Civil Engineering Reference
In-Depth Information
is known about the durability of plastic roof-lights. Normal glass has almost
unlimited durability. Coloured heat-absorbing glass can break if part of it is per-
manently in the shade and the rest is exposed to sun.
Under special circumstances even sealed units can have problems: at low tem-
peratures; low pressure occurs inside them which bends the panes of glass
inwards in the middle, giving a lower insulation value. If the building is not heat-
ed during the winter, the tension within the glass can be so great that the glass
can break, especially if there is a wide space between the panes of glass.
The weak link in these units are otherwise the seals or sealants. Breaking down
of the seals occurs either through vapour getting in or through physical deterio-
ration of the packing. In a penetrating durability test carried out in Norwegian
building research in 1986, one third of metal-sealed windows were defunct after
20 to 32 years. For some of the plastic sealed types, nearly all were failing after
four to five years. Glass sealed panes were without exception useless after 10
years because of wind pressure, vibrations and thermal tensions (Gjelsvik, 1986).
Another important aspect of sealed units is that if only one of the panes of
glass splits, the whole window must be changed, whether it is double or triple
glazing.
In terms of resource use, there is little doubt that the Scandinavian model of
coupled timber windows gives best results, preferably with a window divided
into smaller panes on the outside, where the chance of breakage is highest.
Maintenance costs are small and durability and recycling possibilities are high,
although coupled windows are best used in domestic buildings, as larger build-
ings would incur very high window cleaning bills.
Pure clear glass can be recycled. This is not the case for metal-coated glass
or glass containing laminations of foil, reinforcement etc. Many of these prod-
ucts have to be dumped at special tips, including coloured and metal-coated
glass.
Timber windows
Timber frames used to be made of high quality timber with no knots - often pine
heartwood. When constructing the window, the highest quality was selected for
the most exposed parts, such as the sill. The components were slotted together
and fixed inside with wooden plugs. Windows are still mostly made of pine, but
without the same demands on quality or the same preparation. The proportion
of heartwood used is often very low.
The present methods of sawing timber do not guarantee that the heartwood is
used in the most appropriate parts of the window. To compensate for this, it is
quite common to use pressure-impregnated timber. Adhesive or screws are used
as the binder between the components. The window furniture and the hinges are
usually made of galvanized steel or brass. Between the frame and the casement
