 GLAZING OVERWritten by CAT's free information serviceemail: info@cat.org.uk; tel: 0845 3308373 or 01654 705989 Click here to download a printable version of this sheet (2 sides A4, PDF format, 50KB). Introduction If a house has single-glazed windows, then these can be responsible for about one-fifth of the heat lost from the building. Double glazing should cut this loss in half. Replacement double glazing will be fairly expensive, so if you're on a budget look first at insulating your home as much as possible (see our 'Energy Efficiency in the Home' page for advice) and draught-proofing your existing windows. It is also possible to upgrade existing windows with various types of secondary glazing. Secondary glazing There are a few ways to improve the performance of old windows without going to the expense of double glazing. A simpler, cheaper alternative is to fit secondary glazing, which can be an additional window fitted on the inside of the existing frame, or a magnetic or adhesive pane to fit to the frame, or even just some clear plastic film stuck to the interior frame. Clear plastic film (polythene) should be available from DIY stores. You can attach it round the edge with double-sided sticky tape and then heat it to make it first stretch and then contract back as it cools - to clear the wrinkles. It won't last a huge length of time - probably one year - but is very cheap. Rigid (or slightly flexible) plastic sheets are also usually available from DIY outlets. These will also be stuck on or held in place magnetically to enable easier cleaning. More advanced options could be a single pane in an aluminium frame, and attached on hinges or sliding runners, or perhaps an old and suitably-sized window cleaned up and fitted to the inside frame. There are also many types of more specialist types of films available. Some of these improve the performance of the window by letting through certain wavelengths of light in only one direction - so improving the insulating properties of the glass. The most advanced type of secondary glazing is to fit a second window inside the existing one. This will often be a better and cheaper option than replacing the windows altogether. You could hire a local joiner to make them, or there are certain proprietary systems available - for example from DIY stores. Specifying new windows When buying new or replacement windows, we would recommend going for double-glazed, low-e coated, argon filled, timber framed units. The gap between the two panes of glass should be about 20mm. A smaller gap will have a slightly poorer thermal performance, but a bigger gap will not make much difference (except that it will give better sound insulation). Windows with lots of glazing bars (e.g. Georgian type) will usually give a poorer performance, as the multiple bars conduct heat out. Low-E ("low emissivity") coating is required by current building regulations. A very thin layer of metal on the outer surface of the inner pane reduces heat transfer across the gap in a double-glazed window. Using argon gas between the panes also improves the performance of the window, as the gas does not conduct heat as well as air. Krypton gas is also sometimes used, especially if a narrow gap between panes is needed - as it will give a better performance than argon (but is more costly). 'U-values' are a measure of thermal performance - the lower the value, the better a material or building element is as an insulator.
Timber, uPVC or aluminium frames? For replacement windows, the authors of CAT's 'Whole House Book' recommend good quality timber frames, as these require little energy to manufacture, can last for over 50 years, and can then be renovated rather than replaced. In contrast, uPVC window frames require more energy to manufacture and lead to higher pollution problems at manufacture and disposal. Aluminium manufacture involves a lot of energy use and pollution, and some frame coatings make recycling difficult - not ideal for such a high-value material. If you have to use aluminium, look for anodized frames. Choosing wood framed windows Unfortunately, wood frames have had a poor image in the past, as UK-made softwood windows were often of low quality. However, we are learning from the Scandinavians and it is now possible to get windows manufactured in the UK to high standards of airtightness, using homegrown, durable, untreated wood. FSC-certified temperate (rather than tropical) hardwood or a durable softwood should be the first choice. This will last longer than a cheaper less durable softwood window, and the preservatives needed to protect the latter will cause some pollution when the wood eventually needs to be disposed of. Softwood, or sustainable (e.g. FSC) timber would still be preferable to uPVC. Avoid painting wood, as this increases the environmental impact. Choose untreated durable wood, or a less durable wood that has been pre-treated in manufacture, using a low-impact natural and renewable treatment. If you choose a durable temperate hardwood, then it will not need treating and should last 30 years. These include oak, sweet chestnut or larch. Costs A report by WWF collated various estimates of the relative costs of timber and PVC windows. Overall, it found that even high performance timber windows should not cost more than PVC. Case studies in the report show that timber-framed windows cost 15% to 25% less than uPVC units. Also, sustainable timber, e.g. with the FSC mark, should be no more expensive than standard timber. The WWF 'Window of Opportunity' report is a free download (1.8MB). To access it, right-click and save from the following link: www.wwf.org.uk/filelibrary/pdf/windows_0305.pdf Meeting building regulations Just replacing glass units (e.g. broken windows or faulty double-glazed units), or replacing some rotten wood in the frame or sash, will not need approval from Building Control. If you are replacing the whole of a window frame and the opening parts of a window, or a door that is more than 50% glazed, you will need to get approval from Building Control. By employing a joiner or installer who is FENSA registered (Fenestration Self Assessment Scheme) you will not need to have the work checked. Instead, the installer will issue you and the local council with a certificate to show that the windows or doors comply with Part L of the Building Regulations. Conservation-grade windows For more specialised properties, such as listed buildings, the requirements can be strict. You should be able to find high-specification windows (either replacement or secondary glazing) that are suitable for a listed building. Companies that supply 'conservation glazing' use different coloured strips in the gap between the glass panes, such as black, brown or white. They will supply or design replacement windows (such as sash windows) that replicate the appearance of existing windows, but with much greater levels of insulation / draughtproofing. If you have difficulty finding a supplier of conservation glazing, try English Heritage, Historic Scotland, or Cadw (for Wales). Further Information The CAT book The Energy Saving House is a guide to reducing energy use in the home. For much more advice on eco-building and renovating, including in-depth guidelines on materials choice and techniques, see The Whole House Book. Many more eco-building books are available through http://store.cat.org.uk For one-to-one technical advice, many people planning a big project find it useful to come here and run through their plans with CAT experts. For further details, see www.cat.org.uk/consultancy CAT's several residential course programme includes several on energy efficiency strategies and eco-design. You can also contact us with any further questions. Contacts Association for Environment Conscious Building (AECB). Forest Stewardship Council (FSC) FENSA For advice on conservation glazing: There is some good further reading on the useful National Green Specification web site:
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