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Doors and windows


If you are thinking about changing your windows or doors you will need to think about U-values. A U-value measures of the rate at which heat escapes through a fabric, so the lower the figure, the better.  A U-value of zero means that no heat is escaping.  It is expressed as Watts per square metre (W/m2) but it is really only the number that matters.

Double and triple glazing

Double or triple glazed windows are another form of insulation. However, as you can see from the figures below, it is not generally so efficient as wall and roof insulation. Replacing a single-glazed window with a reasonable quality double-glazed unit will more than halve the heat loss.

The Energy Saving Trust estimate that you will save £165 a year by replacing single glazed windows with B-rated double glazing. You will also reduce noise from outside, and experience less condensation.

However, only about 10 per cent of the total heat loss from the house is through the windows, so there is really no point in putting expensive double-glazed windows in uninsulated walls. Similarly, it's only worth investing in the best triple glazed windows if you have super-insulated the property first.


Standard windows will have air in the gaps between the panes of glass. Higher performing ones tend to have argon or other gases between. They will also have a low-E (low emissivity) coating. This allows the heat from the sun into a room, but reduces the amount that escapes again.


UPVC are a low maintenance option. However, the Centre for Alternative Technology recommend that they should be avoided as their production is energy intensive, and they are polluting at manufacture and disposal.

Alternatives include wood (which needs more maintenance), aluminium and composites which have wood on the inside and aluminium on the outside.

New double glazing is required to have trickle ventilation in the frame (even if the ones you are replacing do not). This is because modern windows are more air tight than older ones.


Some manufacturers label their windows with the standard A-E measures of energy efficiency. In addition, those scoring B or above may carry the Energy Saving Trust Recommended logo.

Secondary glazing

All new build properties must be fitted with double glazing. However, not all old properties are able to upgrade. If you are in a conservation area or live in a listed building, you should consult your local planning department. In these cases the permitted solutions may be prohibitively expensive. This leaves three possible alternative solutions to reduce heat loss (and increase warmth): draught proofing, secondary glazing and thick curtains.

Secondary glazing

This is an additional pane of glass fitted over the window internally. How sophisticated it is will depend on whether or not you want to open the window (you can get glazing that slides open) or not. If you use Low-E glass you will reduce the u-value by more than if you use standard glass. The Energy Saving Trust estimates that you can reduce heating bills by £100 per year by using secondary glazing.

The downsides are that they may cause condensation, and the efficiency is not as good as they are not sealed units. The condensation problem can be mitigated a bit by putting packets of silica gel in the air gap (you will have to change them - or dry them out - occasionally). You can use standard draught proofing strips to make sure you get a good seal.

Detailed advice for older properties is available free of charge from English Heritage. Click the link to download its guide to secondary glazing.


Heat loss through traditional sash windows can be reduced by 41% by heavy curtains drawn at dusk, according to research by English Heritage. They lowered the u-value from 4.3 to 2.5.

To be most effective curtains should be hung flush against the wall (not on a pole that leaves gaps at the top and sides). Ideally there should be a pelmet to help trap the cold air that comes in the window behind the curtain.


You can improve the energy efficiency of existing doors by draught proofing so that it seals better, and using brush draft excluders. You can also draught proof the letterbox.

When buying a new door, uPVC doors tend to have lower u-values. However, as mentioned above, they also use more energy in manufacture and lead to more pollution at manufacture and disposal.

YouGen energy expert Tim Pullen argues: "A typical external door is broadly 1.7 sq m in area. A uPVC door with a U-value of 1.8 will emit about 70W per hour of heat in the coldest months. A solid hardwood door with a U-value of 3.0 will emit 115W. It is not clear, to me at least, that it is worth putting up with all the negative aspects of uPVC for that relatively small advantage."

Specialist manufacturers do make very insulated wooden doors, but they tend to be significantly more expensive.

Building regulations

A few basics. Building Regulations Part L(1A) state that windows must have a U-value of at least 2W/m2, compared to walls that must have no more than 0.3W/m2K. The very best triple glazed windows might achieve a U-value of 0.6W/m2K. So despite anything the double-glazing salesman might say, the windows are always a weak point in your insulation.

To put things in context, here are some comparative U-values:

  • Single glazed: 4.8W/m2K
  • Single glazed with curtain: 3.6W/m2K
  • Secondary glazing: 2.9 - 3.4W/m2K
  • Double glazed (building regs): 2W/m2K
  • Double glazed super low E: 1.3W/m2K
  • The best triple glazed windows: 0.6W/m2K

(Figures for all except the triple glazing from the Home Energy Handbook).

If your door or window installer is not registered with a competent persons scheme such as BSI, CERTASS or FENSA you will need to apply for approval to replace windows or doors from your local building control body.

Green Deal

The Green Deal is a finance mechanism to enable people to make improvements to the energy efficiency of their house without having to bear the up-front cost. Secondary glazing, replacement glazing, sealing improvements and and draught proofing are all items on the list of eligible improvements

At the heart of the green deal is the 'golden rule': as long as the expected financial savings in lower energy bills are equal to or greater than the costs of the energy efficiency measures installed, you are eligible.

The loan to cover installation costs will be in the form of a charge on the property. It will be repaid over a period of 10 - 25 years (depending on what measures are installed) through the property's electricity bill.

An accredited green deal adviser will assess the energy efficiency of the building, using a system that is based on an improved energy performance certificate (EPC). They will draw up a list of measures that can be taken to improve the energy efficiency and indicate which ones meet the golden rule. 

Please see our Green Deal page for more information.

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