Research methods for assessing the thermal and optical performance of building windows

Windows not only mediate thermal transfer, light transmission, noise control, and natural ventilation but also serve as an architectural element affecting a structure's aesthetics and appearance. Since the glazed window was first developed in Egypt in 100 AD, the thermal and optical performance of windows have been primary concerns for architects, engineers, and researchers. Today, windows are of special note because of their relationship to building sustainability and energy-efficient design. According to recent National Fenestration Rating Council reports, due to the relatively lower thermal insulation among all building envelope components, windows are normally responsible for 25-30% of the total heating, ventilation, and air conditioning energy use. This non-neglectable use of energy has motivated researchers to investigate various energy-efficient technologies and design solutions. Windows gain and lose heat through conduction, convection, and radiation. These properties can be measured and rated according to the following energy performance characteristics: insulating value, heat gain from solar radiation, and air infiltration rate. Conversely, the optical properties of windows determine their interaction with light and indoor occupants. The visible transmittance (corresponding to the fraction of impinging visible solar radiation transmitted) and haze value (meaning the clarity of the window) must both be taken into account. Most technology modulating the optical properties of windows also affects their thermal properties. Thus, the process of designing and engineering building windows is uniquely complex, involving both thermal and optical behaviors that affect building energy use, occupant comfort, and architectural aesthetics.