Fenestration and daylighting options

Example simple box models used to evaluate the impact of window-wall ratio (WWR) and overhang projection factor (PF)

Fenestration and daylighting design might have the greatest energy impact of all the early design decisions, and BEM analysis of windows, skylights and shading during conceptual design can provide valuable information. A BEM analysis often compares performance of design concepts developed by the architect, but BEM can also be used to provide valuable information even before any designs exist; see simple box models.

Impact of Fenestration and Daylighting

Example results of a simple box model analysis showing the impact of window area and overhangs on electricity consumption for lighting, cooling and heating. These results show that in this case lighting drops as window area increases, but cooling and heating increase

Fenestration design has multiple impacts due to its influence on heating and cooling loads as well as indoor daylight illuminance. The following are primary impacts that can be evaluated using BEM.

• Energy consumption and energy demand for comfort conditioning.
• Sizing of HVAC equipment.
• Electricity consumed for lighting.

A number of other factors may be important to consider when making fenestration design decisions.

• Occupant thermal comfort
• Occupant well being associated with daylight and views
• The impact on the need for perimeter heating and cooling systems
• Natural ventilation and resilience, with operable windows
• Embodied carbon
• Construction cost

Fenestration and Daylighting Alternatives

Example of alternative skylight layouts for a large single-story building to be evaluated during conceptual design

At the conceptual design phase, a BEM analysis might focus on the impact of these basic fenestration design characteristics.

• Window area
• Window orientation
• Window performance, combined glazing and framing
• Exterior shades
• Skylights
• Daylighting controls

On some projects, it may also be useful to evaluate some of the following features during early design.

• Window head height (often limited by floor-to-floor height) to improve daylight penetration
• Daylight redirecting devices such as light shelves
• Movable shades
• Interior shades
• Switchable glazing

Consider combining a fenestration and daylighting analysis with an analysis to compare massing options.

For ideas on additional options, see the list of energy conservation measures.

Guidance on Modeling Approach

For an early-design study of fenestration and daylighting options, two common approaches are simple box models and shoebox models. An advantage to a simple box model approach is that it provides an integrated result for whole-building performance, while a shoebox model usually represents only a single typical space. On the other hand, a shoebox model can be quicker to develop and it provides a detailed view of performance of that single space.

Examples of shoebox models to evaluate impact of window area and overhang shading on a single room that represents a typical room in a larger building

Regardless of the approach, here are some things to consider when you create your models.

• Pay attention to whether you want to include daylighting controls for the electric lighting in the model, otherwise the results will not capture the energy benefit of daylight to offset electric lighting.
• Find important considerations for modeling fenestration along with baseline values for window-wall ratio and window performance here: define fenestration. 

• Find information about modeling shading here: define shading features.
• To save time and reduce model complexity, consider appropriate window simplification in the model and appropriate shading simplification.
• Include shading from adjacent buildings in your model in cases where it might have a significant impact.

Guidance on Presenting Results

BEM results that are typically found to be useful are described in this page: Analyzing Model Outputs. Since fenestration has such a big impact on heating and cooling loads, consider including peak load results in your presentation. Here are some other important considerations:

• Provide answers and insights
• Present results visually
• Provide an input summary

Example presentation of results that shows the impact of window wall ratio on EUI, both with and without automatic daylighting controls, and showing end-use energy breakdown

Example presentation of simple box model results that compares the peak cooling load and energy cost for different sizes of window overhangs