Tricky things, windows. They divide the in from the out, but should allow controlled transfer between the two. The inside of the building needs light and air, but we also get crotchetty if there's too much of either. Heat can't be lost to the outside very much, and there are regulations that define what that means. We are interested in whether traditional techniques can be used on-site to make good-looking, functional and repairable windows without the embodied energy of gas-filled double-glazing. How better to test this than on your own family?
Up until WWII, it was not unusual for windows to be produced locally by joiners using traditional methods. As the 20th Century wore on, however, increased mechanisation, standardisation and pursuit of efficiency meant windows and doors were made more and more in factories with techniques that moved further and further from what was possible in the joiner's shop.
While there are some benefits to this, they mostly accrue to the large contractor, where repeatability and large order volumes are required. Repair is difficult and wasteful: double or triple-glazed units have to be replaced in toto if they are broken, whereas an old-fashioned single pane can be re-glazed after a trip to the glazier with some measurements.
The energy used to concentrate the gas to go inside the insulated unit (usually argon or krypton, present in the air in very small proportions) is a large share of the energetic production cost. In The Ecology of Building Materials by the Norwegian architect Bjørn Berge, he writes: "In sealed windows, the use of krypton gas will reduce heat loss considerably compared to argon. However, the production of krypton is so energy intensive that this is more than outweighed over a building's lifetime."
He also considers the pollution created during the production of the plastic or metal spacers between the insulated panes, and the seals of petro-chemical, plastic-based mastic. They will fail over time, and there is no accepted method of repair. The failure of these materials to degrade safely is also a concern.
In Scandinavia, it gets quite cold. People have never enjoyed being freezing, so as glazing became more prevalent in the 19th Century, it was realised that more layers led to less heat transfer, and better sealing against draughts. The method still used in many buildings to this day is to hinge together two single-glazed timber windows. They can be opened separately for cleaning or repair, and opened as one hinged casement in normal use. When hooked together, they act as a double-glazed window. The outer layer can be fixed into the frame with linseed putty, and the inner held with wooden beading. Simple, effective and biodegradable.
When we lived in northern Sweden, our 19th Century lodgings had their original windows based on this principle. While I had no quantitative means to measure their effectiveness, my small outlay on thermal underwear suggested they were satisfactory down to -30C, the coldest temperature I experienced.
The conditions in Cornwall are rather different, and temperature rarely drops below freezing. It is warm and damp, rather than cold and dry. Our window tests have therefore been made with certain adjustments. We have decided to mount the panes of glass on wool felt, which is exposed on the inner side. In the damp atmosphere, any condensation that might manifest between the panes will be absorbed by the wool, and allowed to evaporate in the warmer interior air that can support a higher humidity level.
The steel cladding has been continued onto the window frames, giving the house a protective, defensive, hunkering-down character against the rain. We have tried three different methods for the glass/steel interface which I'll detail in another post, testing alternative ways to use sustainable and not so sustainable materials to stop the water getting in.
By the way, the water didn't get into the skylight at the top of this post. That one's sealed with cork gaskets, but more of that later.