Window Energy Ratings Explained

WERs are a real breakthrough because for the first time they offer the window industry an independently verified means of promoting its products to the consumer, in a way that that the customer understands and recognises.

Since the British Fenestration Rating Council (BFRC) launched the scheme in March 2004, the industry has moved quickly and window companies have reacted positively by developing products to ensure they achieve the minimum rating under the new Building Regulations Part L, which is a band E. However, a number of forward thinking companies have gone even further and achieved ratings of C, B and in one case, an A.

The story so far

WERs have been developed to allow a window’s rating to be determined by a formula that combines its total solar heat transmittance (usually referred to as g value), U value and air infiltration. The resulting value is then placed into a band on an A-G scale. This makes the system of rating windows consistent with other products that have energy performance labels (such as washing machines, light bulbs and fridges). This has been successful with consumers and it is a system that the public is already familiar with.

In September 2005 the Office of the Deputy Prime Minister (ODPM) issued four new Approved Documents, which will replace the current two. They come into effect in April 2006 and will be welcomed by those who wish to see a continuation of current window specification and practice. In existing dwellings, for PCV-U and timber replacement windows, the current Part L U value of 2.0 will continue to be acceptable.

However, WERs have been introduced as an alternative to U values for demonstrating compliance. A replacement window achieving band E will be acceptable. Extensions are required to reach a slightly higher standard, where a band D window is specified.

For new buildings, designers and builders will achieve compliance by calculating the total carbon emissions of the proposed building. There will be considerable freedom in the specification of individual components as long as the overall emissions target is met.

Current popular glazing specifications will therefore continue to be satisfactory and windows containing Pilkington K Glass will enable buildings to meet the anticipated target. Irrespective of the target, no window in a new dwelling will be allowed to have a U value greater than 2.2, which means that non low E double glazing will become a thing of the past.

In November 2004, BFRC ratings became recognised under the Energy Efficiency Commitment (EEC). The EEC is the obligation on the electricity and gas supply companies to achieve energy savings in households. The companies must encourage and assist their customers to install energy saving measures, through subsidies or other means. Up until then, windows had never been listed by Government as one of the measures but in its subsequent list of EEC measures published by DEFRA on their website, windows with a rating of band C or better were included.

Then in March 2005, the Energy Saving Trust’s scheme ‘Energy Saving Recommended’, which endorses the best performing products in a category, was extended to windows. Those having a band C or better are able to receive the ESR endorsement.

How is it calculated?

The BFRC Rating combines the three key factors that affect a window’s energy performance – conduction heat loss (U value), solar heat gain (g) and heat loss through air infiltration (L) – into a single performance number. The three factors are linked by an equation, which produces the BFRC Rating:

BFRC Rating = (218.6 x 0.9g) – 68.5(U value + L)

The unit of the rating is kilowatt-hours per square metre per year. The BFRC developed the equation so that, in a typical UK house of average orientation, the value of the rating represents the net useful energy flow across the window throughout the year. If the rating value is positive, it means that the window is likely to be a net contributor of energy over the year.

The solar heat gain (g) value used in the equation is for the whole window, not just the glass. Therefore a window with slim frames will have a higher g-value than one with wider frame sections. Similarly, the U value used in the equation is that for the whole window. The value of L is the air leakage rate under average conditions.

For the consumer the BFRC Rating value is then converted to a letter on scale of A to G.

Who gets a BFRC Rating?

A BFRC rating and label applies to a whole window,not to a frame, system or glass individually. Therefore it is at the point at which all these components come together to produce a whole window that the Rating and label are obtained. It would therefore usually be the window installer’s product that is rated/labelled, although in the case of a factory-glazed window it could be the window manufacturer’s product.

The benefit to the systems supplier is that the one reprot could then be used by it's customers.

Obtaining a BFRC Rating and label involves three stages:

1) A BFRC Certified Simulator produces an assessment report of the window.

2) A BFRC Independent Agency ensures the window company has a satisfactory quality management system, approves the Certified Simulator’s report and informs BFRC.

3) BFRC authorise and inform the window company of the product’s rating and gives permission to use the labels. BFRC places the product on the database on its website.

Although a window system or frame in itself cannot gain a BFRC Rating, it would probably be in a system/window company’s interests to have an assessment report done for a window using its profile, and incorporating a standard IGU. This could be repeated using different IGU variants in the same window. The benefit to the systems supplier is that the one report could then be used by its customers as part of their evaluation by a BFRC Independent Agency.

Who has got a rating?

The BRFC has already issued the first band A rating, an achievement which had not been expected for a few years yet. Created by Piper Double Glazing using Pilkington K Glass and Pilkington Optiwhite, it is the only window so far to achieve the highest energy efficient grade. The company succeeded by using a triple glazed unit, with outer and inner panes of 4mm Pilkington Optiwhite‘, and the middle pane of 4mm Pilkington K Glass. The panes have cavities of 16mm each and are filled with argon gas,

Speed Frame has achieved a B Rating by combining 4mm annealed glass, 16mm Kömmerling TPS spacer, argon gas fill and 4mm Pilkington K Glass‘ along with a Speed Frame envirowindow thermally efficient frame.

A number of companies have been able to achieve C Ratings, again without having to deviate from normal working practices. A band C was awarded to Mcllhattons standard casement window consisting of KBE externally glazed three chambered window system, Pilkington K Glass, argon gas filled cavity and Edgetech’s Super Spacer.

Masterframe was the first company in the UK to achieve a C Rating for its sash window by incorporating Thermix Warm edge spacer bars, argon gas filled units, Pilkington K Glass inner pane and Pilkington Optiwhite outer pane.

Long-term benefits

No matter how low window U values become, they will never approach those of the walls. That is why the response of housebuilders to successive Building Regulations changes based purely on tighter U values has been to reduce window size.

Window Energy Ratings recognise that positive energy can be gained through windows and acknowledges their role as energy contributors. Hopefully it will stop legislators and specifiers thinking in terms of reducing window areas.

The A-G rating system provides a means of promoting the industry’s most energy-efficient products to the householder. Those window manufacturers who receive a BFRC Rating of ‘C’ or above will also be eligible to apply for the ‘Energy Saving Recommended’ logo developed by the Energy Saving Trust (EST).

WERs also give the Government a yardstick enabling it to introduce incentives to increase the uptake of energy-efficient windows and will allow windows to be covered by the same sort of support schemes enjoyed by other products in the past.

The fenestration industry has embraced Window Energy Ratings and should move quickly to capitalise on the opportunities they present. By doing so it will promote the benefits of the industry’s products as a whole.