The Process of Refurbishing (Not Replacing) Sash Windows
The sash window dates back to the 17th century, and the idea behind it is rather ingenious. Rather than just opening the window to let in the fresh air, if the top and bottom are both opened, the hot air can escape at the top and the cooler air can enter through the bottom. It’s actually quite delicious in operation and works very well. A weight hidden in the frame (generally made of lead) acts as the balance and therefore holds the window in place so there is no requirement to prop the window open.
Sash Window Repair
There are, however, downsides to this construction. Over time, moisture gets into the wood and the frames start to rot. Rather than sorting this out when renovating the house, many have taken to painting the sash windows shut and, therefore, you're lucky if both the top and the bottom halves of the window operate properly. The other potential downsides of old sash windows include rattling, drafts, loose glazing, condensation on glass, and maintenance of the chords, wheels, and the timber and paint over time.
Some sash windows may also have a broken equilibrium and no longer stay open. This is because the weights no longer act as a balance as thicker glass has been installed without increasing the weight of the lead. Additionally, the glazing putty will shrink over time and then detach from the timber. This allows the glass to move around in the frame. Although quite expensive in comparison to the casement alternative, uPVC sash windows are available as a replacement. In addition to their cost, I'd also argue that they don't have the intricate detail of the old wooden sashes, which adds to the character.
So, if you want to keep your original sashes, but would like them modernized a little, then the most cost-effective option is to restore them. This can be done on a DIY basis, but patience is required. However, it’s not rocket science! Alternatively, there are companies that will dismantle the original window and refurbish it using modern techniques and materials. Once reinstalled, they will again look fabulous and operate much more smoothly.
Modern equipment will allow the original paint to be removed using infrared technology without the risk of burning or paint fumes. By using a router, a brush can be installed on the horizontal beam of the window (the meeting rail) to stop the windows from rattling and moving in the frame, thus improving thermal and noise efficiency. Replacing putty is straightforward as long as you are careful with the glass—it might be very old!
Modern paint systems offer ongoing protection for the timber and the paint, and a good painter will ensure that a bead of paint is left on the glass so that any moisture runs off the glazing onto the paint and doesn't get between the glass and the timber.
As an example—a few years ago, we restored an Edwardian home that hadn't been touched for 40 years. Due to the age of the property, the original timber was of high quality (slow-grown, properly dried out) and therefore the majority of it wasn't rotten. I would say 95% of the timber was sound and didn't require replacing or filler. Some of the joints were weak and therefore had to be re-glued and set again, but the worst part was removing the old paint. Due to it being a lead-based paint with many layers, it was hard work and we spent hours with a hot air gun melting the paint and scraping it off. These days, there are better options, but that's all we knew at the time. Lead is poisonous, so steps had to be made to ensure we didn't breathe in the hot fumes, although I can still smell it now some six years on.
Once the timber was bare and any repair work done, we used two coats of primer, two coats of undercoat, and one coat of gloss to give protection against the elements. Most of the panes of glass were original, although some were cracked and therefore replaced. As the windows are the "eyes" of the property, it looks so much better when they are all restored and a bright white once again.
The operation and the ventilation that sash windows bring is great. I would always try to restore the original sash windows instead of replacing them. If you live alongside a busy road, there are also specialists that can help with the extra noise by installing secondary glazing. That really isn't a DIY job, though!