The earliest type of double glazing was called Secondary Glazing and in actual fact consisted of a second sheet of glass, with (usually) an aluminium strip fitted to it`s outer perimeter that was screwed to the inside of an existing window frame. This had the effect of trapping a “block” of air between two sheets of glass i.e. DOUBLE GLAZING. This system had many drawbacks and these consisted of :-
- Mould would build up on the inside of the glass.
- Moisture would condense between the sheets of glass
- The system was very unsightly and looked as if it had been added as an afterthought; and in many cases it had!
- In the case of large picture widows it was very cumbersome to remove for cleaning and because of the weight involved; quite hazardous
However in spite of all these problems many homes were converted and some measure of heat insulation was achieved.
Secondary glazing was carried on for 3 or 4 years but was eventually replaced by SEALED UNIT double glazing.
If we start off with the simplest definition of a sealed unit double glazing unit as being a structure to fit in a window frame that has two sheets of glass separated by spacer devices that has sufficient integrity to be physically transported and placed in position.
The biggest problem facing a manufacturer of double glazed units was finding a material that could seal the edge of a double glazing unit against moisture vapour transmission , that was sufficiently thixotropic to be able to be forced into a space and would stay there, that would solidify in a short time so the unit could be moved and that was relatively low cost and easily available and that would stick to glass!
This is quite a tall order, however a small double glazing company in Hull produced a relatively novel new compound which was a building sealant based on an epoxy polysulphide formulation but they did not realise the value of their discovery. They shelved the project and it was only by chance that it came to light some months later.
Now the way was clear and before long companies from Harwich to Hull, from London to Lowestoft and all over the United Kingdom were making double glazed units with epoxy polysulphide sealants in the periphery. This was the start of a major new industry in the British Isles.
Derek Pearson