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Sash window options…


Some back of the envelope calculations to work out the scale of the problem…just over 21% of our 22million dwellings were built before 1919. Obviously quite few houses up until around

1930 also had sash windows, but then quite a few before did not and many have had them replace. If we guess at 9 windows per house then that’s around 43million sash windows.

A single glazed sash window has a U value somewhere between the centre pane U value of around 5.7 and the whole window around 4.5W/m2K.  As a comparison a new wall built to current regs needs to be 0.3 W/m2K i.e heat will be lost through it around 16 times slower per unit area.

The other major problem with sash windows is their inherent draughtiness – a combination of age, the hollow boxes, requirement to slide rather than abut etc.

Solutions for sash windows can be expensive; if you look for total replacement its possible to get little change from £2,000 from some manufacturers.

So below we give some initial thoughts on solutions with some indicative costs, effects and pros and cons..


Historic Scotland looked at a range of accessories including shutters, insulated shutters, different types of blinds and heavy curtains.  They also compared these to secondary glazing and replacement double glazing. The results can be seen on page 13 of their report:

historic scotland windows

Essentially they show that you can easily get 14-50% reduction in heat losses through traditional coverings. The pros are that little intervention to the windows is required, most can easily be done DIY, especially if you are handy with a sewing machine, and hence the costs can be kept down. The cons are they only work when you have them closed, the glass remains cold so can still be a focal point for condensation, and they don’t have a great effect on noise, security and thermal comfort due to radiation heat losses when they are open.


Window pileA range of options are available from sticking some masking tape over unused windows, especially during winter, sealing those that aren’t needed e.g. the side sashes in a bay, surface mounted metal strips with brushes, routed brush strip and compression seals.

Many secondary glazing options may also have a draugthproofing effect but if your windows are very draughty you are liable to significantly reduce the effectiveness of your efforts.

Draughtproofing can have a dramatic effect but care should be taken to make sure you still have adequate ventilation especially in wet rooms, kitchens or rooms that suffer from high humidity levels.   Please see our ventilation blog for ideas and options.

Our advice is avoid the surface mounted metal strips – they are pretty unsightly and not usually very effective.

The routed system requires the windows to be taken out as they need to be routed on all sliding and abutting sections.  You should be able to get these done for around £150-£200 a windows.

Secondary glazing

window filmAgain the options cover a huge range of prices and importantly longevity. If you are just renting and looking to get through a winter in some comfort then the one season only window film might be an option. Essentially you stick it on as taut as you can then then use a hair dryer on to make it even more taut.   Then watch out for sharp elbows til Spring. The pros are that it will cut the heat losses by around a third, can cut down on draughts if stuck on the frame and is removable. The cons are that it is easily damaged, is prone to sagging and renders the window unopenable.

A step up is polyester sheets. Essentially just a little thicker but can be rolled up and used the next year. Usually these are stuck on with magnetic strips. On south facing windows they may soon loose clarity. Acrylic sheets are more robust and have a greater longevity but have greater storage limitations as they are rigid. Both can be done totally DIY from components or can be sent to you ready cut with everything you need based on your measurements. The rigid ones can be put up and down in seconds so getting a blast of fresh air is not out of the question.

Finally you can have permanent or semi permanent glass secondary glazing. Internet kits usually come in thin aluminium frames or you can have a carpenter knock some up. You should expect to pay upwards of a few hundred points for these types of system.

The major downsides are that they are a significant contraption on the inside of your existing windows both visually and in terms of opening to the outside.

Replacements glazing

P1040136For many people, who can afford it, this may be the best option. An added bonus is that they may also be acceptable to all but the most blinkered conservation officers.  Essentially you retain your existing frames and just replace the glass with extremely thin double glazing. It won’t quite bring them up to current regs but will get you as near as damn it really. You can even get it with cylinder, hand drawn or machine glass on the outside – i.e. old wobbly looking glass.

As the windows need to be removed in order to replace the glass, and add new heavier weights, it obvious that a brush draught system should be considered at the same time.

The benefits are significant reductions in heat loss, that they are virtually unnoticeable to all but close inspection from a trained eye and that the original frames and boxes can be retained. The downside is that although the actual glass isn’t going to break the bank on its own, there is a fair amount of skilled work that needs to be undertaken so they aren’t cheap.  Expect to pay upwards of £500 a window.

Replacement windows


The most extreme option is to replace the windows and frames. An important consideration is that a new sash box will be thicker than your existing box.  This will probably cause the window to stick out from the wall on the inside – not necessarily a problem if you are also insulating the wall internally…


If you go for an glossy internet sash window company you’ll probably be paying £1,500 to £2,000 a window – with much of the cost  determined by how much you can afford so dress down! A local carpenter could may do it for half the cost.

Installation can be expensive as you pay a large premium for a FENSA or Certass registered installer. You can also use your normal builder or do it yourself and pay for Building Control to sign them off; a careful eye and a good spirit level and attention to detail along with a can of expanding foam should do the job.

You should make sure the glass is low emissivity coated and has an inert gas fill such as argon or krypton.

Triple Glazing

We recently did some calculations on some triple glazed windows after someone did half a job analysing them. They showed that triple glazed window will save around 42kWh per year over a double glazed window…all good. The problem is that you then really need to follow this through – that’s a whopping £2.10 – and its at best as you will decrease the amount of solar gain and probably both double and triple will be much less if you have some form of curtains – so lets say £1.50. What this means is that unless your triple glazing is less than £30 more expensive then it won’t pay for itself in 20 years.