Facade design: The benefits of early-stage energy modelling

Photo courtesy Tom Arban.

 By Jonathan Graham

In the early stages of design, it is useful to understand how facade design decisions will influence the energy performance of a building. Assumptions about window-to-wall ratio and wall thickness, no matter how preliminary, can limit a project’s ability to achieve certifications and/or comply with the performance path of the National Energy Code of Canada for Buildings (NECB). Indeed, as a project matures, it becomes increasingly difficult, as well as costly, to make effective changes to the performance. In response to this need, KPMB LAB, the research and innovation group at KPMB Architects, has developed a basic energy modelling tool for internal use on projects. This article discusses how the tool works, how it relates to more comprehensive energy modelling software, and how it can be useful to an architect during concept and schematic design phases.

Metrics such as thermal energy demand intensity (TEDI) and peak heating load are strongly correlated to the thermal performance of facades. Given these metrics are part of the performance criteria of green building standards and certifications (e.g. Toronto Green Standards [TGS], BC Energy Step Code, Canada Green Building Council [CAGBC] Zero Carbon, Passive House), it is crucial architects are able to calculate these metrics with respect to the contemplated facade. An inadequate facade can be difficult to remedy past schematic design phase, as it may require a thicker wall or lower glazing ratio.

Energy modelling softwares can calculate TEDI and hourly heating loads for every zone in a building. These models consider hundreds of factors affecting the energy balance of a building, including internal gains, solar gains, thermal storage, air leakage, and mechanical ventilation.1 The trade-off for this comprehensiveness is the complexity of the software itself. The breadth of inputs can be overwhelming to a non-expert, and some of those inputs are bound to be unresolved at the early stages of design (e.g. the type of HVAC system).

At the opposite end of the spectrum, practitioners can use simple hand calculations to estimate TEDI and heating loads. For example, an approximate building energy balance can be modelled using steady-state heat transfer calculations.2 Steady-state calculations represent heat flow through a system, assuming the system has reached equilibrium. This is in contrast to transient calculations, which represent heat flow as a time-varying phenomenon. Fourier’s Law of 1D conductive heat transfer is an example of steady-state heat transfer. The benefit of this approach is fewer technical inputs need to be known. The drawback is the results are less precise and less accurate.

KPMB LAB, the research group at KPMB Architects, has developed a tool that sits between these two opposites. The tool is a simplified energy model written in the programming language, Python. It is designed to quantify TEDI and heating load as functions of facade performance. With this narrow scope, the model excels at providing detailed insights while only requiring inputs that are obtainable during the concept and schematic design phases.

A purpose-built model

The model considers a single room, ventilated at a constant rate, with one external wall and one internal heat source. No computer-aided design (CAD) model is required, instead, the user specifies the room geometry through an Excel spreadsheet. The user provides thermal properties (U-values) for the external wall and its windows, a solar heat gain coefficient (SHGC), a solar transmittance (Tsol), window dimensions, and the orientation of the room relative to true North. The internal heat source represents a terminal device, such as a radiant panel or radiant floor, as identified by the user. The user also provides an EnergyPlus Weather File (EPW), which serves as the weather boundary condition. With these inputs, the model calculates the amount of heat energy needed from the terminal device to maintain an Operative Temperature setpoint (e.g. 21 C [69.8 F]) at the centre of the room. Effectively, it is measuring the heat required to compensate for a cold facade. The result is the heating load of the room, as influenced by the facade design. By repeating this calculation for each hour in the EPW file, the model can also calculate the TEDI.

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