Planners’ calls for ‘authentic’ windows are inconsistent with today’s needs, say Sarah Lewis and Marion Baeli
Deep energy-saving domestic retrofits at the rate of about 13,000 a week are necessary to meet the Government’s 2050 carbon reduction targets. The Government launched the £17 million Retrofit for the Future (RtF) programme in 2009 to address this challenge. Of all possible energy-saving interventions, installation of new windows has been the one measure most consistently adopted on RtF projects. Existing windows are replaced either with very efficient double glazing (sometimes vacuum-glazed) but most often with triple-glazed units with low-e coatings, which enable net solar gains rather than loss from glazed areas.
Windows are not the cheapest measure in a deep energy retrofit and their financial payback time is longer than many other improvements. However, good windows greatly increase indoor comfort. If occupants are comfortable, they are unlikely to turn the heating thermostat up. So if windows are such an important factor in decarbonising Britain, why do we still have so many single-glazed windows in the UK?
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The thermal performance of a single-glazed sash windows is typically 4.3-4.8W/m²K, while the performance of a triple-glazed window is 1W/m²K, more than a four-fold improvement. Moreover, triple glazing also removes the unpleasant downdraughts associated with cold surfaces, eliminates mould growth and air-borne dust mite particles with their asthma-inducing consequences, and avoids wood rot and putty failure due to condensation. Triple glazing contributes to significant improvements in the health and comfort of occupants, as well reducing carbon emissions.
Under current planning regimes, ‘authenticity’ trumps human comfort and energy savings
Under current planning regimes, ‘authenticity’ often trumps human comfort and energy savings. Replacing windows often leads to lengthy and expensive planning negotiations, which are off-putting to organisations and individuals who wish to upgrade their properties. In different planning authorities and conservation areas, guidance varies as to what is an appropriate solution for the same building stock.
The technology of timber windows is developing rapidly to keep pace with the demands of government energy targets. High-performance timber windows that closely replicate historic sash windows are now available. The thin profiles of these advanced, conservation-standard, double-glazed windows have been carefully designed to match heritage sash windows, including their thin glazing bars. Windows which integrate very thin triple-glazed units are now also available. However, despite being almost identical in appearance to single-glazed windows, these new products are being rejected by many local authorities.
The Royal Borough of Kensington and Chelsea is a notable exception. It recently deemed triple-glazed windows appropriate in a conservation area on a Passivhaus retrofit project by Paul Davis + Partners. The window looks identical to existing sashes from the exterior. To ensure airtightness, the bottom sash of the triple-glazed unit operates as a casement with an invisible tilt-and-turn mechanism. From the street, the difference is indiscernible.
Retrofit Insights, a recent report published by the Institute for Sustainability, suggests roll-out of exemplary low-energy refurbishments is possible with current technologies if best practice is employed. This is only possible if local authorities support adoption of high-performance products. Planners’ requirements for windows to be single-glazed, bedded in putty and hand-painted are inconsistent with 21st century needs.
- Sarah Lewis is a director at Bere Architects
- Marion Baeli is an associate at AJ100 Sustainable Practice of the Year 2012 Paul Davis + Partners and author of a forthcoming book on the Retrofit for the Future programme