Old Sliding Sash Windows – Upgrade or Replace?

March 17, 2019

You have a period home with tatty timber sliding sash windows…

They rattle. They hum. They let the rain, wind and flying creatures in. They’re difficult to open, or jammed shut by layers of paint. The sash cords have frayed or snapped. Condensation dribbles endlessly onto your window boards in cold weather. Your heating bills bring a tear to your eye.

But you love them anyway. You have a period home and you don’t want to wreck the look with cheap shiny white ‘mock sash’ casements.

We all love timber. Old traditional sliding sash windows are made of it. Why would we want to replace them with anything else?

The main reason of course is cost. Good quality, high performing, durable, well-engineered timber windows are not cheap. And unfortunately it’s quite difficult to find them. Even though there are a lot of companies who make timber windows, there are not many who do it well.

If you can’t afford to replace the whole window, there are companies who will replace the single glazed sashes with new double glazed sashes.

But is it a good idea? Is it money well-spent? In our opinion, no.

These are the reasons why:

  • The original units were designed to house single glazed sashes – considerably lighter than new double glazed units.
  • The new sashes will be the same thickness as the old ones to fit into the old frames, but the double glazing units will be thicker, leading to less stability in the new sash frames.
  • New, larger weights will be required. Most companies use stainless steel, lighter than the original lead. Steel weights have to be longer and can cause ‘clonking’ as they drop to the bottom of the weight box.
  • If new lead weights are used, they will be more costly, will still need to be longer, and could cause the same problem.
  • The old timber boxes and frames will still require decorating and repairing as often as before.
  • The old boxes and frames are likely to be worn and warped. This can’t be rectified in situ, so the benefits of installing a new sash and new draught-proofing can be limited in comparison to a new unit.
  • Even using a high performing slim-line Krypton-filled glazing unit could reduce the integrity of the frames and new seals, thus negating the benefits.
  • For those on a limited budget, experience tells us that seriously considering replacing the whole unit, with a really good quality non-timber product, would be better than just replacing the sashes (i.e. the moving frames), or carrying out major refurbishment.
Sliding sash windows
North Country recommended UK manufacturer – 12 and 14 years old respectively and no sign of requiring repainting.

Of course if you get in touch with a company that specialises in renovating old windows, they will tell you that the above risks are minimal and they are skilled at ensuring that your renovated windows will be almost as good as new. That’s probably true, for a few years.

They may even have some excellent reviews. The problem with this is that reviews are posted just after the job has been completed, like this one about a well-known ‘specialist’ company:

“We originally picked ********* on the strength and conviction of their salesman who seemed dedicated to restoration and appeared knowledgeable. The cost was expensive …… and [we] assumed the prices would reflect the finished product. The work done was beyond disappointing ….. the job and outcome appeared rushed …… Some windows won’t open …. some ‘fall’ and won’t stay open. Catches wouldn’t close as they were put in the wrong place …. the glazer had to replace every pane of glass as the painter sanded down the windows and scratched every single one! Save your money.”

But it would be better if customers posted reviews at least 5 years later, like this one from a Client of my preferred timber manufacturer/installer:

“We have been blown away ……. The oldest ones have been in over 7 years in full South facing sun with sandblasting from windblown dust from a dirt track in front of house and have needed zero maintenance. The paint is a bit duller but still doesn’t need any attention. Great product.”


Also, look at the guarantees you are offered. Glazing units in part-replacement windows will likely have limited guarantees – less than 5 years is common. They will not guarantee the old sections of windows, and the scope of the guarantees will have caveats to cover the issues above. So before you part with your money, read the small print.

Non-partisan information, such as that on the link below, will show that what they do not say is revealing: lists of things you can do to improve old sash windows does not include replacing the sashes with double glazed units.



Non-Timber, i.e. uPVC or ‘Composite’

These days, a good non-timber unit can look remarkably authentic. There are very few that look truly traditional, but they do exist.

Sliding sash replicas
For non-timber traditional sliding sash replicas cut the mustard, but one or two excellent ranges exist. Detailing and options vary, but they can look almost indistinguishable from the real thing.

The units we would recommend will last at least 25 years and require little maintenance, whereas many of the cheaper hardwood products we see these days start to require maintenance and even repair in less than 5 years!

Sliding sash window ranges
Slim profiles & meeting rails, through horns & butt joints that mimic timber window construction, lend a truly authentic aesthetic to a few good quality sliding sash ranges available in the UK.

Repair & Stick with Single Glazed

There will be fewer issues if you just repair the existing windows and single glazed windows, although it will not reduce your maintenance costs over the years. If done badly though – which is common – it will prove costly over time.

Professionally restored bay window: the ‘knotty’ pine timber will not last long. Once painted, it may look ok, but in a few years the knots will ‘bleed’, joints between old and new timber will show, let moisture in, and the whole lot will need re-decorating. It amazes me how often this cheap timber is used, but even with a top quality, engineered timber, this is not a long-term solution. maintenance costs (and the hassle will eat up any savings over replacement.

Secondary Glazing

If you take the repair route, good quality secondary glazing will upgrade the thermal performance of your windows to almost the level of a new window. This can be far better, and cheaper, in the long run – in our view – than replacing the sashes. They are not the best option aesthetically, although there are good products that are slim and relatively unobtrusive.

Secondary glazing listed homes
Secondary glazing: improves thermal efficiency & security, reduces condensation & noise ingress. A good option for listed homes, but it won’t reduce maintenance of the original windows.
Robert Goldsbrough

Robert Goldsbrough

Robert has been developing and building period & contemporary homes since 1995, and has installed all types of external home improvement products.