Window-to-wall ratio parametric studies
Window-to-wall ratio, often abbreviated as WWR, is a common metric used to describe the window area in a building and is typically defined as the fraction of above-grade exterior wall area covered by windows and glazed doors. A parametric analysis using BEM can provide designers with very useful information about the complex impact of windows on building performance.
See also related information in the discussion of conceptual design fenestration and daylighting options, guidance on defining fenestration, and guidance on defining shading features. And a WWR study might also be combined with a comparison of massing options.
Impact of Window Wall Ratio
As also discussed in Impact of Fenestration and Daylighting, window area has several important impacts that can be evaluated directly using BEM.
- Energy consumption and energy demand for comfort conditioning.
- Sizing of HVAC equipment.
- Electricity consumed for lighting.
There are other important design considerations related to the choice of window area.
- Energy code compliance. Most energy codes set an upper limit on WWR in their prescriptive compliance path for commercial buildings and high rise residential buildings. For ASHRAE 90.1 and the California Energy Standard the limit is 40% and for the International Energy Conservation Code the limit is either 30% or 40% depending on the presence of automatic daylighting controls. Designs that exceed the WWR limit must use an energy modeling approach for compliance.
- Daylighting performance. Window area is an important factor in the performance daylighting design and will affect achievement of daylighting design metrics.
- Visual comfort. Window area, together with glazing performance specifications and shading features, are primary factors in occupant visual comfort, affecting daylight illuminance and glare.
- Thermal comfort. Increased window area increases the possibility of negative thermal comfort impact on occupants near windows, including the radiant effect of cool or warm window surfaces, direct solar radiation on occupants, and downdrafts created by cold window surfaces.
- HVAC zoning and system type options. Window area may affect the opportunity for an HVAC system approach that avoids the need for a perimeter heating system to maintain comfort.
- Adjacent building impact. Reflections from windows can affect solar heat gain and glare on adjacent buildings.
Alternatives
A WWR parametric analysis will at a minimum include simulations representing a range of potential window areas, from low (or zero) to high. If there is no information available from the design team regarding a starting point WWR, then see defining fenestration for some tables of values by building type.
Window performance and shading
The inputs for window performance and window shading in the model are important considerations, because those assumptions will affect results and may affect conclusions drawn from the analysis. Consider running the WWR alternatives with two or more different glazing types to test the sensitivity. Consider also running the WWR alternatives with and without exterior shading.
Orientation
Because the impact of window area will vary by orientation, useful information can be gained by running alternatives that vary the window area on one orientation at a time. For example, create models with low and high window area on the north side of the building while keeping area constant on the other facades. Then repeat the exercise for the other facades.
See also this page for other architectural energy conservation measures.
Guidance on modeling approach
The basic approach to a WWR parametric study is straightforward: develop a model and create alternatives with varying window area. At the schematic design phase, it is likely that one or more options for the building form are defined and can provide the basis for the model geometry.
In most cases, the goal is to develop a model with an appropriate level of detail that provides reasonably accurate representation of the relative performance with different window areas. Some simplifications are generally appropriate for this type of analysis.
Window inputs
Make sure to include the impact of window framing. See defining fenestration.
Thermal zoning
Some simplifications to thermal zoning are generally appropriate because the actual HVAC zones may not yet be defined. See Defining the building geometry for guidance.
Internal load inputs
The choice of whether or not to include automatic daylighting controls in the model can have a significant impact on results. See https://bemcyclopedia.com/wiki/Daylighting.
While exact internal load inputs are generally not required for this analysis, some attention is warranted due to their impact on cooling and heating loads. As an example, excessively high internal heat gains in the model will offset some of the potential heating load caused by larger window areas. See Define internal loads (occupants, lighting, equipment) for guidance.
HVAC system inputs
At the schematic design phase, the HVAC system may not yet be selected, and there are likely many details not yet defined. See Selecting appropriate HVAC systems for guidance on choosing a system type and Preparing Model Inputs for guidance on appropriate HVAC inputs.
Since window area affects HVAC system sizing and associated HVAC system cost, pay some attention to HVAC sizing inputs in the model to ensure the impact is reasonably represented.
Thermal comfort impact
Windows affect the mean radiant temperature experienced by occupant in perimeter zones, and the effect is magnified in extreme climates and with large window area. Some BEM tools can report mean radiant temperature and operative temperature results, which can be used to compare thermal comfort impact of varying window area.
Impact of adjacent buildings
Include shading from adjacent buildings in your model in cases where it might have a significant impact such as in dense urban neighborhoods.
Guidance on presenting results
In addition to energy results, consider presenting results for peak electricity demand and HVAC system sizing because both can affect project construction cost. Consider also showing impact on daylighting performance, visual comfort and thermal comfort.
General BEM results that are typically found useful are described in this page: Analyzing Model Outputs. Other important considerations:
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