Five key considerations in designing the vertical fenestration of a building
Designing the vertical fenestration of a building is a critical aspect of energy-efficient building design. Vertical fenestration, which includes windows, doors, and other glazed areas on the walls of a building, can significantly impact a building’s energy consumption and occupant comfort. In this article, we will discuss five key considerations in designing the vertical fenestration of your building, taking into account compliance requirements for all climatic zones and incremental energy efficiency levels.
- Window Wall Ratio (WWR) The first consideration in designing the vertical fenestration of your building is the Window Wall Ratio (WWR). WWR is the percentage of the wall area occupied by windows, and it is critical to strike a balance between providing sufficient daylighting and views and minimizing heat gain and loss. For all climatic zones and incremental energy efficiency levels, the maximum allowable WWR is 40%. This requirement applies to buildings showing compliance using the Prescriptive Method, including Building Envelope Trade-off Method.
- Visual Light Transmittance (VLT) The Visual Light Transmittance (VLT) of glazing is another critical consideration in designing the vertical fenestration of your building. VLT is the percentage of visible light that passes through the glazing, and it is essential to strike a balance between providing sufficient daylighting and views and minimizing heat gain and loss. For all climatic zones and incremental energy efficiency levels, the minimum allowable VLT is 0.27.
- U-factor and SHGC Requirements The U-factor and Solar Heat Gain Coefficient (SHGC) are critical factors in determining the energy efficiency of vertical fenestration. The U-factor measures the rate of heat transfer through the fenestration assembly, while the SHGC measures the amount of solar radiation that passes through the glazing. For ECBC buildings, the maximum U-factor for vertical fenestration is 3.00 W/m².K, and the maximum SHGC is 0.27 for non-North facing fenestration, 0.50 for North-facing fenestration with a latitude of ≥ 15°N, and 0.27 for North-facing fenestration with a latitude of < 15°N. The requirements for ECBC+ and SuperECBC buildings are specified in Table 4-11 of the ECBC 2017 code.
- Assembly U-factor and SHGC In addition to the U-factor and SHGC requirements, it is also essential to consider the assembly U-factor and SHGC. The assembly U-factor includes both the frame and glass area-weighted U-factors, while the assembly SHGC includes both the frame and glass area-weighted SHGC. By considering both of these factors, designers can ensure that the fenestration assembly provides the desired level of energy efficiency.
- Fenestration Orientation Finally, it is important to consider the orientation of the fenestration when designing the vertical fenestration of your building. Vertical fenestration on non-cardinal directions should be categorized under a particular cardinal direction if its orientation is within ± 22.5° of that cardinal direction. By categorizing the fenestration in this way, it is possible to ensure that the fenestration is designed to optimize its energy performance in relation to the sun’s position throughout the day.
Here are the requirements for vertical fenestration for a building in tabular form for five key factors:
| Factor | Requirements |
|---|---|
| Maximum Window Wall Ratio (WWR) | 40% |
| Minimum Visual Light Transmittance (VLT) | 0.27 |
| Assembly U-factor | Includes both frame and glass area weighted U-factors |
| Assembly SHGC | Includes both frame and glass area weighted SHGC |
| Maximum Solar Heat Gain Coefficient (SHGC) and U-factor requirements | See Table 4-10 for ECBC buildings, Table 4-11 for ECBC+ and SuperECBC buildings |
| SHGC for fenestration with permanent external projection | Equivalent SHGC may be calculated using Projection factor (PF) and Shading Equivalent Factor (SEF) as per the applicable shading type |
The specific values for the maximum U-factor and maximum SHGC vary according to the composite and climatic zone of the building, as outlined in Tables 4-10 and 4-11. Exceptions to the SHGC requirements are also outlined for fenestration with a permanent external projection.
Designing the vertical fenestration of a building requires careful consideration of several factors, including the Window Wall Ratio, Visual Light Transmittance, U-factor and SHGC requirements, assembly U-factor and SHGC, and fenestration orientation. By considering these factors, designers can ensure that the vertical fenestration of the building is energy-efficient, provides sufficient daylighting and views, and contributes to occupant comfort.