Prescot THI Blog

Sash Windows for the 21st Century

They say you should never bring your work home with you. But on the other hand, they say you should practice what you preach. With the latter in mind I thought I’d use this blog to share how sash windows can be made as good as new, but with better thermal performance, by repairing the draught-stripping them rather than replacing them all together. I’ve used my own house as the example – the works were done in Summer 2013.

For over a decade, with my conservation officer hat on, I’ve been telling people the importance of maintaining sash windows, how easy they are to repair and how improving them can be far more cost effective than replacing them. Don’t take my word for it – there’s even a Glasgow Caledonian University study that backs these claims.

The house dates from about 1910 and is a standard brick terraced house in a short row of five. It’s not listed, nor is it in a conservation area, and it’s not a remarkable piece of architecture either. Even so, the fact that the house still had its original one-hundred-year-old sash windows front and back was to us a big plus because we feel they really give the house its character (unlike the uPVC door!).

The house had been empty for six years before we made it our own, so understandably the windows were not as well maintained than if the house was occupied.

We chose a sash window specialist who had lots of local examples of their work. When they quoted they had a good look at each window, inside and out and explained how much work would be done to each window.


Before the work started: You can seen the bubbling and peeling paint on the cil and some fo the rotten timber. Out of the entire window (5 feet by 2.5 feet), this one piece of timber was the only piece that needed replacing.

Even though the windows were in less than perfect condition, there wasn’t a drastic amount of work to be done to them. In all three timber sills were replaced and one of the sliding frames needed replacing. Each window was capable of being overhauled, made to open and close smoothly, and have draught-exclusion strips fitted into the frames, along with new locks, safety stops and other window furniture. Three cracked panes of glass were swapped for matching imperfect bobbly panes of glass that had been recycled from other buildings and kept in stock by the contractor.

The relatively minor amount of timber replacement just goes to show the quality of the timber used. People always say to me that timber rots or warps too easily, but really it’s down to the quality of the timber you use. Good slow grown softwood like Douglas fir can last for many decades, if not longer, if maintained. Cheapo bargain basement fast grown softwood will have a wide grain and will soak up moisture like a sponge, and will be prone to rot unless it is very well-maintained. In short, you get what you pay for.


One of the rotten sills that was taken out. Both of the pieces of timber were a single piece. It was cut in two so the sill could be removed and a new sill put in. You can see that even though part ofthe tmber has usffered from rot, the quality of the timber means that the rot didn’t spread across the entire window sill. That neat end sticking out on the left has been inside the wall of the house for 100 years but was in as-new condition when it was removed due to the rot further down the sill (on the right).

Even things like which direction the grain runs through each piece of timber and whether the joinery has rounded or ninety degree edges will influence how quickly rot or warping will occur – even if the same species of timber is used. This is why it is important that whoever selects the timber to be used and makes or repairs the window should know about the qualities of the material they are working with.

(By the way, a ninety degree edge to a piece of timber will be more prone to rot because the sharp edge will never get a proper coating of paint. This makes the edge of the timber piece more prone to soaking up moisture. Even a slight rounding to the edge of the timber piece will mean there is a proper paint covering and hence proper protection from the elements).

The contractor worked through the house room by room, window by window so that by the end of each day (there were six days’ work in all) we had secure, fitted windows. One of the sills that needed replacing was only in a bad way because of a lack of paint. The density of the timber meant the rot was very slow to spread and hadn’t reached any other part of the window. If it was a lesser quality timber, the rot would have spread much faster and to other parts of the window. This rotten cill was simply cut out and an entirely new matching timber cill slotted in its place.

The other two rotten sills had been made worse by a poorly thought-out repair. They were both on a wall that faces the prevailing wind and gets a lot of direct sunlight in summer. I imagine these started to show the signs of rot due to the paint failing quicker on this more exposed part of the house. Instead of being fixed, somebody put uPVC slips over the timber. The problem was, this only made the sills deteriorate a lot faster – the plastic would have trapped moisture in the timber sills making ideal conditions for rot because the timber probably rarely if ever dried out. In one of these two windows the rot had spread from the sill into one of the sash frames. Because of the way sash windows are designed – each component can be replaced without disturbing the rest of the window – the rotten parts could be replaced by the contractor very quickly and easily.


Before the works: one of the windows that was more exposed to the elements the sill was so west that the rot spread to the frame of the sliding sash, shown here. This was the ‘biggest’ job, but it was done in a day and the vast majority of the entire window was kept.

Sash 3

Before the works: a combniation of windows being painted shut and broken sash cords meant the windows were either had limited opening or could not be opened at all. We had no idea until they were fixed that the top halves of any of the windows could actually be slid open!


During the works: one of the sash window frames taken out so it can be overhauled. This included planing off the paint that was causing the windows to jam. As you can see the timber underneath the paint is in very good condition despite a century of weather.

The draught stripping was installed while the windows were taken out and sanded down, re-hung and re-balanced. The sanding down took off all the excess paint that had accumulated over the years, and this, with the re-balancing and re-hanging of the sliding sashes, made them far easier to open and close. This included the upper sashes that were painted shut probably many years ago. You can only see the draught-stripping when the windows are open and even then it can barely be seen. Big strips of it can be taken off and slotted back in when the windows are re-painted.

The draught-stripping is fitted in such a way that when the windows are closed and locked, the frames fit snugly together, minimising the amount of draughts coming in around the edges of the window and where the two halves of the window meet at what is called the meeting rail. As well as making the rooms warmer, the draught-stripping cuts out a lot of noise from the busy road outside. Likewise, getting those cracked panes fixed cut out draughts.


During the works: the unpainted bit is the new window sill that about to be painted. The rest of the window is the original wood that has just been overhauled and has new draught-stripping (NB we have since taken that wallpaper and pelmet down!)


After the works: the finished article. The frames have been draught-stripped, re-hung, repaired, new locks, new stops, and re-painted. As good as new despite 85 to 90% of the fabric being original.

The other thing people often level at me about sash windows is that they are unsafe on the basis that a child could fall or climb over the sill and out the open window. This won’t happen if you open the top half of the window only (but for some reason in my experience the top halved of sashes have usually been painted shut), or like us, you have a stopper fitted. The stopper means that the lower sliding sash only opens a few inches (but you can always open them fully with a key that removes the stoppers).

I didn’t realise when I started this how much I would be writing about windows – but people’s queries about windows take up a surprisingly high amount of conservation officers’ time. Unfortunately people see uPVC or double glazing as the only ‘green’, ‘money-saving’ or ‘energy efficient’ solution to cutting out draughts or noise from outside. It’s a shame really, because in historic buildings, even common unassuming ones like our house, the windows make such a big contribution to the overall character and appearance of the house, and in our case, the terrace.

Since getting the windows fixed, draught-proofed and overhauled, we’ve got through a mild but very wet winter with no problem keeping the house warm. If we have very cold winters like we did a couple of years ago, we might consider putting secondary glazing in, but don’t see the need to so far. And fixing and improving the existing windows worked out cheaper than replacing all five of them.

For what it’s worth, we got through those recent super-cold winters in a flat in a Grade II Listed Georgian house with Victorian sash windows and secondary glazing – no draughts, no complaints!

The best guidance for windows I have read is available for free from Historic Scotland and English Heritage.

In conclusion here are my top tips:

  • Get the right person to survey your windows. Most firms do this for free. There are companies out here who specialise in the type of work I’ve just described. A general builder or joiner may not be up to this type of work and will quote accordingly. The big national and local uPVC window installers will always say that wooden windows need replacing because it’s in their interest to say so.
  • Give more weight to independent research, the advice of a conservaiton officer or guidance written by historic building experts rather than advertising, sales pitches, or advice written for post-1945 buildings.
  • The right firm will keep as much of the timber as possible. They should be able to show you where the window is rotten as they are quoting.
  • When someone says an entire window needs replacing, it is very rarely the case. Sash windows are designed to be easily repaired. If it’s old timber any rot would progress very slowly through the timber.
  • Addressing all of the window’s problems in one go: condition, opening, draught stripping, cracked panes, will have a far bigger impact on how draughty the room inside is than if you address only one or two of the problems.
  • Maintenance, maintenance, maintenance. Timber windows can last indefinitely if they are kept in good working order and are painted. Some of the earliest known sash windows in the North of England are still in place 300 years later!

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