Fenestration: Energy Impacts
Window Design Guidance for New Homes in a Mixed Climate
by The Efficient Windows Collaborative
The energy impact due to windows in a home depends on several design decisions, including climate, window orientation, window area, shading conditions, and window frame and glazing type. Homeowners and designers need to know the answers to the following questions. What is the best window type to reduce energy use in a particular location? Does window area and orientation affect energy use? Are shading devices effective in saving energy?
Unfortunately, the answers to these questions are not quite as simple as they seem. For example, there is a general perception that homes with larger window areas use more energy than homes with smaller window areas. This may be true for windows with conventional clear glazing, however, with high-performance windows, a home with a large window area can use the same amount of energy or even less energy than a space with a small window area. The best option is not always obvious, so it is important for homeowners and designers to be aware of the available advanced technologies and to use calculation tools to optimize design choices for energy-efficient performance.
In a mixed climate, it is necessary to reduce solar heat gain in the cooling months and it may be beneficial to take advantage of the passive solar heat gain in the heating months. In warm climates, facing windows to the south where overhangs can be designed to keep out most of the hot summer sun and allow for passive solar access when the sun is lower during the winter months is an effective strategy. West windows are subject to the full force of the strong afternoon sun, at a time of day when temperatures generally climb to their peak. East windows have the same problem in the morning hours, but air temperatures tend to be cooler. In spite of energy concerns, a house may have a spectacular view or other amenity to a direction other than south. Fortunately, the traditional patterns of avoiding east-facing and west-facing windows are not as critical when high-performance windows are used.
Another traditional guideline to reduce solar heat gain is to reduce the home’s total glazing area. This can be effective with any type of window, but it is particularly important when less efficient windows are used. Because of the need for daylighting, views, and natural ventilation, significantly reducing window area may not be a realistic or desirable strategy. As windows have improved considerably, high-performance windows can equal the performance of an insulated wall during a winter heating season. Consequently, the strategy of reducing window area to reduce energy use is no longer significant if highly efficient windows are used.
Any effort to shade traditional windows has had great benefits in terms of comfort and energy use. The best place to shade a window is on the outside, before the sun strikes the window. Exterior shading devices have long been considered the most effective way to reduce solar heat gain into a home. The most common approach is the fixed overhang. For south-facing windows, overhangs can be sized to block out much of the summer sun. Overhangs have the advantage of reducing heat gain and glare without diminishing the view. Other exterior devices include grills, awnings, shutters, roll-down shades/shutters, and canopies. The choice of shading strategy is often distinctly regional, based on local traditions. The drawback of some shading devices is that they block light and view.
Most homeowners use some form of interior window treatment such as drapes, blinds, or shades on their windows. In addition to their decorative aspects, drapes and curtains are used by homeowners to control privacy and daylight, provide protection from overheating, and reduce the fading of fabrics. To most effectively reduce solar heat gain, the drapery used to block the sunlight should have high reflectance and low transmittance. The impact of drapery on the solar heat gain is proportionally lessened as the window is shaded by other methods, such as exterior shading or reflective glass. The main disadvantage of drapes and other interior devices as solar control measures is that once the solar energy has entered the room through a window, a large proportion of the energy absorbed by the shading system will remain inside the house as heat gain.
Blinds and shades primarily provide light and privacy control but they also can have an impact on controlling solar heat gain. These include horizontal Venetian blinds, miniblinds, vertical slatted blinds, pleated and honeycomb shades, and roll-down shades, all of which can be made of various materials. Unlike other strategies to reduce heat gain, such as overhangs, interior shades generally require consistent, active operation by the occupant. Unfortunately, when shades are down, daylight and view are diminished or excluded completely. It is unlikely that anyone would operate all shades in a consistent, optimal pattern as they are often assumed to be operated in computer simulations. Motorized and automated shading systems are widely available to solve these operational problems. The control systems can be automated using sensors, time clocks, mobile, apps, or a home automation system. They can also be directly controlled by the occupants.
By using high-performance windows to provide the necessary solar control, there are two important benefits: there is less need for operating the shades, and the window is covered less of the time, resulting in increased daylight and unobstructed views. If your goal is to minimize cooling energy use, or you live in a house without air-conditioning in a hot climate, then the combination of good shade management with low-solar-heat-gain windows will be the best strategy.
A broad-leafed tree is good at providing cool shade in the summer. In addition to shading the building from direct sun, trees have been found to reduce the temperature of air immediately around them by as much as 10ºF below the temperature of the surrounding air due to evaporation of moisture. A window shaded with vegetation can have full shade in the summer, while enhancing the view and perhaps the ventilation. Trees and bushes can provide strategic shade from low east or west sun angles that are extremely difficult to shade architecturally.
Thermal comfort is that condition of mind that expresses satisfaction with the thermal environment. There are large variations, both physiologically and psychologically, from person to person, so it is difficult to satisfy everyone in a space. Windows generally do not insulate as well as opaque wall elements. In winter, when outdoor temperatures are cold, window roomside surfaces will be cooler than the adjacent wall. Cold glass can also create uncomfortable drafts as air next to the window is cooled and drops to the floor, creating an air movement pattern that feels drafty. High-performance windows with lower U-factors will result in a higher interior window temperature in winter and thus greater comfort.
Direct sun has obvious impacts on thermal comfort. During cold periods, limited solar radiation can be a pleasant sensation. But during warm or hot weather, it invariably causes discomfort. Just as people turn up the heat to compensate for cold windows in winter, they may use more air-conditioning to counter the effects of warm window surfaces and sunlight in summer.