Although the house is not listed, one of the conditions attached to the planning permission was that we maintain the original facade, including the orginal window frames, made of Belgian oak.
At the same time, if we are to achieve the level of energy efficiency needed to qualify as a "Low Energy" house, one of the most obvious improvements was to add double glazing.
Happily, it is quite straightforward to add double glazing to old windows, providing the frame is thick enough. There are a number of companies in Brussels who specialise in this, including Xylonis, who did the job extremely efficiently and proficiently.
First, they take the window off its hinges, and repair or replace any peices of wood that need it. They also place a rubber seal around the frame, to prevent drafts.
Then they fit the panes of double glazing, which has been specially made to measure.
And voila! It took all of four days.
Given how simple this operation was, I find it extraordinary that most windows in London - a city I know well - do not have double glazing. It is not uncommon, in a London flat, to feel the draft coming from a closed window. The usual excuse is that double glazing cannot be fitted into the traditional sash windows, but I find that hard to believe...