In commercial buildings windows are a much larger component of the design element compared to residential architecture. From buildings that appear to be made completely of glass to structures with small, purely design style openings, windows can make or break the appearance of a commercial building.
Commercial Window Design
In residential applications, a windows design has more to do with the frame style and shape and how it opens. Commercial windows rarely open (fixed windows) which gives designers greater flexibility to design a window more for appearance and performance rather than function. Additionally, commercial windows often use different reflective qualities and colors to change the look of a building.
Depending on the look desired, commercial windows are often highly reflective - often mirror like, darkly tinted – almost completely opaque, and the reflectiveness and tinting are available in various colors. The look of many of these windows changes as the angle of the sun changes – time of day and season of the year. Most commercial windows would be considered picture windows.
Commercial Windows and Architecture
Luckily for office workers everywhere, architects have moved away from the small windowed institutional office building look full of interior, windowless offices in favor of airy and open designs that let in lots of daylight. In buildings such as these, windows play a much larger role in the look of the building from the exterior and the interior.
Architects are also paying more attention to how a building is oriented in relation to the sun when choosing windows and other design elements. Even government and other traditionally boring commercial buildings are now designed with architecture and function in mind. You will often find commercial buildings with different window treatments (shade structures, selective Low-E coating, etc.) on different sides of the building based on how and when the sun hits that side of the building. What often looks like a significant design element of a building is often as much for function (shade) as it is for appearance.
Aside from appearance, other factors that should be considered when choosing commercial windows include: structure/construction technique and building structural requirements, moisture control, acoustical characteristics, security, cost and energy efficiency (heat loss and heat gain). Fortunately, sophisticated commercial windows planning and design tools are available to help balance all the variables.
Heat Transfer Properties
Heat transfer is affected by two components of a window, the glass (or glazing), and the frame. The number of panes (double pane being the most common), any treatment on the glass (Low-E coatings, tinting, reflective coatings) and any gas filling (argon or krypton) determine how energy efficient the glazing is. The frame material and construction technique determine the heat transfer rate of the frame.
Heat is transferred through a window in three ways: conduction, convection, and radiation. Heat which travels through the glass and frame is “conducted” to the other side. Heat transferred by movement (through gases and air) is referred to as “convection”. A window’s ability to resist conduction and convection heat transfer is measured by its U-Factor. Heat which is given off by an object that does not rely on conduction or convection is called “radiation” or radiant heat. Much of the recent improvements in the energy efficiency of windows is related to better management of radiant heat loss.
Measuring Energy Efficiency
Fixed windows are generally more energy efficient that functional windows and perform better in harsh climates. The seals and cracks of functional windows provide a significant point for heat transfer, air leakage and moisture problems.
Standardized window ratings established by the National Fenestration Rating Council (NFRC) make it easy to compare the energy efficiency of windows. The four most common window ratings include:
- U-Factor – provides a measure of a window’s insulting value
- Solar Heat Gain Coefficient – measures a window’s ability to limit solar radiation heat gain
- Visible Transmittance – measure the amount of light that passes through a window’s glazing (more light passing through window means less electricity is needed to provide light inside the building)
- Air Leakage – measures the amount of air (in square feet or square meters per minute) that pass through a the cracks and seams of a window and its frame
Codes
The most common energy code in the United States is ASHRAE Standard 90.1-1999. The code provides guidance for U-Factor, Solar Heat Gain and Visible Transmittance ratings based on climate zones. It also limits the amount of window area a building can have as a percentage of its gross wall area.
Other Factors in Commercial Windows
More and more studies are indicating the benefits of lighting, and in particular, natural daylight in improved workplace effectiveness, doing better in school and improved retail sales. However, it isn’t as simple as just adding more windows and letting more light in. It is a balancing act.
Too much daylight is not good and the wrong kind of light and too much glare is not good either. Regardless of the amount of light desired, even distribution of that light is crucial. The amount and type of light also varies depending on the task being performed. For some tasks, the more light the better but for other tasks,such as working at a computer, too much light can make it hard to read the computer monitor which increases worker fatigue. A lot more planning and analysis goes into commercial windows than residential windows.