Shading feature design analysis

Overhang and sidefins shading the lower view window and daylight redirecting louvers in the upper daylight window. Research Support Facility, NREL, Golden, CO. Source: NREL

The schematic design phase is a good time to explore options for exterior shading. BEM can provide information about the impact of shading and will help designers evaluate tradeoffs between building performance and construction cost.

Impact of exterior shading

By blocking, reducing or redirecting solar radiation, exterior shades have several potential impacts that can be evaluated directly with a BEM analysis.

• Cooling. Reduce cooling energy peak demand and consumption.
• Heating. Potentially increase heating energy consumption.
• HVAC sizing. Reduce peak cooling load and the airflow requirements in zones with windows.

Exterior shades can also provide these potential benefits:

• Visual comfort. Improve visual comfort by reducing glare from direct solar radiation.
• Thermal comfort. Improve thermal comfort for occupants near windows by blocking or reducing direct solar radiation.
• Daylighting. Improve distribution of daylight in perimeter zones with light redirecting surfaces.
• Views. Potentially reduce the need to deploy interior shades that block views to the outdoors

There are several factors that affect the impact of shading devices

• Size and configuration of the shading devices
• Reflectance of shading surfaces
• Glazing performance (solar heat gain coefficient and visible light transmittance)
• Fenestration orientation
• Building latitude
• Weather (sky conditions)

Window overhangs at the second floor and overhang/light shelf at the lower floor. Carnegie Global Ecology Center, Stanford University

Alternatives

Exterior shading with overhangs and sidefins. Iolani School, Honolulu, HI

Overhang shading devices at NREL Energy Systems Integration Facility, Golden, CO. Credit Dennis Schroeder/NREL

Many different configurations and materials are used for exterior shades. Most BEM software can represent the following types of shades:

• Window overhangs. Horizontally oriented surface above a window.
• Window sidefins. Vertically oriented surface to either side of a window.
• Other building shading surfaces. Additional surfaces attached to the building that shading fenestration and/or opaque surfaces.
• Site shading objects. Surfaces representing objects such as adjacent buildings or trees.

Depending on the BEM software, it may be possible to model other features of exterior shades.

• Partially transmissive shading materials
• Scheduled shade transmission, which, for example, can be used to represent seasonal changes in shade from deciduous trees.
• Movable shades, with controls based on signals such as the level of solar heat gain or glare or indoor space temperature.
• Light redirecting devices such as slat-type blinds

When defining alternatives for analysis, consider evaluating strategies separately by orientation, because impacts can vary significantly on, for example, south-facing and west-facing facades.

See also this page for other architectural energy conservation measures.

Guidance on modeling approach

As discussed on the page on window-to-wall ratio parametric studies, a first step is to develop a base model with an appropriate level of detail with respect to inputs for building geometry, thermal zoning, internal loads and HVAC systems. Then apply exterior shading to the base model to evaluate potential savings. See the page define shading features for guidance on approaches to modeling shading.

Additional considerations when evaluating exterior shading:

• Simplification. To minimize model complexity and simulation time, consider approaches to simplify or approximate the representation of complex shades such as louvers. This page shows one example: shading simplification.

• Daylight modeling. When using the BEM tool to simulate daylight illuminance and lighting energy savings, be sure to understand the tool’s calculation approach and check whether it will accurately reflect the impact of exterior shades. See daylighting for more information.
• Integration. Consider combining a shading analysis with a study of the impact of WWR and glazing performance.
• Supplemental analysis. Consider using other software tools to provide information to supplement the BEM analysis, such as 3D modeling tools for shadow studies and daylight simulation tools for glare and visual comfort studies.

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.

General BEM results that are typically found useful are described in this page: Analyzing Model Outputs. Other important considerations:

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

Example presentation showing the impact of overhang size on peak cooling load and energy cost savings.

Resources

Sun Control And Shading Devices, Whole Building Design Guide