Simulations of energy performance

Window Energy Ratings (WERs) are controlled by the British Fenestration Rating Council – BFRC. Originally established in 1999 with assistance from Government and the major fenestration trade associations, BFRC was part of a research project to develop a Window Energy Rating scheme for the UK. BFRC Ltd was established in 2006 to take over and further develop the activities of the original BFRC. BFRC Ltd is now part of the Glass and Glazing Federation – GGF.

Window Energy Ratings are based on simulations of energy performance and can only be issued by an authorised BFRC simulator. Usually the WER scheme is used in the replacement window sector of the market. The new build market operates slightly differently and will be looked at further below.

What is a Window Energy Rating?

WERs are a method of testing various windows, including the glass, to enable the energy performance to be assessed and compared against all products on the market. Three major components are looked at by a BFRC simulator:

1. U-value of the window including:

• Frame material – upvc, metal, timber
• Re-enforcing material – steel, plastic, etc
• Sealed unit glass make up – glass type, cavity gas, etc
• Warm edge spacer systems
• Sealant depths

2. Solar gain

• Often referred to as g-value, this is the solar heat energy that passes into the room through the glazing

3. Air leakage

• If air can leak through frame gaskets and joints, then heat can too
• This has to be reduced and is a measure of quality manufacturing

All the window parameters are put through a computer simulation process that will give the window a value placing it into a band rating A – G. This process is based on a benchmark window, enabling different products to be directly compared. This offers consumers an independent performance reference.

The familiar ‘rainbow’ label indicates the performance band of the window and can help consumers choose products based on price and associated performance from different suppliers.

• Building regulations 2010 document L, was the first time WERs were included in the regulations, giving the scheme some credence.
• The regulations state that replacement windows in dwellings must be at least a WER rating of ‘C’ or maximum u-value of 1.6 W/m2.K.
• Many suppliers offer windows from ‘C’ up to higher performing ‘A’ and ‘A++’
• In broad terms, the higher the band the window is in, the better energy saving performance the window should offer.
• It is not an indication of how professional a supplier may be or an assessment of the quality of the manufactured product. It is purely an energy performance rating.
• Most windows pre dating 2010 but installed after 2002 are likely to be in band E.
• Pre 2002 installations would likely be G band or not even on the scale, meaning they are poor energy performers.
• Greatest improvements will be when replacing old windows – pre 2010 and pre 2002 with WER A – C bands.
• Swapping between WER A – C bands offers little saving in real terms.

New Build Sector

New build housing has its own set of building regulations, potentially making their window requirements different to replacement windows in existing homes.

House builders are faced with many options that will help them attain energy saving targets and windows are just one. Others include:

• Energy efficient heating systems
• Low energy lighting
• Additional wall/floor/roof insulation
• Solar panels
• Layout of estate in relation to roads and lighting

All these options and more, may help reduce the energy consumption of houses. With 25% of heat potentially escaping through windows, choosing the right window can be a low cost method of meeting building regulations.

Here, the builder is generally more interested in whole window u-values with WERs being of little importance. Triple Glazing can offer low U-values and is in demand more and more in new build properties.

WERs v U-values

Contradictory to WERs, lower U-values can often be superior in terms of saving energy, than top band WERs. This is because of the solar gain factor that has influence on the WER rating but not the U-value.

The solar gain issue is a highly contentious subject that has varying opinions across the industry. The facts however, are self-explanatory.

• Lower u-values = lower heat transmission = lower heat loss.
• Some high performing glass products such as Planitherm One can help achieve whole window U-values of 1.3 W/m2.K or less, but be WER ‘B’.
• Many ‘A’ rated windows have a U-value of 1.4W/m2.K making them worse performers.
• The better performing glazing products have lower g-values meaning that they block solar heat gain, pushing them down the WER bands.

A house builder will generally opt for the lower U-value window as this will lower the energy consumption of the property.

A home owner looking to buy WER rated replacement windows, needs to consider some basic issues regarding solar heat gain.

• In summer, energy consumption by central heating is low if not even switched off. This is the part of the year that solar gain is at its greatest with long days and high levels of sunshine. Solar gain is unwanted and many people will close blinds, open windows and switch air conditioning on. All are efforts to reduce heat in the home.
• A home owner may spend more on energy to reduce heat gain in summer.
• In winter the days are short and the sun is lower in the sky, reducing solar heat gain to almost nothing. Only large south facing glazed areas will achieve any solar gain and this too may be unwanted. At this time of year, the home heating system is on and all efforts should be toward reducing heat loss by having lower U-value products.
• It is unlikely that the sun is going to heat your home and offset your energy consumption in winter.

WERs are not always the best option but a low U-value will always mean reduced heat loss and better insulation.