Energy Efficiency

According to the updated Part L building regulations, all replacement windows should meet the requirements for new 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 rate of heat loss through windows and other areas of the home are measured by the U value. 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.

To find a more environmentally friendly window in terms of minimal heat loss you should look for a low 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.

British Fenestration Rating Council:

The BRFC is responsible for the promotion and monitoring of the UK rating system for "the thermal performance of fenestration products". As well as explaining their proposed system for a clearer understanding of energy efficiency in windows, they also offer impartial information on the most energy saving manufacturers or types of glass and window frames.

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. The U value is only a part measure of the window's energy effectiveness, but the Window Energy Rating (WER) aims to provide a full picture of its performance.

Window Energy Rating:

The WER measures the total performance of the whole window for its effectiveness at gaining and retaining heat. Unlike the U rating, the WER scale takes into account the complete energy flow in a window and looks for energy saving initiatives in each area. A window's energy flow comprises three main elements:

1. Solar Heat Gain Coefficient (SHGC) - radiation or heat caused by sunlight

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. Products like windows can now be rated under WER, and accorded with the WER label as well as an EER label as a mark of superior energy efficiency.

The Domestic Window Energy Rating initiative grades each residential window type on a scale from 1 to 100; the higher the number indicating higher energy efficiency. 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. Tinted windows will have a lower LT.

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. Long wave should be contained to prevent energy losses and high heating bills.Short wave radiation is the heat energy caused by the sun. It passes through the window and is absorbed by your home furnishings, where it transforms into long wave radiation. Long wave radiation however, wants to flow from warm to cool and so in cooler weather the energy will try to pass back out the glass.Low E coating reflects back in the heat in cool weather so that the overall U value of the window sees an increase in winter. In summer, the short wave radiation entering is reduced as it is filtered through the coating.

There are however, two kinds of low E glass available:

Hard coating (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. It can also make the glass appear slightly hazy.

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. Its almost invisible coating allows for window clarity combined with a very low E rating.

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.