How to Save Energy On Sash Windows

Sash windows are an elegant way to add character and class to your period home, but for some they can be a cause for concern. Although they are inherently inefficient, modern thermal upgrades have been developed that can have a huge impact on the ambience and overall energy efficiency of your home, without the need to replace the entire window.


Why are sash windows so inefficient?

Original sash window design and construction allowed for 3mm gap or tolerance. This is to help the sash travel smoothly within the pulley stiles. While 3mm doesn’t sound like all that much, it can end up equating to an overall 3 inch² opening on a well fitted window. For some older sash windows that have been well over painted and suffered from significant movement, this can increase to as much as a 6 inch² gap.


How to make your sash windows more energy efficient

The most cheapest and most straightforward way to solve the issue is a sash window draught proofing system. By placing a series of brushes hidden within the mechanism you can virtually eliminate draughts to an accuracy of at least 95%. The weakest and most vulnerable part of the sash is where the parting bead meets the bevel of the sash. Since this is effectively a hole it takes a slightly larger seal to catch it, but it’s normally achievable.

If your windows are rattling, or you can feel cold draughts as you stand by your window then it’s almost certain that sash window draught proofing would considerably improve the comfort of your home. Given sash window draught proofing starts at around £250 a window, it’s by far the most costeffective way of saving energy whilst maintaining the original sash windows.

On occasion it’s possible to install double glazing units into the original sash window. For this the sash must have a depth of more than an inch and a half. This allows for profile to remain in tact and the joints firm and costs in the region of £700-£800 per window.

Another decently priced cost option is to manufacture a pair of double glazed sash and install them into an original frame. This provides most of the efficiency of single glazed sash windows without the need to entirely replace. By carefully replicating the profiles and decorative horns you can barely tell these windows have been replaced by wooden double glazed sash like for like. Here’s a look at double glazed sash windows manufactured like-for-like:

As you can see, the new double glazed sash windows (right) look almost identical to their original single glazed counterpart (left). Aesthetically, this kind of window is has come a long way since the finish is now achieved with a precision factory spray, as opposed to traditional brush. The spray system is also more durable than a traditional hand finish, and repainting generally won’t be required for over 10 years.

Historic England standards state that “12-14mm or more double glazed units can now be installed”, but our friends at London Sash Window Repairs Ltd go further. They always aim to install a 16mm (4-8-4) Argon filled, low-E Pilkington thermally efficient glazing to all standard like-for-like double glazed sash window replacements into the original frame. Pilkington website clearly mentions the benefits of using low-E glazing are reduction in energy consumption and money saving.


What is the best way to save energy on sash windows?

Double glazing sash windows provide the biggest energy saving, however in terms of value for money it’s hard to compete with sash window draught proofing.

The most expensive option is an entire sash window replacement. This provides similar performance to double glazing sash windows using the existing frame but far more costly. On occasion, if a sash window has considerable movement it might be beneficial to entirely replace as poor sealing of gaps will diminish the returns of double glazing.

You can find the most comprehensive list of sash window draught proofing, sash window repair, and sash window double glazing prices on the London Sash Window Repairs Ltd price list.


To learn more about sash windows you can visit the London Sash Window Repairs Ltd Website.


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