Should I get double glazing?

Glazing in CAT's new WISE buildingWhy double glazing?

When buying new or replacement windows, we recommend 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 gives a slightly poorer performance, but a bigger gap won’t make much difference (except that it will give better sound insulation). Windows with lots of glazing bars (e.g. Georgian) are poorer: the multiple bars conduct heat out.

Low-E coating and argon/krypton filling

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. Argon gas in the gap between the panes improves the performance of the window, as argon does not conduct heat as well as air. Krypton gas is better but more costly – it’s particularly useful if a narrow gap is needed.

U-values: Lower is better

'U-values' measure thermal performance, a lower U-value gives less heat loss.

Specification U-value
Single glazing 5.5
Standard double glazing 2.7
Double glazing, low-E coating 1.8
Double glazing, low-E coating
and argon-filled
1.6 to
Triple glazing, multiple low-E
coating and argon-filled
less than

Wood, metal or PVC?

For replacement windows, CAT's eco-building experts 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.

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 possible to get UK-made windows with high standards of airtightness, built from homegrown, durable, untreated wood. FSC-certified temperate (not tropical) hardwood or durable softwood would be first choice. A durable temperate hardwood (e.g. oak, sweet chestnut or larch) won’t need treating and should last 30 years. The preservatives used to protect softwood will cause some pollution when it eventually needs to be disposed of. If you choose a less durable wood, look for one pre-treated with a low-impact natural, renewable treatment. Any sustainable (FSC) timber is preferable to uPVC, but avoid painting wood as this increases the environmental impact.

The WWF found that high performance timber windows should not cost more than PVC, and that sustainable timber (e.g. FSC-certified) should cost no more than standard timber. Their Window of Opportunity report can be downloaded (1.8MB) from

Planning and building control

Replacing either glass units (e.g. broken windows or faulty double-glazed units) or some rotten wood in the frame or sash will not need Building Control approval. When replacing an entire window frame and the opening parts of a window, or a door that is more than 50% glazed, you will need approval - but by employing a FENSA registered (Fenestration Self Assessment Scheme) joiner or installer you don’t need to get the work checked. Instead, the installer issues a certificate to verify compliance with Part L of the Building Regulations.

The BFRC energy label for windows gives an overall assessment of performance, so can be a help when choosing windows. There's also useful further reading on the National Green Specification site.

For properties such as listed buildings the requirements can be strict, but high specification windows (or secondary glazing) are available. Specialist replacement units (e.g for sash windows) replicate the appearance of old windows but have much greater levels of insulation & draughtproofing. To get help finding suppliers of conservation glazing, ask English Heritage, Historic Scotland, or Cadw (for Wales).

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