Sash windows are a must-have for many self-builders, renovators and converters as they provide an unparalleled glimpse into past traditions, but it is as important to consider the function of the window as much as it is the style.  

An inherent part of British architectural history, any styled property from the 17th to 19th centuries would be lacking without them so if you’re renovating a Georgian, Regency or Victorian property, this item should be high on your shopping list.

large sash windows in living room

The homeowners of this Georgian-style self build chose Mumford & Wood timber sash windows 

The word ‘sash’ refers to a single frame for glazing and the traditional sliding sash window is usually made up of two sashes that slide up and down.

Styles of Sash Window

When deciding on sash windows for a period property, you must be careful to get the time right as there were several developments and style changes in sash windows over the years.

  • Venetian windows consist of a central sliding sash with two fixed panes
  • The Queen Anne Revival styles incorporates several panes in the upper sash but just one or two in the lower.
  • For Regency or Gothic revival periods, sashes were often arched instead of rectangular and it some regions it is popular to have horizontally sliding sashes.
shash windows in school conversion

This Victorian school conversion made sure to include traditional sash windows to complete the period look

Be careful not to make the mistake of removing the original period timber sashes and replacing them with new models. Where possible, existing sashes should always be repaired and waterproofed, but if the windows are beyond repair, or there aren’t any left in place, there are many companies who will manufacture authentic replacements.

What Material Should I Choose for a Replacement?


For traditionalists or for those living in a Conservation Area or listed building, genuine timber sash windows are likely to be the first and only choice. Sadly, the same tactile effect can’t be realised in plastic.

Wood is durable and an excellent insulator and, if taken care of properly, a timber frame can last for centuries. A common misconception is that timber windows are high maintenance, but with the use of modern finishes – available in almost any paint colour or stain imaginable – they really don’t require a lot of upkeep, just some periodic maintenance and to be examined once a year for cracked, flaky paintwork and decay.

period windows on exterior of house

Jeld-Wen’s softwood sliding sash windows provide timeless design with modern performance engineering


PVCu is often used as a substitute for painted wood. Though most commonly seen in white, it comes in a wide range of colours and finishes, including a photo-effect wood finish. It is low maintenance and energy efficient, though it can’t be recycled.

When bought off-the-shelf, PVCu is usually far cheaper than timber, which can cost as much as 40% more. Many conservationists are opposed to plastic windows, but the better quality models can be attractive and hard-wearing, though they are difficult to repair.


It is becoming popular for sash windows to be composite in their construction, with timber on the interior, but clad with aluminium on the exterior. This ensures they retain the classic look of wood on the inside, but outside are extremely low-maintenance and are able to withstand all weathers.

shash window as original feature in victorian home

In the Victorian era, sash windows in with a single pane in a bay format were popular, as this terrace flat renovation demonstrates

Glazing in Sash Windows

Sashes traditionally consist of a number of small panes, or ‘lights’ that are held together by glazing bars to create a larger glazed area. This is because glass advancements at the time didn’t allow for very large expanses of clear glazing.

The number of panes depended on the era: ‘six over six’ is quintessentially Georgian, though larger ‘eight over eight’ windows were also common. In Victorian times, ‘two over two’ reigned supreme, but throughout the whole period, many other configurations were seen, as well as the inclusion of sidelights.

sash windows in period style bedroom

This traditional-style self build includes large format sash windows to complete a period interior style

Modern building regulations make it near impossible to have single glazed windows on a new house, so you may have to sacrifice true authenticity. It is, however, possible to install single glazed windows on many renovations.

Dividing up small units of double glazing with thick bars tends to look clumsy, but there are ways to effectively recreate fine glazing bars. The best way is to bond mock bars onto either side of a single double glazed unit. You could also incorporate spacer bars between the glass sheets, at a higher cost, to add to the effect.

Articles like this Comments
  • Clare Wright

    You mention that old sashes can have insulation fitted. What type of insulation and how please? How do you stop the pulley getting caught?

  • Lindsey Davis

    Hello Clare,

    Secondary glazing is a common way to decrease draughts around sashes and increase insulation. These come as panels fitted to the inside of the sash and therefore don’t affect the look from outside. They are also pretty inconspicuous from inside.

    Most are sliding so you can still open the sash in summer for ventilation, and you can also get ones which are fitted with magnets so you can take the panel out altogether if need be.

    This article may help

    Hope this is helpful.


  • Clare Wright

    HI, I was referring to where you say "Insulation can be retrofitted in the pockets of older windows" as I have never heard of this being done. Please can you describe the process and what type of insulation to use that won’t cause condensation within the frame? Thanks a lot

  • Lindsey Davis

    Apologies Clare. I misunderstood.

    A sash window or period window specialist will be able to fit insulation in the weight pockets. They use foil faced strips of rigid foam which seal gaps and help reflect heat.They will also re-putty the windows if need be to help make them more draught proof.


  • Barnaby Dickens

    Many companies now make fully-finished double-glazed sliding sash windows that combine the performance of a modern wood window with the authentic looks of a true period window. This means you can have slim glazing bars, traditional mouldings and horns etc., combined with A energy ratings, long maintenance intervals (generally a minimum of 10 years before the first redecoration) and an expected service life of 60 years. You can have traditional cords and weights, or, if space is an issue, a modern spiral balance. The only compromise is the glass: unlike period glass, modern glass is much smoother; and you can see some depth in the double-glazing. Many think this is a small price to pay for the added comfort. For more info and manufacturer/installers visit Incidentally, the jury is still very much out on slimline double glazing units. Conservation officers often see them as a way out of the argument between climate change and conservation. But they are an expensive option, less energy-efficient than standard units – and more likely to fail. At all costs, avoid fitting any double glazing unit with putty!

  • Lindsey Davis

    Agreed Barnaby, I think we generally advise against trying to fit double-glazing units into old sash windows as the original frames simply weren’t produced to take that depth of glass. If you want to keep the old windows but increase insulation, you’re best to keep the single glazing and insulate the frame, and use secondary glazing for extra measure.

  • Clare Wright

    Barnaby, I hadn’t thought of insulating sash pockets until I read this article. I have sash windows and I am deciding whether to replace with complete new box sashes, or just new sashes in the existing boxes, which would be preferable as would change the look less. But in a new sash do companies put insulation in the sash pockets? My windows have very large sash pockets in places, as there are 3 narrow sash window beside each other, with stone pillars separating on the outside and on the inside there is one big box with the 3 sashes in (25cm of wood between each window). So I think there is a lot of air in there between the weight mechanisms. What would be done if recreating this to insulate it? And if I don’t rip this out what can I do to insulate the gap in the middle between the windows? Thanks

  • Clare Wright

    Also, Lindsey, wouldn’t using foil faced foam against the wood in the sash pocket increase the risk of condensation there as it isn’t a breathable material?

  • Jonathan Buck

    I just stumbled on this old thread because I saw the link to retrofitting insulation to sliding sash windows but am not convinced. I presume the suggestion is to try and affix insulation to the inner and/or outer linings via the sash pockets. Putting aside the problem that any foam type insulation would be restricting the space that the sash weights need to slide up and down in ( the weight boxes- not sash pockets as on the illustration) is anyone really suggesting that it is practicable to insert a foil-backed foam (for example) through the narrow inner and outer sash pockets in the bottom half of the window and get it up into the top half of the window- and then fix it so it stays put? I guess someone who builds ships in bottles could do it. I suggest that the energy saved on this, even if it could be done effectively, would be minimal. At Bradford Woodworkers, we retro fit new double-glazed timber sashes into existing sash windows- provided that the sash boxes are in good condition- along with draught stripping. Because the new sashes are now heavier than the old ones, then heavier, and hence bigger counterbalance weights are required but may not fit through the sash pockets. This can mean using lead instead of cast iron weights as it is denser. If fitting complete new boxed sash windows with spiral sash balances instead of weights, then no pockets are required as the balances are let into the sashes or the frames. The spaces left in the checks (ie rebates) of the masons openings could then be filled with insulation foam.

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