Advice on windows

Questions answered on this page:

Double and triple glazing

How much will I save from double/triple glazing

It is often said that 25% of the heat loss from your house is through the windows; fortunately there is a lot of scope for reducing this. You lose heat through the windows and frames directly, and through air leakage especially around opening windows. Draught strip will usually fix badly fitting windows. Upgrading to single to double glazing will cut direct loss by at least 50% and triple glazing by around 70%. The very best windows can reduce heat loss by 85% (1).

The frames are important as well as the windows. Metal frames are generally more heat leaky than wood, though these days they will have a thermal break which makes a big difference. (You can see a picture of thermal break here). Wood frames and PVC are good.

On sunny days you gain heat through windows too. This can be a problem in summer but is a good thing in winter. Double glazing reduces heat gain but not as much as it reduces the losses: double glazing reduces solar gain by 10% to 25% and triple glazing by 20% to 30%. South facing windows benefit most from solar gain in winter, and south facing double or triple double glazing may even give you a net gain. For other orientations it is much more important to reduce heat loss.

(1) Typical U-values: single glazing 4.8W/m2/K, double glazing (12mm gap) low-e soft-coat 2.1, triple with low-E soft coat 1.5, Green Building Store Ultra windows 0.75.

Is secondary glazing as good as double glazing?

Secondary glazing involves a second layer of glass inside the existing panel, often fixed to the frame. There are many different kinds of varying cost and durability such as:

  • Plastic film stuck to the frame edges - very cheap but does not last long, usually replaced each season. (e.g. this from Wickes)
  • Acetate sheets attached to the existing frame with magnetic strip - easy to remove for the summer (e.g. this from Tubeway)
  • Fixed units with sections that slide open.(e.g. this from trackglaze)

Many types of secondary glazing can be fitted DIY from a kit.

Secondary glazing is almost as good as double glazing at reducing heat loss, as long as it is well sealed (1). If it is not well sealed you can get problems with condensation. The larger gap is also effective at blocking out noise.

(1) Typical U-values: double glazing low-E soft coat 2.1, secondary glazing 2.4.

Can I have double glazed sash windows?

Most sash windows can be fitted with double glazing but it is a big job. The sash weights will need to be adjusted because the double glazing is heavier. This means opening the sash boxes and replacing the weights.

See also Should I replace my sash windows (The Green Age)

Secondary glazing is often cheaper. There is more advice here on My Green Pod

How efficient are patio windows?

In most respects patio windows are basically large windows. However, if they are the sliding type then there is potential for draughts, especially under the bottom edge. There should at least be a brush strip to reduce leakage. Also, it is worth checking how they close at the sides. Good patio doors should not be draughty.

Compared to walls, patio windows are almost certain to lose more heat, but if they face South you can get useful heat gains too.


Why am I getting condensation on my windows in the morning?

Condensation on the inside of your windows used to be the norm with single glazing. This means your windows are cold on the inside and you are losing a lot of heat. Ice was not unusual!

Condensation on the outside of your windows is normal with double/triple glazing. This means the outside of the window is cold, which shows that you are losing little heat. Just as dew forms on grass when the air gets cold overnight, so it will form on windows too.

I have condensation inside double glazing. What can I do?

Condensation between the panes of double glazing suggests that the seal between the panes is ‘blown’ i.e. air and vapour is leaking in. This is bad, because apart from reducing visibility it also implies that the heavy gas that often fills the space between the panes (usually argon) is leaking out. Performance will be reduced. It should be possible to replace the glazing unit without replacing the frames as well, if the frames are still in good condition.

I have condensation inside secondary glazing. What can I do?

Secondary glazing is never totally airtight, which can lead to condensation. Typically the air inside your house is more humid than outside, in terms of total water content. (This is because we generate moisture by breathing, drying clothes, cooking etc.) When this air leaks into the air gap in the secondary glazing it gets colder and vapour can condense on the inner surface of the outside pane. To avoid this problem you need the inner window to be more airtight than the outer window.


How effective are curtains to keep heat in?

Medium curtains will cut heat loss by about 15% when closed. Roller blinds will save more. Thermally insulated curtains will also save more. However these savings are when the curtains are closed. There are no savings when the curtains are open!

There is a discussion of research on this here on Nicola's blog.

To be effective the curtains should fit well into the sides of the window recess and go right down to the sill but not hang over the edge. Never let curtains hang in front of a radiator, because this directs the warm air up behind the curtains instead of into the room!

We have often seen condensation behind roller blinds, sometimes with mould. This is a sign of how cold it is behind the blinds, which is good, but the mould is not. You either need to improve the windows so the surface is not so cold, or increase ventilation behind the blind, which will increase heat loss but it is better than developing mould.

How effective are curtains/blinds to keep heat out?

Venetian blinds can reduce solar gain by around 50%. They can even be fitted between the panes of a window where they can’t get dusty - this is sometimes called integral blinds. This product from KJM group is one example).

Other blinds and curtains are also effective. Ideally they should have a white backing (facing the window). Reflective backing materials are even better.

There is further advice from the US dept. of energy

How can I reduce solar gain through my windows in summer?

Solar gain can be big factor leading to overheating in summer, especially for East/West facing windows where the sun shines in at a low angle for much of the day. Keeping curtains closed will reduce heat gain substantially, as described above. However shading on the outside can be even more effective. A final option would be to apply a reflective film.

There are various techniques for shading windows. For the ground floor, deciduous trees and bushes have the advantage that they shade very well in summer but much less in winter. You can also fit awnings. Here is some advice from Cambridge Carbon Footprint on how to do it DIY. In hot climates external shutters are common and there is no reason why we should not have them here too.

See also our fact page on Keeping Cool


When and why should I need vents in the windows?

You need some ventilation in your house, but not too much. Modern homes tend to be fairly air tight. Also, if you have insulated the walls (either cavity or solid wall insulation) this will tend to reduce air leakage, so it is a good idea to make sure you have ventilation from somewhere. Window vents are required by building regulations, unless you have an alternative mechanism such as mechanical ventilation with heat recovery. New homes also need extractor fans in kitchens and bathrooms because these rooms get a lot of moisture from cooking and hot water.

If you do not have enough ventilation you may get condensation on windows and other surfaces and mould growth. This is very bad.

See also our fact page on ventilation and our fact page on Damp

How can I stop heat leaking out of windows when there is a radiator underneath?

The first thing is, never let curtains hang in front of a radiator! This is because they guide warm air from the radiator behind the curtains where they will warm the windows but not the room so much.

Secondly you can guide the warm air away from the window by widening the window sill to go over the radiator. This is called a radiator shelf. Making one is normally a job for a carpenter but it can be done DIY if you are handy.

Also, if you have a radiator under a window it must be on an outside wall and you are heating this wall; if it is a solid wall without insulation it can lose a lot of heat. You can reduce this by fitting a reflector panel behind the radiator to direct heat away from the wall. See our fact page on central heating

It used to be the norm to have radiators under a window to reduce draughts but this is much less of a problem if you have double glazing. With single glazing, air in front of the window would get cold and drop, making the floor cold and bringing in more warm air near the ceiling and so you get a circulation round the room. Putting heat under the window blocks this effect as it makes the window warm. However, with better windows that lose less heat the circulation will be weaker regardless.

Can uPVC windows be recycled?

Window glass cannot be recycled the same way as bottle glass because it has a different chemistry. However a lot of glass ends up ground into hard core which is better than going to landfill.

uPVC windows contain a good deal of steel which can be recycled. However, for a long time the actual plastic was considered a lost cause. This is no longer the case - VEKA recycling has a plant in Germany.

See also Recycling solar panels and uPVCwindows