What about those windows?

According to the National Insulation Association windows and doors typically account for 20% of all heat loss in a home. A leaky single glazed window will also have a big impact on comfort, given the draughts, not to mention the cool temperatures adjacent to the window. This is frequently borne out in our energy surveys; barely a report goes out that fails to make some recommendation to improve windows

The means of heat loss from windows is more complex than one might assume. But there are two characteristics which largely determine how well or poorly a window conserves heat; the U values of the glazing and timber and the degree of air tightness of the window. The latter can be particularly decisive with period sliding sash windows. Research undertaken by English Heritage found that 60% of overall heat loss could be attributed to air leakage.

So in very plain terms there are two objectives, improving the insulation values of the window and reducing any uncontrolled air movement. But what is the best way of meeting these goals? There are a good many options available which vary significantly in effectiveness and cost. Do you refurbish or replace?

Air Tightness

  • Draught proofing can make an immediate difference which won’t cost the earth. You could try doing it yourself. But unless you’re an undercover joiner I’d leave this to the pros. There is a world of difference between a well installed draught proofing system, routed into the wood itself, and some sticky backed rubber strip.
  • It can be quite staggering to see how timber windows, seemingly beyond repair, can be refurbished to look like new. Doing so will also help to reduce air leakage and is often done at the same time as draught proofing. Regular maintenance of timber windows is essential to help them realise their full life time. A well maintained timber window should last longer than most people will be alive. Then it’s someone else’s problem. In contrast a UPVC window can need replacing after twenty years.


  • An increasingly popular solution for single glazed sash windows is to keep the box and sashes and install new double glazing. It some cases this is feasible but we would urge caution as many of these windows date from the early 20th century or before and simply weren’t designed to cope with the additional weight of the double glazing.
  • Secondary glazing will improve the U-value of the window but not to the same extent as modern double glazing. However if noise is a problem, secondary glazing with a large air gap is the better solution. Also in its favour is that it is a lot cheaper than installing new windows and in some conservation areas and in listed buildings, it’s the only option.  Ventilation must also be considered.
  • Upgrading the glazing to either modern double or triple glazing is the surest way to significantly improve the U-values of a window. The typical U values for a single, double and triple glazed window are 5.0W/m2, 1.7W/m2 and 0.8W/m2 respectively. It will also secure an improvement in comfort. This is exemplified by a table often seen in Passivhaus presentations. It depicts the internal surface temperature of a window with an external temperature of -5°C and an internal temperature of 21°C. Under this scenario the temperature adjacent to a single glazed window is 1°C, an older double glazed window sees it rise to 11°C, a more modern double glazed window has it hitting 16°C and finally a triple glazed window has an adjacent temperature of a balmy 18°C.

Very crudely speaking, air tightness can be significantly improved without having to replace a window. However if you’re looking to make inroads into the heat loss through the glazing you should really be considering replacement windows. Incremental improvements can be achieved with the use of curtains, shutters and blinds. On the other hand there would be little point in installing Passivhaus standard triple glazed windows if the other elements of the building are a much poorer thermal standard. It also seems pretty wasteful to replace windows that are in a good shape. Of course the most natural time to upgrade your windows is when they need replacing anyway. But maybe that’s too long to be cold.

Deciding what to do with your windows is not clear cut, there will undoubtedly be a number of things you’ll need to factor in. Just make sure you know what all the options are. And make sure you choose a tradesman you can trust; otherwise missed delivery times will only be the start of your worries!


Inigo Harrison