Sash windows are a must-have for many self-builders wishing to give their homes a traditional appearance, but it’s not always easy to get the style right. This guide not only explains how sash windows work, but how to make sure they work for you.

10 Things To Consider Before Buying Sash Windows

1 Sash windows are an inherent part of British architectural history: they were introduced to England in the late 17th century and remained a sought-after fashion item for over two centuries. Any styled property from this era would be lacking without them, and this is why it’s so important if you are renovating a Georgian, Regency or Victorian property, or building a new house in one of these styles, that you include them.

2 The word ‘sash’ simply refers to a single frame for glazing. A traditional ‘sliding sash’ window is usually made up of two sashes that slide up and down, one in front, and one behind, in vertical grooves, counterbalanced by lead weights on cords — though in many modern windows, the weights have been replaced with springs. Sliding sash windows can be opened at the top or bottom, or both, depending on the design, and though traditionally they have no outward swing, many modern designs tilt in and out for easy cleaning.

3 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.

4 Sash windows for period properties must be chosen carefully to ensure you get the right period, as there were several developments and style changes in sash windows over the years. Different designs included the Venetian, which consisted of a central sliding sash with two fixed side panes, and the Queen Anne Revival style, where there were several panes in the upper sash, but just one or two in the lower sash. In the Regency and Gothic revival periods, sashes were often arched instead of rectangular; and in some regions it was always popular to have horizontally sliding sashes.

5 All too often, renovators make the mistake of removing original period timber sashes and replacing them with new models, when if they had simply been restored to their former glory, they would have been perfect. 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.

6 Genuine timber sash windows are likely to be the first and only choice for traditionalists and those living in either a Conservation Area or a listed building. You simply cannot achieve the same effect with plastic. Wood is very 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 highmaintenance, 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.

7 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 lowmaintenance 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.

8 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.

9 Traditionally, sash windows were single glazed with fine glazing bars to hold the panes in place; but 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.

10 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.

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How Sash Windows Work

1. Top Rail: The top horizontal framing member of a sash. Rabbeted (SEE GLAZING BAR) on the outside.

2. Box Frame: Sometimes referred to as a ‘jamb’, the main box frame consists of three TIMBER LININGS.

3. Sash Cord: Runs over the PULLEY WHEEL and holds the WEIGHTS. It can be bought in rolls when it needs replacing (the waxed type is best). Chains are an alternative.

4. Weight Pocket: The WEIGHTS hang in the pockets created by the TIMBER LININGS on each side of the window. Insulation can be retrofitted in the pockets of older windows.

5. Parting Bead: A long, narrow vertical seal that fits in the BOX FRAME to form separate channels for the upper and lower sashes to run in, and holds the top sash in place. Newer beads often have built-in draught seals. On the upper sash, the bead is fitted facing inwards; on the lower sash it faces outwards. The fit should be snug to prevent heat loss.

6.Staff Bead: A moulded piece of timber made up of four sections, nailed all around the BOX FRAME, which aids airtightness and also holds the bottom sash in place. The staff bead is removable, but rarely salvageable, for the maintenance of the sashes.

7. Meeting Rails: The horizontal framing members which meet the two sashes together in the middle. Their adjoining faces are bevelled so they close together tightly.

8. Stile: Vertical side framing member of a sash — there is one each side.

9. Apron: A decorative panel or cladding beneath the window.

10. Cill: A horizontal board fitted internally at the base of the sashes. Shaped so that water flows away outside.

11. Bottom Rail: The bottom horizontal framing member of a sash.

12. Weight: A pair of lead weights hung on the SASH CORD counterbalance each sliding sash. The sash and weights must weigh the same.

13. Wag Tail: A strip of timber inside the BOX FRAME that separates the WEIGHTS. Also known as a ‘parting slip’.

14. Timber Linings: The sections that form the casings of the BOX FRAME – which must be hollow to house the WEIGHTS – are made in three parts: the ‘outer’, ‘inner’ and ‘pulley’ linings.

15. Pulley Wheel: A pulley mortised into the top of the PULLEY LINING; the SASH CORD passes over it to counterbalance the WEIGHTS. Cheaper pulleys have plain axles; better quality ones are made with roller or ball bearings.

16. Glazing Bar: A vertical or horizontal framing member that divides the panes within the STILES and RAILS. Each is rabbeted – a cut which forms a groove – to hold the single-paned glazing. The quintessential Georgian sash window has ‘six over six’ panes, but other configurations were seen over time.

17. Soffit Lining: The window’s head is made up of the main inner and outer TIMBER LININGS as well as a separate soffit lining; glued triangular blocks provide strength.

NB: A ‘sash’ is a single glazing frame. The full name of a sash window is a ‘vertical double-hung box-framed sliding sash window’.

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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