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Free Building Energy Software — EnergyPlus, THERM, Radiance and the Rest

Free Building Energy Software — EnergyPlus, THERM, Radiance and the Rest

Assessing a building's energy performance — heating and cooling loads, thermal comfort, daylight, thermal bridges — is exactly the field where the free tools are the engines underneath the commercial software. EnergyPlus, THERM and Radiance are developed with US government funding, and most paid products on the market are interfaces around them.

Here is the full chain, in the order you actually work in.

The energy simulation engine

1. EnergyPlus

The US Department of Energy's simulation engine: an energy balance at sub-hourly time steps covering envelope, HVAC systems, ventilation, lighting, occupancy, thermal mass and a full weather year. It is the de facto standard for annual consumption calculations and for code compliance.

What is missing: the input is a text IDF file. Without an interface on top, the work is close to impossible — which is exactly what the next item solves.

2. OpenStudio

The working environment above EnergyPlus: geometric modelling, system and construction libraries, scenario and result management, and an SDK that lets you write "measures" which modify the model automatically. This is what turns EnergyPlus from a research tool into a working one.

Envelope: glazing and thermal bridges

3. THERM

Two-dimensional heat transfer analysis of a section — a window profile, a wall-to-roof junction, a balcony edge. This is the tool that shows exactly where a thermal bridge forms and what it costs, with a colour temperature field you can put in a report.

4. WINDOW

Computes U-value, SHGC and visible transmittance for a complete glazing assembly — layers, coatings, gas and spacer. It works together with THERM: WINDOW for the glazing, THERM for the frame, and the result feeds into EnergyPlus.

Daylight and glare

5. Radiance

The most accurate lighting calculation engine there is, and certainly the only free one at that level. It computes illuminance, luminance and glare on a physical basis, including dynamic skies across the year. Almost every professional daylight analysis — including those run inside commercial software — runs Radiance underneath.

Ventilation, airflow and indoor air quality

6. CONTAM

A multizone airflow network model: infiltration, stack pressures, contaminant and smoke spread, and system pressures. The right tool for natural ventilation and smoke control questions, where full CFD would be overkill.

When full CFD is needed — flow around a building, or pedestrian-level wind comfort — the tools are in the CFD category.

Renewable energy

7. OpenFAST

From NREL: full aeroelastic simulation of a wind turbine — aerodynamics, structural dynamics, control, and for offshore machines hydrodynamics and mooring as well. The reference tool for load calculations in wind energy.

How this fits into a workflow

The practical order: build geometry and systems in OpenStudio; compute the glazing in WINDOW and the frames in THERM and feed those values in; run the year in EnergyPlus; check daylight in Radiance; and check ventilation and airflow in CONTAM. The whole chain is free and documented.

Two things worth saying in advance: the weather file determines the result — make sure you use the file for the nearest station rather than an American default. And the codes built into the tools are not your local code; the engine is right, the threshold requirements are local.

More in this area: architecture and building · environmental engineering.

Further reading