Site conditions

Shadow cast on project (shown in blue) by adjacent building modeled as a shading object (shown in pink).

Evaluating site conditions is important at the very earliest stages of a project. The site may impose some restrictions on the design. From a BEM practitioner's standpoint, climate analysis and developing an understanding of how a building's surroundings will impact its performance are among the most important components to consider. This article focuses on analyzing the building's surroundings and their impacts on how wind and solar resources affect the building.

Terrain and elevation

EnergyPlus input options for site terrain

Most energy modeling software tools ask for inputs related to the project's location. Weather data is discussed on the climate analysis page, but inputs related to the site's terrain and elevation are input separately.

Terrain refers to the the building's location with respect to other structures or natural features that may shelter the project from wind. In essence, the selection input by the user will impact how the wind affects the building facade by taking the wind speed from the weather file and altering it for the simulation to account for wind shielding by surrounding structures (buildings, trees, etc.).

Elevation (above sea level) is another input that affects the wind impacts on a building. It also impacts the outdoor air barometric pressure and air density which will, in turn have impacts on the HVAC system performance such as the effectiveness of heating and cooling coils.

Surrounding buildings and vegetation

A building's surroundings can affect both airflow (from wind) and available solar resources at the site. They can also impose constraints on the design itself.

Wind Impacts

As discussed above, the wind impacts related to the site are accounted for by defining a terrain input. This is a fairly general way to account for changes in airflow due to nearby buildings and trees, but BEM tools are not capable of accounting for this more explicitly. Computation fluid dynamics (CFD) tools can model this explicitly, but they are not annual simulations and are generally used to look at snapshots of performance at different times of day/year.

Solar Impacts

BEM tools can account for solar impacts due to shading from nearby buildings and trees. This is done by modeling them as shading objects. Typically, the shading objects are modeled using a low amount of detail - for example, a nearby building may be modeled as a box or series of planes rather than accounting for all of the detailed geometrical features. Similarly, a tree (or group of trees) may be modeled as a triangular shading object (for a single tree) or a plane (for a group of trees or forest surrounding the site). BEM simulation engines will calculate the percentage of each surface being shaded by adjacent objects, and reduce the direct solar radiation received by the surfaces by that percentage.

Some BEM tools allow for shading objects to be "scheduled." This feature is designed to model operable shades, but can also be used to account for deciduous trees that lose their leaves in the winter. The schedule could be defined so that the trees are "opaque" during the spring and summer (when there are leaves on the trees and therefore they cast shadows), and "transparent" during the fall and winter when the leaves fall and shadows are no longer cast. More detailed schedules could be defined that vary the amount of transparency in the different seasons and months to account for gradual growth and fall of leaves.

Design Constraints

Example Site Constraints, Including Maximum Dimensions and Adjacent Structures

Adjacent buildings may impose height limits, or footprint constraints. Local ordinances may also limit the building height. The owner may also have other requirements such as access to views that may influence the design and orientation of the building and placement of windows.

Additional Resources

• Google Project Sunroof - In the early stages of a project, if a model has not been developed yet, you could use an online tool like this to assess solar availability at the site that accounts for nearby buildings and trees.

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