Glass and Windows

Summer sun shines from the east in the morning and the west in the afternoon at low angles, which creates glare and a large air-conditioning load for rooms facing east and west. South windows admit solar heat during the middle of the day, coming from high in the sky.

Glass can be a friend or enemy of energy efficiency in any building. The choice of glass is a crucial one for comfort, natural lighting, and heating and cooling costs. Metal coatings have revolutionized the energy performance of glass in the past 20 years. Metal-coated glass is known as low-e glass because it has low emissivity meaning that it reduces heat flow. Low-e glass is packaged in insulated glass units (IGUs), which are double-pane glass assemblies. You can still buy IGUs with uncoated glass but this is seldom a wise choice. Low-e glass is usually a better value and there are two common kinds to choose from. The first type is a standard variety and is the best choice for cold climates, where heating costs dominate your seasonal energy use. The second type is for hot-climates and is designed to block solar heat.

You can also buy metal-coated films that apply to single-pane glass, to reduce heat loss in winter and solar heat gain in summer. The choice of metal-coated films is similar to the choice of glass. It depends on whether your heating or air-conditioning costs dominate your seasonal energy use. There are cold-climate and hot-climate varieties of the films. The films may reduce the life span of IGUs by increasing temperature and are therefore not recommended for installation on double-pane units.

You have to decide how you want to control the heat flow through the windows in your commercial building. This decision is based on your climate, the exposure of your windows, and the size of your building. Small commercial buildings are like homes because their heating and cooling costs are determined by outdoor temperature and solar radiation. Larger buildings’ cooling costs are governed more by internal heat gains and glass may actually help purge heat from the building.

If you’re planning a new building or new windows, consider carefully the U-factor, solar transmittance, and visible light transmittance of the glass units you specify because the glass is a key design element of any building. Also consider exterior shading devices, which are more effective at shading than the best interior window treatments.