“Keep the weather, noise, and danger outside. Maintain thermal comfort and safety inside. Let the view and light pass through at will.”
These are the qualities that schools expect from a classroom window — providing the desired amount of daylight, sheltering students from heat or cold, and providing a shield of protection. But a window cannot yet manage itself. Those who choose school windows, and those who use and maintain them, need to make sure the windows are meeting needs effectively.
How large should a window be? Should it have double panes or special treatments to keep out heat, cold and noise? Can the window be opened and closed?
TOO HOT TO HANDLE
Transparency to light is a window's most obvious characteristic. Controlling the amount of light entering a room can be as simple as opening or closing shades. But a window also transmits, reflects and absorbs sound and heat.
Overheating is a common problem in classrooms. Gather 20 or more students, add several computers, and the temperatures in a classroom can climb quickly. The thermal environment can become more like that of a modern office. Drastic solutions, such as reducing class size, or intermittent room use are not feasible for most schools. In an older building, opening the window may be your only choice.
In general, classroom designers try to avoid the individual opening of windows for cooling. Randomly open windows make it too difficult for a building's HVAC systems to function optimally. As a compromise, some schools are considering windows controlled with keys. Indicators tied to the HVAC systems within each room can designate times when opening the window is most beneficial.
Thirty years ago, a typical classroom window had single glazing. Now it is common to find double-glazed windows. When the glass industry introduced double glazing in the 1970s, its strategy was twofold: add a sealed air space with a second pane of glass to impede heat exchange and increase window size to admit more daylight. Both panes of glass reflect, absorb and transmit varying quantities of the sun's heat and light. They can even be different types of glass. If daylight enters without heating up a room, a school can reduce its lighting load without increasing HVAC costs.
Another method for controlling heat transfer is low-emissivity (low-e) glass. The concept of low-e has been in use in stove doors since the 1930s. The glass coating causes heat to be reflected back to its source.
With low-e double-glazing, the heat is reflected to the exterior in summer and (in cold climates) back to the interior in winter. Low-e glass helps heat an occupied classroom in winter — unless the class is overheating. If a room in the winter is overheated and needs to be cooled, the low-e glass is causing the cooling load to increase.
Depending upon the shade or blind materials used, and the length of time involved, pulling shades over a low-e glass window in winter could improve room conditions. Shielding low-e glass from the room with a drawn shade will reduce the heat gained in the room by reflection until the shade itself becomes hotter than the room. The conditions in a room may be so time-specific that only trial and error will determine the best way to keep comfortable.
BLOCK THAT NOISE
When a question is raised in a classroom, does the teacher ask, “Who said that?” That's a sign of poor acoustics. Double-glazed windows can help. A side benefit of double-glazing is the added acoustical insulation. This could make a difference, especially in schools plagued by traffic noise.
Exposed large expanses of glass in a classroom can cause acoustical problems of another sort. Sounds within the space are reflected from the hard glass surface back to the room. If a wall of glass is exposed on one whole side of a classroom, speech clarity may be troublesome. Once again, pulling the shades can alleviate the problem. However, pulling the shades nullifies the daylighting value of the windows. Increasing the sound absorption of other surfaces in the room, such as walls, floors or ceilings, may allow a school to improve a room's acoustics without having to diminish the benefits of daylighting.
TOO CLOSE FOR COMFORT
One obvious hazard of windows: they can break. The hazard can originate inside or outside a classroom. In areas where a person can collide with a window, building codes require railings or special safety glass. A window near a classroom floor is an especially serious problem for younger students. Schools in regions prone to hurricanes or tornadoes also must protect their windows from airborne debris.
Even where safety is not a concern, windows can cause students discomfort. A window may be permanently closed, but if it is large enough or students are sitting close enough, they may feel uncomfortably cold. Schools should position students' desks and tables so they are not affected by the cold. But schools should balance this need with the beneficial effects of daylighting. Even with additional light-reflecting devices, as the desk of a student moves away from a window, the daylighting advantage diminishes.
Daylighting is desirable in most cases, but since the amount of natural light varies from day to day, all classrooms still need artificial lighting. High-efficiency light fixtures and energy-efficient luminaries, as well as control mechanisms, such as motion detectors and light sensors, can reduce lighting expense dramatically.
However, in climates with prolonged snow seasons, windows may not be able to deliver the benefits of daylighting. Reflection may result in too much light, and a school will have to use window blinds or tinted glass to keep the distracting glare away from students.
Rush, AIA, is an architect with The Office of Michael Rosenfeld, Inc., West Acton, Mass.
Balancing needs
Every function of a window is opposed by some other function. In addition, one set of functions implies that the window would benefit from being larger. The opposing functions equally imply that the window should be smaller. Sometimes, using a window-shading device that can alter the effective opening is all that is needed.
If there are paint fumes in the space, open the windows. If traffic noise is too great, close the window. School officials need to decide which is worse — odors or noise.
If a classroom has expensive computers, close and lock the windows. If the room is filled with smoke, unlock and open the windows. Which is more important — safety or security?
If heating costs are too great, cover the windows with thick wool drapes. If lighting costs are too great, remove the drapes, and paint the room white. Which cost is greater — heat or light?