Calculating the energy implications of a façade and choosing between façade and system options can be different stages of the same decision. Energy modelling calculates building behaviour; optimisation searches for suitable options within defined objectives and constraints. The studies can work together. Understanding their relationship helps the client clarify the question to be answered and the outputs needed.
Energy modelling: assessing a defined design
Energy modelling calculates the annual energy use, heating and cooling loads and system behaviour of a given design from selected climate data and use assumptions. Envelope heat transfer, solar gains, ventilation and system efficiencies are considered together. The U.S. Department of Energy describes building energy modelling as a versatile, multipurpose tool with a broad range of uses, from new building and retrofit design to code compliance. U.S. Department of Energy — Building energy modelling.
A modelling study may assess a single design, a baseline and proposed design, or several alternatives. The questions the calculations need to answer are defined at the outset. When the design changes, relevant inputs and results are reviewed. For code or certification assessments, the comparison method is selected from the applicable requirements.
Optimisation: comparing many options
Optimisation defines variables such as glazing type, glazing ratio, insulation, shading or system selection alongside objectives and feasibility constraints. The calculated options form a comparison that supports decisions. Multi-objective assessment can also examine trade-offs between goals such as energy and comfort.
Parametric analysis investigates how defined variables affect results. Optimisation searches for solutions suited to selected objectives; the method does not need to calculate every possible combination. Their common basis is the use of modelling calculations to assess the options’ energy results.
For a clear comparison, fixed assumptions and changing properties should be distinguished. If an operating schedule or weather condition changes as part of a scenario, this is recorded explicitly. Differences can then be traced to the relevant design or use decision.
Which one, at which stage?
The answer depends on the project stage and the decision question; there is no single rule for every project. When many design options are still under consideration and the team needs direction, an optimisation study comparing them under consistent conditions provides a meaningful picture. When the options have settled and the objective is the assessment of a specific design, a single modelling study may be sufficient.
The order of the two studies in the same project is also not fixed. In some projects a broad comparison comes first, followed by a detailed modelling study in the selected direction. In others the design is already mature and only the assessment is needed. Where code compliance or a certification application is in question, the assessment method and documents follow the requirements of the relevant programme.
How do the two run together?
The direction selected through optimisation is detailed in later modelling reports. For the two studies to support each other, assumptions must stay consistent: results calculated with the same use profile, climate data and system assumptions can be read together. When assumptions change, the option that looked best in optimisation may give a different result in the later assessment, and the reason stops being clear.
The interpretation of results is shared as well. The output of both studies is the expected performance under defined assumptions; it is not an implemented and measured saving. Standard 209, developed jointly by ASHRAE and the International Building Performance Simulation Association (IBPSA), systematically places simulation in the design process, defining modelling cycles corresponding to stages such as conceptual design and HVAC system selection. ASHRAE — Standard 209, Building Performance Simulation Process.
The two services at AECO
At AECO, these two scopes run as separate services. The building energy modelling service covers the assessment of a defined design and preparation for code and certification applications. The optimisation service compares design decisions under consistent conditions. Our energy modelling and design article covers where the method sits in the design process; our building optimisation and parametric study article covers parametric comparison in detail. Sharing your project stage, the decision question ahead and your objective helps assess which scope is meaningful at that stage.