In the previous installment we considered the thermal envelope the collection of surfaces separating and protecting a building's conditioned interior spaces from the environment. In this installment we consider those special, hard-working components of the envelope windows.
Windows deserve a chapter of their own (in fact, two, including the next installment on their energy performance) because inexperienced homeowners too often make their selections on the basis of appearance alone. In doing so, they ignore many other important functions.
The Functions of Windows
Our word window derives from the Old Norse word vindauga, meaning eye of the wind, or an opening through which one can see and the wind can enter. Since operable windows didn't yet exist, one can surmise that the first windows contained no glazing, as in the castle window above. Windows have come a long way from that simple hole in the wall. Today we expect windows to admit:
Winter solar heat
Us (if egress is required by fire code)
At the same time, we expect our windows to keep out:
Types of Windows
Many species of windows have evolved from the seminal hole in the wall, each better at a particular suite of functions. Stock windows you will find at today's lumberyards and home centers include fixed, casement, double hung, bow, bay, awning, hopper, gliding, patio door, statement (non-rectangular), roof (skylight).
Fixed window If you are looking only for view and for daylight, buy a fixed, non-opening window. With no hardware and no weather stripping, the fixed window is the least expensive per square foot. Having no operable joints to seal, it is also the most airtight.
For design flexibility, window manufacturers offer most of their windows in modular sizes, thus allowing different types to be combined with a uniformity of appearance. Very often a fixed unit will be flanked or stacked with operable casement or double-hung units.
Before installing a bunch of low-cost fixed windows, however, consider the reasons for operable windows. The first is ventilation. The second is fire. Fire codes call for two exits from every living, dining and sleeping room (unless one is a door leading directly outside). One of these exits may be an egress window any manually operable window with a sill less than 4 feet above floor level, a clear opening of at least 5 square feet, and a minimum opening dimension of 22 inches. Manufacturers usually state whether a particular model qualifies as an egress window.
One appropriate application for a large area of fixed glazing is a sunspace. While a sunspace requires some ventilation to prevent overheating, 90 percent of its glazing can be fixed. This is the perfect application for patio-door replacement glazings. These are what the glass man brings when a neighborhood vandal throws a rock through your patio door. All glass stores stock them and, because they are manufactured by the millions, they are surprisingly inexpensive. These insulated glazing panels come in three standard sizes:
28 wide x 76 high
34 wide x 76 high
46 wide x 76 high
Great savings can be realized by purchasing the unframed panels and framing them onsite. Your glass dealer can provide the required glazing tape, setting blocks and instructions for a proper installation.
Casement window The casement window is hinged on one side and opens outward. Being side-hinged, it can reach out and scoop in a summer breeze like one of those old-fashioned automobile windows. A single casement unit should be hinged (right or left) to be most effective. In a double unit, such as a fixed unit flanked by casements, a pair of right- and left-hinged casements can be positioned to scoop in the breeze from either direction.
Being tall and narrow, casements are relatively easy to clean from inside the house. Similarly, the inside-mounted screens are easily installed and removed.
Double-hung window If you travel outside of North America, you may notice that the double-hung window, so ubiquitous in the United States, is not so universal elsewhere. In Europe the side-hinged casement window prevails. The reason is climate. The double-hung window originated in England, where wind-driven rain is common. There the casement's tendency to scoop wind proved a liability. In contrast, double-hung sashes could be raised or dropped to create a narrow horizontal slot for ventilation. Particularly under a wide roof overhang, a double-hung top sash dropped a few inches and admitted air but no rain. When the English settled in North America, they brought the only windows they knew the double-hung.
Unless the sashes pivot (a tilt sash), the double-hung window is difficult to clean from the inside. Screens are mounted outside the sashes but may be installed and removed from the inside.
Bow & bay windows These consist of multiple glazing units assembled into a single unit. The bow is a segmented, outward-projecting arch of five or more units. It usually consists of casements at the ends and three or more fixed units of the same size in the center.
The bay window typically contains a single large fixed window in the center and operable casement or double-hung windows at each side.
Every homeowner wants at least one bow or bay window. Except for the sunspace, these are about the most striking architectural features you can incorporate into an exterior wall. On the inside they create pleasant sunny spots for plants and sitting. The depth of the window sill is a function both of the flanking unit widths and the angles they form with the center unit. A wide sill can provide an ideal space for either sitting or growing plants.
Awning & hopper windows The awning window derives its name from being hinged at the top and opening outward. If hinged at the bottom, the same window would be called, appropriately enough, a hopper window. Obviously, hopper windows should swing inward in order to shed, rather than catch, rainwater.
The awning window has a single strong point: the ability to shed water when left open in a rainstorm. If you are away from home a lot, or often forget your windows are open, awning windows may be a good solution.
Because of its short height, the awning window is often installed over a kitchen sink, where counter height limits the sill height; in a bathroom, where you wish to limit the neighbor's view to above shoulder height; and in combination with and below fixed windows. A combination of tall fixed glazings over awnings near floor level is common in "window walls and sunspaces.
Gliding windows The gliding window is essentially a short version of the patio door. Another way to view it is as a double-hung window on its side. Like the double-hung, it is ineffective at scooping breezes and can be opened only halfway. Tall, narrow gliders tend to jam, and windows greater in width than in height are not aesthetically pleasing, so gliding windows are not very popular.
Patio doors Depending on its use, the patio door is either a pair of all-glass doors or a gliding window large enough to walk through. Either way, because it has become so standardized (widths of 5, 6 and 8 feet) and because it sells in such great numbers, it is a relative bargain in cost per square foot. Being of identical size and shape, a patio door works well architecturally with site-built, fixed windows using the patio-door replacement glazings described above.
The patio door can also be a very effective ventilator. With 20 to 25 square feet of floor-to-ceiling opening, it has the ventilation potential of three or more windows of any other type. Positioning a patio door therefore involves solar gain and heat loss, as well as ventilation considerations and traffic patterns.
Statement windows These are what I call non-rectangular windows whose primary purpose is aesthetic rather than functional. The category includes trapezoids, ovals and half-circles. A half-circle over a fixed window of the same width forms a Palladian window.
Most statement windows are fixed. In spite of that, because they are difficult to manufacture and because fewer are sold, they tend to cost twice as much or more per square foot as the standard varieties.
Roof windows If natural daylight is your objective, the fixed roof window, or skylight, is your answer. Light falling on the roof is more intense and more uniform throughout the day than that on any wall of the house.
If you want ventilation, as well, install ventilating skylights. An open skylight coupled with open windows and doors on the ground floor will result in a stack-effect breeze, even when there is no wind. Short of a whole-house fan in the ceiling, there is no more effective way to flush hot summer air from a house.
Although roof windows cost twice as much per square foot as their wall counterparts, they are more than twice as effective in both lighting and ventilating, so they justify the extra cost.
Placement for Light and View
In a previous installment we cited two architectural patterns relating to view: windows with small panes and low window sills. The illustration on page 61 shows the window geometry for these patterns.
First, a large picture window generally frames a view poorly. Like a novice photographer with her first camera, it indiscriminately includes excessive visual information. Breaking the view into smaller scenes with horizontal and vertical muntins creates more interest. Furthermore, these small scenes change continually as the viewer moves around the room. To the extent that there are primary points or lines of interest (such as the horizon), we should space the horizontal muntins so that they miss the line of sight of both a standing person (average eye height 60 inches) and a sitting person (average eye height 36 inches).
Second, events in the foreground are of interest to a building's occupants. How many times have you risen from your chair and gone to the window to investigate a sound outside? We can visually connect to the near surroundings even while seated if we place the window sill at 12 inches rather than the customary 30 to 36 inches.
The third dimension is the standard height of the tops of exterior doors and windows, 80 inches. Strict observance of this standard is key to the appearance of a house. If you doubt this, drive around the neighborhood and note the visual discord resulting from violation of the rule.
The illustration also serves for a discussion of the function of admitting natural light to the interior. Once again I refer to several architectural patterns cited in a previous installment that have to do with the value of natural daylighting. The first called for a place by a window. My favorite place to write is a comfortable chair nestled into a window corner framed by two tall windows. Not only is the lighting extraordinary, but I feel both indoors and out at the same time.
The second pattern calls for windows on two sides of every room. People and objects illuminated from a single direction appear flat and unnatural like cardboard cutouts. Daylight comes both directly from the sun and indirectly from the sky vault, so windows on at least two walls provide more natural, three-dimensional lighting.
The illustration specifies a final rule: The width of a room should not exceed 2.5 times the height of the windows providing the illumination. The standard window height of 80 inches thus limits the lighting width of a single-story room to 200 inches, or just over 16 feet. In the case of an open floor plan with windows on opposite walls, however, the total width can be up to 32 feet.
Depth and distribution of illumination are also improved by painting the ceiling and, to a lesser extent, the walls a light color. Finally, glare a painful sensation caused by the pupil of the eye responding only to the average light level in spite of extreme differences is minimized by painting the walls housing windows a light color.
Placement for Natural Ventilation
Despite of its invisibility, the air in a house is a real substance occupying a fixed volume. It cannot enter a space through one opening unless it can find an escape through a second opening. This is also the reason bathrooms and bedrooms with windows on a single wall stagnate. Remember as you locate ventilating windows and doors that the breeze will enter openings facing the prevailing breeze and then proceed as directly as possible to openings facing downwind. Rooms and corners of rooms not directly in these paths will not be well ventilated.
Remember also that the object of ventilation is not the structure, nor is it the rugs on the floor it is people, lying in their beds, sitting at the kitchen or dining-room table, sitting in front of the television. So in planning, place the ventilating windows and doors vertically as well as horizontally to direct air precisely across the people places. I once owned a small cabin with a windowless sleeping loft and spent many a sweaty summer night in that stifling, airless loft while, downstairs, cool air swept from front to back door. I finally installed an operable window at the precise level of my pillow and closed the downstairs door on the same wall. Ah, sweet relief! As the breeze now proceeded from loft window to back door, it had no choice but to run across my face.
But what if it's really hot inside and there is no breeze? Obviously we could use one or more window fans to force ventilation. But we could also take advantage of something we learned as children: Hot air rises. A hot house is like a hot-air balloon, and like the balloon, the house will lose its hot air out the top if we merely cut a hole in the roof. A single open skylight at the top of a stairway can vent several volumes of house air per hour, at the same time inducing a breeze through open downstairs windows and doors.
As a final ventilation exercise, mark on your floor plan those places where you anticipate sitting or lying on hot summer nights, then draw arrows from the windows and doors on the wall facing the prevailing summer breeze to the windows and doors on the downwind side. This exercise will show you which of your windows should be operable in order to ventilate the people places and, by elimination, which can be fixed.
In the next installment we will take a hard look at the energy aspects of windows: heat loss, solar gain, the performance of different special glazings and how the windows are constructed.