Fenestration 101: WWR and WFR

Fenestration refers to the design, arrangement, and placement of openings in a building, such as windows, doors, skylights, and curtain walls. It affects both architectural aesthetics and building performance. Fenestration allows natural daylight to enter the building. It facilitates airflow for natural cooling and air quality, influencing heat gain and loss, which impacts the building’s thermal performance. Numbers and arrangement would enhance the structure’s appearance and character, providing a visual connection to the exterior environment. Thus, the fenestration percentage per total wall area shall be carefully analyzed to achieve the design intent, facade performance, and functionality.

The range of building fenestration, or proportion of openings, can vary significantly depending on the urban context. In commercial zones, ground-floor facades typically have a higher fenestration ratio to create an inviting and transparent environment. This can range from 30% to 90% of the wall area. The fenestration ratio is generally lower in residential buildings to ensure privacy and energy efficiency. The upper-story facade might have a 20% to 70% fenestration ratio.

In densely built urban areas, the window-to-wall (WWR) and window-to-floor ratio (WFR) are often adjusted based on these factors, such as building orientation, shading from adjacent structures, and the need for natural light and ventilation must be considered in fenestration design. The ratio of glazed to opaque surfaces (WWR) significantly affects buildings' thermal and lighting performance, influencing energy demands for heating and cooling. The increase in WWR generally results in higher cooling demands due to increased solar radiation, but its impact on heating varies depending on orientation and obstruction.?

In the northern hemisphere, larger windows on the south facade may help capture solar energy during winter, reducing heating demand. North-facing windows may need to be smaller to minimize heat loss. The windows from the east and west may require careful management due to potential overheating in the morning and afternoon.

Generally, in cool climates, where heating is the primary concern, increasing WWR may improve natural heating through solar gains. Windows facing the south (winter sun) allow more sunlight to penetrate the building, warming the interiors and reducing the need for an artificial heating system. Larger windows facing south maximize natural light, reducing the need for artificial lights. However, it is important to understand that larger windows must be well-insulated. If not, they can become a source of heat loss, negating the benefits of solar gains.?

On the other hand, buildings are designed to reduce heat gain in hot or tropical climates. The design priority is to keep the interior cool and reduce the load on the air conditioning system. A smaller WWR with shading devices reduces heat gain and cooling demands. Smaller windows reduce the amount of solar heat gain inside while limiting the amount of direct sunlight entering the building. Thus, in hot climates, Low-E glass or reflective coating is often used to prevent solar heat from entering while allowing natural daylight inside. This glazed coating reflects significant infrared radiation (heat) while admitting visible light, lowering cooling demand.??

Surrounding buildings or obstacles, such as trees or other urban structures, can block sunlight and affect the energy performance of windows. In dense urban areas, careful calculation of WWR and WFR is necessary to account for shadows and reduced daylight access.?

This reduced daylight access directly impacts the window’s energy performance. It diminishes the effectiveness of natural light for interior illumination, increasing reliance on artificial lighting, which leads to unsustainable practices. A higher WWR or WFR may not necessarily translate to better energy performance if shadows significantly reduce the available sunlight.

Window-to-wall ratio (WWR) is a measurement that represents the proportion of a building's exterior area that is covered by windows. It is expressed as a percentage to determine natural daylight and solar heat gain in a building.

Av = Sum of the area of all the windows on the facade

At = Height of the wall x Width of the wall

WWR = (Av/At) x 100

Example:

Av = 50 sqm.

At = 200 sqm.

WWR = (50/200) x 100 = 25%

Result Indicator:

• Higher WWR: More natural daylight, but potentially higher gain or loss could affect energy performance.
• Lower WWR: Less natural daylight reduces lighting efficiency. Improves thermal performance depending on climate conditions.
• The ideal WWR balances maximizing natural light and minimizing unwanted heat gains or losses.?

Properly designed fenestration maximizes natural daylight, enhances ventilation, and creates a strong connection between the interior and exterior spaces. Ideal WWR maximizes daylighting and minimizes unwanted heat gain or loss, tailored to the building's climate and use.