help you evaluate the energy efficiency of your window choice, including U-value and window energy rating scheme.

U-Values

The U-value is a measurement of windows’ ability to retain heat and thus keep the building at a stable temperature. The lower the U-value of a window the more energy efficient it is.

A Rated Windows

Many window manufacturers also use the British Fenestration Rating Council (BFRC) window (and door) energy ratings scheme to show the energy efficiency of their products. The scheme uses the familiar ‘A’ to ‘G’ rainbow label, similar to those for electrical appliances. 

More recently A+ and A++ (launched May 2015) ratings have become available. The whole window (the frame and the glass) is assessed on its efficiency at retaining heat with ‘A++’ being the most efficient. The BFRC ratings take both the U-value and the solar gain provided by the windows into account.

Windows rated by the BFRC have label that displays the following information:

 
  1. The rating level – A++, A+, A, B, C etc
  2. The energy rating eg: -3kWh/m²K the product will lose 3 kilowatt hours per square metre per year.
  3. The window U value eg 1.4 W/m²K
  4. The effective heat loss due to air penetration eg: 0.01 W/m²K
  5. The solar heat gain eg: g = 0.43

In simple terms the rating label displays how well a product will perform. 

The newer A+ and A++ have advantages over the lower rated windows (B, C etc) such as:

  • A++ rated windows and A+ have notably lower U-values than those lower down the A to G scale so they can help to reduce overheating during the summer months whilst helping to keep your home warmer in winter.
  • Furthermore due to their lower U-values, they allow for warmer internal surfaces and hence can help eliminate the risk of internal condensation.

Installation 

Finally, one important aspect not to be overlooked is the installation of your new windows. There is little point in choosing the most energy efficient windows if they are not properly installed. Your window installer should ensure the following:

  • Your windows are correctly fitted to preserve air tightness in the building, and prevent draughts and thermal bridging.
  • If the building uses external insulation, this should overlap slightly on the outside of the window frames.
  • Lintels and sills must be insulated on the inside or outside, to prevent thermal bridging.

Since April 2002 all replacement windows (and doors) must comply with current Building Regulations standards. If you live in England and Wales it may be worth contacting FENSA (Fenestration Self-Assessment Scheme), a government authorised Competent Persons Scheme for a list of approved installers. On completion of the work your FENSA registered installer will issue you with homeowner certificates (which is a proof that your home complies with current Building Regulations and will be required if at a later stage you decide to sell your home). 

You can use our ‘find a…’ service to find a highly recommended installer in your area. It is also worth asking friends, family and neighbours for recommendations. Additionally, ask your window suppliers/installer for references from previous customers and ask to see examples of previous work (photos or visit nearby properties if possible). Finally, don’t forget to ensure that any guarantee covers the window products and the installation and clarify the length of the guarantee.

In summary, when selecting energy efficient windows the following should be considered:

  • Type of window frame,
  • Double or triple glazing complete with inert gas sealed between the panes,
  • A low-e coating,
  • BFRC A++, A+ or A rated windows (low U-value) and,
  • Employ a reliable and reputable installer.

Choosing energy efficient windows and having them properly installed can contribute to a more energy efficient home, a warmer home, with less intrusion from outside noise and crucially in today’s economic climate they can help save money on energy bills!

Photo: SuperHomes

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Comments

1 comments - read them below or add one

Graham Marshall

Graham MarshallComment left on: 4 July 2015 at 9:52 am

The local joiner I plan to use to install replacement windows is offering a product produced by a local manufacturer who has shown me sample windows and smaple profiles. He claims the windows are A rated but cannot tell me the U value. He claims the profile used for the frames is energy efficient because it has several seperate chambers but there is no insulation fill. We are in Scoland where your article suggests there is no  FENSA scheme. What should I do to ensure the windows installed are the best available?

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