The window has a strong association for humans. It serves as a practical way to look through walls and a metaphorical way to look into the human soul. It speaks to obstruction or transparency when we say, “you make a better door than a window.’’
In architectural terms, windows serve to ensure that we can have shelter but still have the borrowed space of the out-of-doors. While keeping out the cold and the wind, they also allow the warmth and spirit of the sun to penetrate our dwellings.
The early windows in buildings were slits or eye holes, mostly for seeing out, as opposed to letting in light. Sometimes they were covered by animal horns, skins or cloth and often protected by shutters.
Later, glass was created and small pieces were connected with leading to cover larger openings. This allowed the creation of stained glass windows, artful and sometimes religious. Windows took on new roles.
Glass manufacturing became more advanced and a single pane of glass could cover a much larger opening. Dividing bars became decorative.
With rising expectations about comfort came the realization that a single pane, while effective at keeping out wind, dust and insects, is still cold, so exterior units, called storm windows, were added.
Placing two layers of glass close together and sealing the edges was determined to be even more effective at heat retention.
Sashes, the part that holds the glass, and frames, in which the sashes slide or hinge, were traditionally wood. The big drawback of wood has been maintenance, but this has been resolved by claddings of aluminum and vinyl.
Although its market share is half what it was two decades ago, wood is still used primarily in new installations. Windows are available that are made from sustainable forestry wood and are certified by the Forest Stewardship Council. If clad, wooden windows are still considered a wise choice by PassivHaus, an international body that focuses on highly energy-efficient housing.
Aluminum framed windows are still used commercially because of their durability, but aluminum is a great conductor, so heat loss is considerable. As a result, they have dramatically lost their share of the market.
Vinyl windows, which have a similar energy performance to wood, now represent about two-thirds of installations because they are economical and easy to maintain.
They do have one major flaw, however: a high level of thermal expansion. In other words, they shrink and swell more than the glass they contain, which can cause seal breakage and cracks at corners and connections.
The other downside is that PVC is one of the more obnoxious plastics in terms of environmental impact.
Fibreglass as a window framing material is about 20 years old, but still represents a small percentage of installations. It is often found in high performance windows. Seal breakage is reduced because its co-efficient of thermal expansion is low, close to that of glass.
Fibreglass is expected to outlast PVC or wood, but because the manufacturing process is slow, the windows are likely to remain pricier than vinyl.
Framing is only part of window performance. The other part is glazing. It is interesting that heat loss is greater if panes of glass are either too distant or too close.
The appropriate distance depends on the filler between the panes, from 1/4 inch for krypton to 1/2 inch for air or argon gases.
Most windows have air in the space, but in higher performance units it can contain the gases krypton, xenon, or argon, the latter being the most common.
Glass coatings have been around for more than 30 years. Low emissivity coatings allow the passage of short wave lengths or visible light, but block the passage of long waves, which are responsible for heat radiation.
Sunlight enters the room and strikes objects, but the resulting heat is blocked from exiting. Low-e coating can almost double the performance, expressed in U-factor. The lower the numbers the better.
Spacers between panes are also a cause for design attention because they can act as thermal bridges, such as aluminum, or thermal barriers, such as rubber and silicone.
The greatest heat loss from buildings is from the movement of air: warm air out, cold air in. As a result, it is essential that window assemblies be airtight. Even the best double or single hung windows or any slider window is not as tight as a hinged window.
Divided lites are multiple sections of glass in one integrated window unit and are often used in traditional windows. They are less efficient because the ratio of frame to glass is increased and frames are bigger heat losers.
Many new windows have faux divisions, which are bars that appear to divide the glass but are only on the surface. These avoid the heat loss issue, though not the issue of cleaning those small areas.
Windows let in light but not all light is the same.
Direct lighting can have a significant impact on heating a building’s interior, which is called solar gain. However, this is not the case with for indirect lighting.
Every pane of glass results in re-duced light transmission, so there is a good argument for using dual pane windows for direct-lit southern windows and having triple pane for all other directions. Windows on the south side of the house should have different solar heat gain co-efficient ratings than windows for the north, east or west.
And when comparing window products, it is important to remember that there is a glass rating and a whole window rating.
Installing windows with proper papering and sealing is essential, both to ensure that any water coming down the wall does not penetrate the wall cavity and cause degradation and to ensure that there are no air leaks.
The proportion of window area to wall area is important to building energy performance. Huge banks of windows create extremes for a building’s interior. Hot days and cold nights in winter and truly excessive heat gain in summer are good examples in Western Canada.
Good building design allows direct light entry in winter when heat gain is desirable and shading over the windows in the summer, resulting in energy savings in both seasons.
Windows have progressed from having thin pieces of animal horns to triple panes of glass, surface coatings, gas fillings and alternative frame materials.
And after all these centuries, not surprisingly, we are still seeking to be comfortable and to see outside, keeping warm, checking on the neighbours and watching the vivid sky as the sun goes down.