Windows are a key element in Eco design. Depending on where they are positioned, how large they are and how they are shaded, they play an important role in passive solar strategies, in natural ventilation and in natural day lighting. Another crucial factor is how they are constructed and of what materials they are composed.
Ordinary single-paned windows are responsible for up to 10 per cent of the heat lost from the interior. In older properties, where windows may fit poorly, warm air escaping through the gaps around the frame can account for much of that heat loss. Standard single-glazed windows also serve as cold bridges. Ideally, a window should have the same thermal performance as the adjacent wall. When it
doesn’t, heat takes the path of least resistance, travelling through materials with the highest conductivity, and a cold bridge occurs. Materials with high conductivity include glass and metal – which means metal-framed single-glazed windows are particular offenders in this respect.
You can improve the energy efficiency of your home to a significant degree by replacing existing windows with highly insulated varieties. Most people are familiar with the principle of double glazing, where an insulating layer of air is trapped between paired panes of glass. The most common double-glazed units are framed in PVC and such products have been heavily sold over the past few decades, often unscrupulously, making double-glazing salesman’ a byword for sharp trader. On aesthetic grounds, they can also be something of an eyesore, especially where they have been substituted for original sash windows in a period house.
Modern insulated windows are much more sophisticated and much better looking, too. Those with
the highest green credentials are framed in wood or in composite frames made of wood clad in al
Double glazing is better than single glazing, but falls short of the efficiency of triple-glazed units. In Germany, double glazing has recently been banned in new construction.
Very hight-performance insulated windows are triple-paned with the gaps between filled with argon or krypton gas. Some of these units incorporate integral blin ween the panes, which can be controlled
ide of the window to regulate heat gain and cut down on maintenance. They may also include frame or trickle vents to prevent condensation. Others are lockable in an open position for secure ventilation at night-time.
Many of the windows with the highest specification and best thermal performance are manufactured in Sweden. Some are available off the shelf in standard sizes and shapes, but bespoke designs are also possible. Larger panes are more efficient than windows composed of multiple small panes because perimeter length is minimized and there are fewer places where a cold bridge can occur.
In some cases it is possible to upgrade traditional windows. such as sash windows and other period designs, by retro-fitting them with double glazing. There are a number of companies which specialize in this work and which are able to advise in individual cases. However, a sash window retro-fitted with double glazing will not be as efficient as a new insulated unit. This is particularly true if the window is
subdivided into multiple panes. If your windows can’t be glazing, fitting brushes and other types
of draught-proofing can help to cut heat loss.
Choice of material for window frames has a direct bearin on thermal performance. At the same time, there are other factors to consider, such as durability and maintenance, as well as the environmental impact of sourcing and production. Common materials used for window fran include hardwood, softwood, aluminium, steel and PVC.
Because the heat lost through a window frame over the lifetime of its use is likely to have a greater impact than energy consumed to produce it, the general advice is to opt for a material that has least conductivity, which means wood. The second least conductive material is PV and the most conductive is metal.