Why townhouse schemes fail Homestar by accident
Most townhouse developments that miss their Homestar target don't miss it because someone aimed low. They miss it by accident, through a small change made late in a design that was sitting right on the line. Here is how that happens, and how to stop it.
Townhouses live close to the line
Standalone houses usually have room to spare in the energy model. Townhouses often do not. Smaller floor areas, exposed walls, tighter glazing options and less roof to work with mean these schemes tend to sit right on the 6 Homestar threshold with very little margin. When there is little margin, small things matter.
The accidental fail
Here is the pattern we see. A scheme is designed, modelled, and it passes at 6 Homestar. Then, somewhere between concept and consent, a change is made for a perfectly good reason. A window is enlarged for a better outlook. Glazing is added to a living room for more light. The glass specification is changed to bring the price down. None of these look like sustainability decisions, so the energy model is not rechecked. But each one moves the numbers, and on a scheme with no margin, one of them quietly pushes it under the line. Nobody notices until the rating is run again, by which point the design is locked and the fix is expensive.
Why glazing is the usual culprit
Glazing is the one that catches people out most often, because changing a window feels like a design or budget decision, not a thermal one. But glass is the weakest part of the thermal envelope by a wide margin. A wall can sit at an R-value several times higher than the window beside it, so every extra square metre of glazing, and every change to where it faces, moves the energy model far more than most people expect. Enlarge a window, add glazing for a view, or switch to a cheaper glass specification, and you can shift the result even when the plan is otherwise identical. On multi-unit projects this compounds, because a single glazing change can ripple across every unit of that typology.
How to avoid it
The fix is not complicated, it is a matter of timing and discipline. Model early, before the design is locked, so you know where the margin is. Then treat the energy model as a live document, not a one-off test. When a change is proposed to the glazing, its size, position or specification, check the model before committing, not after. The same goes for orientation and the building form. We build an early check into our medium-density work around resource consent for exactly this reason, and keep testing as the design develops. It is far cheaper to protect a rating as you go than to rescue one at the end.
The takeaway
Townhouse schemes rarely fail Homestar on ambition. They fail on a small, sensible change made without checking what it did to the model. Design the rating in early, keep an eye on it, and the accidental fail simply does not happen.
Planning a townhouse development? Read our practical guide to Homestar, or get in touch to talk through a feasibility check.
