Whole-building energy simulation with EnergyPlus
EnergyPlus is the US Department of Energy's whole-building energy simulation engine. It models heating, cooling, lighting, ventilation and equipment loads hour by hour across a full year, using the real building geometry, construction build-ups and Auckland weather data. Every energy figure on this page comes out of an EnergyPlus run of the actual project model.
Before any comparison means anything, you need to know what a normal New Zealand home actually does. That is not a flattering picture, and it is measured rather than assumed — BRANZ's Household Energy End-use Project 2 (HEEP2) monitored temperatures in hundreds of real homes.
of winter morning readings in occupied bedrooms sat below the WHO healthy minimum of 18°C. The median bedroom overnight was just 16.4°C.
HEEP2 SR495, winter 2023 sampleof monitored Auckland bedrooms were classed as overheating in summer — the worst rate of any region, against a national average of 36%.
HEEP2 SR502, CIBSE 1b, summer 2023/24So the typical home is too cold in winter and too hot in summer. Fixing that costs energy — which is why the number that matters is kWh per square metre per year, and why lower is better. It is the yardstick used in every chart below.
A building built to do no more than the Building Code requires. This is the baseline every other option is measured against.
The usual way to bring the number down: thicker insulation and better-performing windows. More insulation, same lightweight structure.
Cross-laminated timber walls with an insulated cavity. Timber insulates where concrete does not, so the assumption is that CLT must have the better wall — worth testing rather than accepting.
200 mm in-situ concrete with 60 mm PIR. On these build-ups it carries an R-value of 3.4 m²K/W against CLT's 2.9 — and far more thermal mass.
Does the Simplicity in-situ concrete system genuinely use less energy than a timber alternative — and if it does, is that down to the insulation, or to the thermal mass of the concrete itself?
Here is the answer. The same building, on the same site, modelled four ways — and split by end use so you can see where the difference comes from. Lake Road, Northcote stands in as the Simplicity building. Every bar is annual energy in kWh per m², with the total printed above it.
Key Finding: The Simplicity build system demonstrates the most cost-effective and energy-efficient balance when considering both initial construction and operational performance. Our in-situ concrete approach provides superior thermal mass benefits while minimising construction complexity.
The same comparison repeated across the four projects, from EnergyPlus simulations of the actual building models. Each successive design carries forward what the last one proved.
The geometry that goes into EnergyPlus, coloured by space — every apartment, corridor and service room the simulation solves for. Drag to orbit, scroll to zoom.
A single Northcote N12 apartment (~65 m²) as the unit of analysis — showing where electricity is consumed, what that means in operational carbon, and how the choice of building fabric (in-situ concrete vs CLT) changes the heating and cooling load over the building’s life.
Individual appliances roll up into the four end-use categories EnergyPlus reports against — consistent across every project, and comparable to NZ benchmarks. Figures are per apartment per year.
To complement the whole-building energy model, we are developing Therm thermal-bridge analysis for each key junction detail — wall corners, slab edges, window reveals. These isothermal simulations quantify how heat migrates through the fabric beyond what simplified R-value calculations capture. The corrected U-values and linear thermal-bridge psi (ψ) values feed directly into the EnergyPlus construction definitions.
The two diagrams below illustrate the concept: temperature isotherms flowing from the warm interior through the wall construction to the cold exterior, and why in-situ concrete mass with a well-placed insulation layer changes the gradient — and therefore the annual heating/cooling load — so significantly compared to a CLT or timber-frame alternative.
At a wall corner two exterior surfaces converge, creating a zone of higher heat flux than mid-wall. Therm quantifies this as a linear thermal bridge (ψ value) applied as a correction to the EnergyPlus whole-building model.
In-situ concrete is embodied-carbon intensive at construction — but it pays back through reduced operational carbon across the 50+ year building life. Higher thermal mass means the building absorbs heat during the day and releases it overnight, cutting the hours the heat pump runs and lowering annual electricity demand and associated grid emissions.
The complete Nezo EnergyPlus analysis for each project — annual consumption breakdown, monthly profiles and HVAC performance. Choose a project, then a building within it.