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6 ways to keep the heat in with sash windows

Posted by Chris Newman on 24 January 2014 at 10:15 am

Sash windows are inherently draughty. A combination of age, the hollow boxes and the requirement to slide rather than abut all contribute.

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

So, how big is the problem? A back of an envelope calculation: just over 21% of our 22 million dwellings were built before 1919. Quite few houses built up until around 1930 also had sash windows, but quite a few before did not and many have had them replaced. If we guess at 9 windows per house then that’s around 43 million sash windows.

Ways of improving the energy efficiency of sash windows can be expensive; if you look for total replacement you'll get little change from £2,000 from some manufacturers. Below we give some initial thoughts on solutions to the draught problems with indicative costs, effects and pros and cons.

1. Ways of keeping the heat in with sash windows

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

It shows that you can easily get 14-50 per cent reduction in heat losses by using traditional coverings. The pros are that

  • you don't need to do much to the window,
  • most can easily be done DIY, especially if you are handy with a sewing machine and
  • the costs can be kept down.

The cons are

  • they only work when you have the curtains 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.

2. Draught-proofing sash windows

A 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 (see below) may also have a draught-proofing 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. 

We advise avoiding the surface mounted metal strips. They are unsightly and not usually very effective.

The windows will have to be taken out to install a routed draught proofing system as the material needs to be routed on all sliding and abutting sections.  You should be able to get this done for around £150-£200 a window (see picture).

3. Secondary glazing 

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 until 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. These are a little thicker and 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 are more difficult to store 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.

4. Replacing the glazing in your sash windows

If you can afford it, this may be the best option. An added bonus is that it may 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 pretty close. 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 makes sense to install a brush draught system at the same time.

The benefits are

  • significant reductions in heat loss,
  • they are virtually unnoticeable to all but close inspection from a trained eye and
  • 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, a fair amount of skilled work that needs to be done, so they aren’t cheap.  Expect to pay upwards of £500 a window.

5. Replacing your sash windows

The most extreme option is to replace the windows and frames. Double glazing will mean 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 (although this won't be a problem if you are also insulating the wall internally…).

If you go for an glossy internet sash window company you’ll probably pay between £1,500 and £2,000 a window (with much of the cost  determined by how much you can afford so dress down!). A local carpenter might be able to 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.

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

6. Triple glazing

We recently did some calculations on triple glazed windows. We found that a triple glazed window will save around 42kWh per year over a double glazed window, which sounds good ... until you follow it through. That's a whopping £2.10 at best, as you will also decrease the amount of solar gain, and the saving from both double and triple glazing will probably be less if you have some form of curtains. So let's sayyou save £1.50 per window. Unless your triple glazing is less than £30 more expensive than the double glazed equivalent it won’t pay for itself in 20 years.

Pictures: Parity Projects

More information on draught-proofing and windows

The YouGen guide to energy saving

From the blog

A case study of secondary glazing

How can I stop heat loss from our bay window?

The best way to draught proof an old window or door

About the author: Chris Newman leads on development and delivery of the Parity Projects Home Energy Masterplan.

If you have a question about anything in the above blog, please ask it in the comments section below.

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3 comments - read them below or add one


JonTopNotchSashComment left on: 2 June 2015 at 7:20 pm

These suggestions all work really well. It is also worth nothing VAT should be charged at 5% for any energy effeicney improvements you are making to your home.

Make sure your home is not listed and check with local planners before making any of the above suggestions beyond draught proofing or secondary glazing installations,

It is also worth noting that you can have removable secondary glazing, which I find my clients often opt for.

Regading new joinery for sash windows though, a good hardwood should last well beyond 25 years. I often make repairs on windows that are over 100 years old, and will last for another 100 with the right care and attention. Where possible make sure your sash window specialist is giving you either a hardwood or Accoya as a timber option. As some softwoods don't work out sustianbly from both from an economical or longevity point of view.

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Zac Comment left on: 23 January 2015 at 9:18 pm

Depending on the age and overall condition, some sash windows can be refurbished and improved to include glazing which increase the centre pane U value. There are however considerations, the most important being that the window is also fully refurbished to include using brush piles and adequete draught sealing. Having all of this work done to exsisting windows can be quiet costly and quiete often a false economy when compared to new double glazed replacement sash windows which are build to the modern day standards and have much better and measureable overall thermal performance. This option also carries a better longevity through 5/10 year warranty, although new timber sash windows can be exptected to last at least 25years plus.  

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KatyDComment left on: 27 October 2014 at 10:49 am

You could also fit thermal blinds - I invented them specifically for sash windows. Take a look at the technical specification on the website - equivalent to triple glazing on a single glazed sash.

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