Maximising Energy Efficiency for Windows in Green Buildings

In the pursuit of sustainable architecture, green buildings play a critical role in minimising environmental impact while maximising energy efficiency. Windows, often regarded as the “eyes” of a building, are crucial elements that influence both energy performance and occupant thermal comfort. The Leadership in Energy and Environmental Design (LEED) v4 rating system, which serves as a global benchmark for green buildings, places significant emphasis on the role of windows in residential construction. This article explores how various aspects of window design - shading, size, air infiltration, glazing quality and orientation — contribute to energy efficiency in line with LEED v4 requirements.

Proper shading is vital to control solar heat gain, which is the amount of solar radiation that enters a building through windows. In green buildings, external shading devices such as overhangs, louvers, and vegetation are often employed to reduce direct sunlight during peak hours. According to LEED v4, effective shading can reduce cooling loads, thereby lowering energy consumption. By blocking excessive solar heat in the summer and allowing passive solar heating in the winter, shading devices enhance the thermal comfort of interiors while optimising energy efficiency all year round.

The size of windows directly impacts both energy efficiency and natural lighting within a building. LEED v4 encourages the use of windows that balance day lighting needs with thermal performance. Large windows can provide ample daylight, reducing the need for artificial lighting and thereby reducing electricity consumption. However, oversized windows may lead to excessive heat loss during the winter or unwanted heat gain during the summer, similar to what we see in Amman where huge windows cause havoc in heating and cooling.

Therefore, a well-calculated window-to-wall ratio (WWR) ensures that windows provide sufficient daylight while minimising energy losses. LEED v4 recommends designs that optimise daylight penetration without compromising the building’s overall thermal integrity. Geographic location and height above sea level ought to be considered in the criteria.

Air infiltration through windows can significantly affect a building’s energy performance. Uncontrolled air leaks result in heat loss during winter and heat gain during summer, leading to higher energy demands for heating and cooling. To meet LEED v4 standards, windows must have low air infiltration rates, typically achieved through high-quality sealing, weather stripping and advanced window frame technologies. In Jordan, unfortunately no tests are available to measure air exchanges per hour.

Glazing quality plays a pivotal role in controlling the thermal and optical properties of windows. Some of the key parameters include reflectance, transmittance, and emissivity:

Reflectance refers to the ability of the glass to reflect solar radiation. High-reflectance glazing can reduce heat gain, making it ideal for warm climates. However, in colder regions, low-reflectance glazing may be preferred to allow more solar energy to enter the building. Ultra Violet light reflected is also an advantage.

Transmittance measures the amount of visible light that passes through the glass. High-transmittance glazing allows more daylight into the building, which can reduce the need for artificial lighting. LEED v4 encourages the use of glazing that maximises daylight while minimising unwanted solar heat gain.

Emissivity refers to the glass’s ability to emit absorbed heat. Low-emissivity (low-E) coatings on glazing are particularly effective in reducing heat loss, as they reflect interior heat back into the room while allowing visible light to pass through. This technology is crucial for maintaining energy efficiency, especially in colder climates. However, it is costly and need to applied in the proper location in the window frame.

The U-value of a window is a measure of its thermal conductivity, with lower U-values indicating better insulation. LEED v4 emphasises the selection of windows with low U-values to minimise heat transfer, thereby reducing energy consumption for heating and cooling. Windows with advanced glazing technologies, such as double or triple glazing, argon gas fills, and thermally broken frames, are preferred for their superior thermal performance.

The orientation of windows is a critical passive design consideration in green buildings. Proper orientation maximises energy efficiency by optimising the building’s exposure to sunlight. In the northern hemisphere, south-facing windows are ideal for capturing solar heat during the winter, while minimising direct sunlight during the summer through appropriate shading. LEED v4 encourages strategic window placement to enhance passive solar heating, day lighting, and natural ventilation, all of which contribute to reduced energy consumption and comfort.

In conclusion, windows are an integral part in energy efficiency of green buildings, influencing both thermal performance and occupant comfort. According to LEED v4 for residential buildings, careful consideration of shading, window size, air infiltration, glazing quality and orientation is essential to achieving optimal energy efficiency. By adhering to these principles, architects and builders can design windows that not only meet the stringent requirements of green building certifications but also contribute to a sustainable, cleaner and energy-efficient future.