Determine design/operation features required by owner (OPR)

Contents of the owner's project requirements (OPR). (Source: IBPSA-USA BEMP Training Workshop)

The design intent refers to the vision or purpose behind the architectural and engineering decisions made during the design of a building. It defines how the building should perform, function, and appear, and includes considerations like aesthetics, energy efficiency, thermal comfort, and operational goals.

Design intent is commonly outlined in the Owner’s Project Requirements (OPR) or Basis of Design (BOD) documents and serves as the foundation for all subsequent design decisions. ASHRAE Standard 209 provides a framework for the type of details that could be included in an OPR.

Intent of an OPR

• Identify and describe the owner’s energy efficiency and sustainability goals and expectations for the project
• Clearly communicate the required outcomes and goals to the entire project team
• Ensure alignment around the goals

Content of an OPR

• Energy Use Index (EUI) Goal
• High level goals (building certifications, all-electric, carbon, zero net energy, etc.)
• Project constraints (cost, operations, existing conditions, etc.)
• Renewable energy goals (types of renewables, storage, backup power, etc.)
• Top-down building performance commitment from the owner
• Initial list ECMs (renewable energy, HVAC systems, envelop features, etc.)

Relevance to BEM practitioners

The energy model must align with the overall design intent to ensure it supports key project decisions. Misalignment between the design intent and the energy model can lead to:

• Inaccurate energy performance predictions.
• Inefficient design solutions.
• Missed opportunities for energy savings.

For example, if the design intent is to maximize natural ventilation, but the energy model assumes a fully mechanical ventilation system, the results will be misleading.

Step 1: Review Key Design Documentation

Start by reviewing project documentation that outlines the design intent. The Owner’s Project Requirements (OPR) and the Basis of Design (BOD) typically contain this information.

• Owner’s Project Requirements (OPR): Outlines the owner's vision and expectations for the project, including energy performance goals, sustainability objectives, and operational requirements.
• Basis of Design (BOD): Details how the design team plans to achieve the goals in the OPR, including the selection of HVAC systems, building envelope characteristics, and lighting strategies.

Step 2: Identify Key Design Features Influencing Energy Use

Key features of the design will have a direct impact on the building’s energy performance. Identify which elements of the design influence the model and the key energy-consuming systems.

• Building Orientation and Form: Understanding how the building’s orientation impacts solar exposure and shading strategies.
  • Example: A design that prioritizes passive solar heating will require modeling attention to solar exposure and window placement.
• Envelope Design: The materials and components used in the building envelope—such as walls, roofs, windows, and insulation—directly influence heat transfer, infiltration, and overall energy performance.
  • Example: The design intent might focus on a high-performance envelope to reduce heating and cooling loads, requiring the model to evaluate insulation R-values and glazing U-Factors.
• HVAC System Selection: The mechanical system strategy—whether it’s centralized or decentralized, or relies on traditional heating/cooling or heat pumps—plays a key role in how the energy model is structured.
  • Example: If the design prioritizes an energy-efficient heat pump system, the model will need to reflect part-load performance and equipment COP under varying conditions.
• Lighting and Controls: The inclusion of daylighting strategies and advanced controls can significantly reduce energy use. The energy model should reflect the effects of these strategies.
  • Example: A design focused on daylighting should evaluate how lighting controls adjust artificial lighting based on daylight availability.

Step 3: Align Design Features with Energy Modeling Assumptions

Once key design features are identified, align them with the assumptions used in the energy model. Ensure that the energy model reflects the design’s intent and the intended operation of the building.

• Example 1: If the building design aims for extensive daylighting, ensure the model includes daylight-responsive lighting controls that adjust based on available natural light.
• Example 2: If natural ventilation is a key part of the design intent, ensure the model includes parameters for operable windows and calculates ventilation rates appropriately.

Step 4: Collaborate with the Design Team

Energy modeling is an iterative process. Continuous collaboration with the design team ensures that any changes to the design are reflected in the model.

• Communication Points:
  • Discuss design choices that directly impact energy use (e.g., insulation levels, HVAC system efficiency).
  • Ask questions to clarify design intentions (e.g., "Are the windows designed to be operable year-round or seasonally?").
  • Highlight any assumptions in the energy model that deviate from the design intent.

Impact of Design Intent on Energy Model Results

Design choices based on the intent of the project can have a major impact on the energy modeling results. If the energy model assumptions are misaligned with the design intent, the results can be inaccurate.

• Example 1: A project designed for passive solar heating needs more accurate modeling of solar heat gains, including window placement, shading, and materials.
• Example 2: If the design intent includes achieving high thermal comfort standards (ASHRAE 55), the energy model needs to simulate varying occupancy and internal loads to ensure comfort is met.