Window Energy Efficiency

According to the updated Part L building regulations, all replacement windows should meet the requirements for new window energy efficiency measures. Excess heat lost from buildings is argued to be one of the major causes of global warming, as well as causing homeowners to waste money on their heating bill. In an attempt to slow down the rising environmental crisis, from 2002 all wood and UPVC windows should achieve a U value of 2.0W/m2K, with aluminium reaching a level of 2.2W/m2K.

U Value:

The U value measures the amount of heat lost through windows and other areas in your property. U is deduced from the amount of heat loss in one square metre of the window; against every degree difference in the temperature either side of the window. The end result is described in units of watts per metre squared per degree Kelvin (W/m2K), with a lower number equalling better heat retention.

If you want a more environmentally friendly window then you should look for the windows with the lowest U rating. However, according to the Energy Saving Trust and British Fenestration Rating Council (BFRC) there are further measures that can be taken to provide consumers with energy saving frames and glazing.

What Do The British Fenestration Rating Council Do?

The BFRC is responsible for the promotion and monitoring of the UK rating system for "the thermal performance of fenestration products". They can provide advice on the most energy efficient window manufacturers and what kind of frames you use to be most energy efficient.

The U value only determines how much heat a window loses, but the aim of the BFRC is to introduce a measure that also represents how much heat a window can gain. Although the U value gives you an acurate idea of how much the window loses, they have now introduced a rating called the Window Energy Rating that gives a better indication of overall performance.

Window Energy Rating (WER):

The WER measures the total performance of the whole window for its effectiveness at gaining and retaining heat. So unlike the U rating looking at how much energy is lost through the window, it looks at the whole energy flow around the window. A window's energy flow comprises of three main elements:

1. Solar Heat Gain Coefficient (SHGC) - radiation of heat gained from trapping the suns heat

2. Convection, conduction, and radiation throughout the window's composition that is not caused by sunlight

3. Ventilated and infiltrated flow through the window

The Window Energy Rating system as defined by the BRFC is implemented in the Energy Efficiency Recommended (EER) label scheme established by the Energy Saving Trust. Windows can now be rated under both WER labelling and EER labelling so you can tell products of superior quality easily.

The Domestic Window Energy Rating grades domestic windows on a scale of 1 - 100 for effectiveness, with the higher the number being the most effective. Domestic windows certified by the BFRC will be labelled with the initials DWER, and display its various performance ratings for the following criteria:

U Value - Heat loss should be lower than 2.0W/m2K for wood and plastic, or 2.2W/m2K for aluminium.

Solar Heat Gain Coefficient (SHGC) - The protection against heat caused by sunlight. This is rated between 0 - 1, with 0 offering the greatest barrier.

Light Transmittance (LT) - Measurement of how much light is transferred through the window; rated between 0 - 1. The lower the LT the greater the tint in the window.

Infiltration - This is included in the WER calculation but is not put onto the BRFC label. It is a measurement of how much air escapes from the window when it is shut.

Low E Glass:

Low Emission glass is an energy efficient glass with a metal coating layered on one side. Because of this coating, Low E glass has high insulating properties that prevent the passage of heat or coldness through it. Consequently, this type of glass helps to keep your home warm in winter by retaining the internal heat but cooler in summer by keeping the external heat outside. Low E glass offers year round energy efficiency as well as cheaper bills.

The heat in our homes can be classified into long wave and short wave radiation. Long wave radiation is the heat that occurs within the home from your heating system. This heat is held in by the reflective properties of the glass and means you don't lose heat from your heating system through the windows. As well as the reflective property it allows heat from the sun to pass through while reflecting it back into the room, so the heat from the sun is also kept in the room. Each of these properties means you can save energy on both heating and cooling systems, saving you money on energy bills and helping the environment.

There are two kinds of low E glass available these are:

Hard coated energy efficient glass (pyrolytic coating) - This offers medium performance insulation and is fairly durable. The coating is sprayed onto the surface at high temperatures during the float glass process, and so can be used on single glazing. When compared with soft coating however, hard coating produces higher U values and a higher SHGC. One down side to hard coated glass is it can sometimes make the appearence of the glass slightly tinted or hazey.

Energy efficient soft coating (sputter coating) - Soft coat glass offers the highest level of performance. The application involves multi layering of transparent silver coating which is held between metal oxide layers in a vacuum chamber. Inevitably, this more complex process makes soft coat more expensive, but it does allow for much more visible light with reduced UV. So with this process you can have glass that is as clear as a normal pane but with the added benefits.

Unlike the hard coat, sputter coating can only be used in sealed doubled glazed units and must be sensitively handled. There are also nominal colour variations that can occur with the coating process.