Sorry for being quiet on the blog for a couple of weeks – things have been going a bit crazy with the house renovation and I’ve been preparing for a conference that I’m speaking at on Tuesday (Sustainability of Traditional Buildings). The topic of my talk is about retrofitting slim double glazing into old windows and I thought a blog post about it would be good seeing as it’s so fresh in my mind.
Up until recently it’s been hard to find companies to repair and retrofit old windows. Joinery companies are all too willing to make new windows in their workshops but ask them to work on existing windows on site and they go quiet and block your calls. But I like to think that this attitude is changing – not only are homeowners seeing the value and beauty of their timber windows but smaller joinery firms are offering the service of replacing the single glazing with ultra thin double glazing. Not only does this dramatically reduce heat loss through the glass (resulting in a much warmer room) it also removes the woes of condensation which can lead to mould and timber rot. The picture below is the thermal image of two adjacent houses. The house on the left has single glazing where the red indicates high levels of heat loss from the windows. The house on the right has had double glazing fitted where the green/yellow indicates greatly reduced heat loss.
But before I go any further, I have to say that I’m only in favour of replacing modern glass with slim double glazing. If you’ve got old glass in your windows (you can tell because there will be small air bubbles or a wobbliness to it) then don’t go replacing it because it is beautiful and becoming increasingly rare. Instead, find other solutions to conserving energy such as secondary glazing or installing shutters.
But why, you might be thinking, is replacing the glass better than having a new window made?
- Firstly, it’s cheaper possibly even by 50%.
- Secondly, old timber is vastly superior to new, fast grown timber. Many old timber windows have lasted 150 years and will last another 150 if they are properly maintained.
- Thirdly, replacing glass is much less disruptive than fitting a new window. It doesn’t disturb the plaster or brickwork surrounding the window and it can be done within a day.
- And finally, if you care about the environment, it is more sustainable because it cuts down on waste and reduces embodied energy.
Retrofitting slim double glazing into old windows is more common in Scotland, particularly Edinburgh. In England it is easier to get new windows made but retrofitting is gaining credibility. There are a number of firms specialising in it, including Envirosash, CR Carpentry and Mill House Window Workshop. My blog post on Improving the U-value of windows, 1 February 2017 gives tips on what you should specify for the glass.
Something you should think about if you’ve got multi-paned Georgian style windows is to have them fitted with a single pane of slim profile glass and have the glazing bars applied to the glass rather than as an integral part of the structure. I’ve been won around to this idea by an engineer/boat builder called Ken Endean who has designed a fantastic window for his Grade II listed house. He has used traditional joinery techniques to fix the glazing bars to the perimeter bead and has also managed to recess the internal spacer bars behind the glazing bars so they are only visible at a very oblique angle. Although Ken had new windows made, there would be nothing to stop the design being used for existing windows although you may have to sacrifice the glazing bars.
The top sash has slim double glazing whilst the bottom one has not yet been fitted with it.
This photo shows how traditional joinery techniques have been used to fix the glazing bar in place.
Window on the left has slim double glazing whilst one on the right is single glazed.