To paraphrase Le Corbusier, the history of architecture is the history of the window. Windows are the eyes of the home, letting in light and framing the world outside. They can make or break a house, but aesthetic value isn’t the only factor to consider when buying windows: thermal performance, frame type, durability and maintenance are also important. Thermal performance, or insulation, has become increasingly significant since the government’s Part L emissions regulations were introduced in 2002 and became even more stringent in April 2006. They add to the regulations covering new-builds as well as replacement windows in existing homes.
Materials for framing
Durable, biodegradable and with good thermal performance. If you choose hardwoods such as lark, oak and sweet chestnut there is no need for painting or preservatives, making this a very low maintenance – and beautiful – material. And if the wood is Forest Stewardship Council (FSC) rated, it will be eco-friendly as well.
This shares many of hardwood’s attributes, including good thermal performance, and is much cheaper to buy, but when bought from non- certified forests its logging can cause habitat destruction. Softwood cannot be left untreated, so if eco issues are a concern, go for plant- based paints to finish. Synthetic paints are toxic during manufacture and application, and have high levels of VOCs which pose a health hazard.
Steel and aluminium
Used for ambitious glazing projects, steel is structurally versatile, durable, recyclable and very low maintenance. Steel currently contains around 20 per cent recyclate. Its downside is its poor thermal performance, and the toxic by-products of its manufacturing process. Ditto aluminium, which is a secondary material in many wood and uPVC frames as spacers. Aluminium has fallen out of favour as a framing material and is comparatively expensive to install, although the final results can be beautiful.
Despite its durability, competitive pricing and thermal performance, the uPVC window frame has not added too much to the world of glazing. White PVC can discolour with age, the frames are large in comparison to wood or metal and often require reinforcement – and the chlorine and petrochemicals used in their manufacture pose risks to the environment.
Style of frame
If your house was built before 1900, it will probably have sash or casement windows. It’s often more economic, and in keeping with the style of the house, to have dilapidated and broken sash or casement windows repaired rather than replaced. In many cases, having windows repaired rather than replaced also sidesteps the building regulations, saving time and money. Secondary glazing, where a second, detachable frame is inserted inside the existing frame to increase insulation, also avoids the need to meet building regs as the original windows can be retained intact. This is particularly useful for listed or historic buildings. If your house has original metal windows, companies such as Crittall and Clement Steel Windows produce Part L-compliant replacement windows.
Until the Eighties, manufacturers improved energy performance by adding additional layers of glass. However, three recent developments in glass technology have steered purchasers away from triple-glazing. Firstly, the thickness of the air space: increasing the distance between double-glazed panes has improved heat retention, reducing the need for an extra layer of glass. Then, as customers and architects demanded thinner profiles, filling the space between the panes with low- conductivity gas became popular. Gases such as argon, krypton and – ironically – CO2allow a much thinner gap between panes, as they insulate more effectively.
Finally, the invention of low- emissivity (low-e) glass has had a huge impact on glass production and performance. Thin, invisible coatings of silver or tin oxide reflect infrared heat radiation back into rooms. This, combined with modern tinted glass to reduce heat gain, has helped create glass tailored to particular climates or sites – cutting out heat gain in south-facing conservatories, or insulating on north-facing elevations for example. Consumers are, at present, opting for low-e rather than choosing triple-glazed glass to meet the new Part L regulations.
Nanotechnology has invaded glass production: the best of the bunch is Pilkington’s Activ self-cleaning glass, which uses an active layer of titanium dioxide that reacts with UV light, causing water to spread across the surface of the glass.
When searching for new windows, it is often difficult to find accurate quotes. If your house is a new-build, seek the advice of your architect or project manager. For replacement windows, if you cannot get a personal recommendation, look for a company registered with the Fenestration Self-Assessment Scheme (0870 780 3989; fensa.org.uk). Your windows will have the required certificate needed at the point of selling your property.
Part L of the building regulations cover emissions in relation to replacement windows.From 2002, all new windows have had to achieve a particular level of heat loss (the U- value) dependent on the material of the frame. Glazing within critical locations must be replaced with safety glass; and existing measures for background and natural ventilation, and existing measures for means of escape, must be retained (if the window pattern is to be changed this may affect means of escape). In April 2006, Part L was updated to take into account new methods of calculating heat loss, based on the dwellings carbon emisssons rate (DCER). The DCER must not exceed the target carbon emissions rate (TER), which is about 20 per cent lower than for a housecomplying with Part L before 2006. There is an overall BFRC energy rating for a complete window, including all components – glass, spacer, frame etc – and the regs now also cover solar gain to guard against buildings that overheat. The regs are so comprehensive that wags have suggested it may soon be illegal to leave your windows open.
A structural engineer will be needed for any project requiring large sections of glass. To customise a new window in your home, contact a glass artist such as Andrew Moor, who specialises in one-off bespoke work. Or look for decorative films which can revamp existing windows and increase privacy. For curved glass, Floatglass and Balcony Systems both have large-scale products that can be adapted for the domestic market.