For years, thermal performance in New Zealand has been framed around one goal: keeping homes warmer in winter.
That focus was necessary. But it also created a blind spot.
A home can be highly insulated and still be uncomfortable in summer. In fact, BRANZ’s recent monitoring found indoor summer temperatures have risen by 6–10% over the past 20 years, with average evening temperatures in living rooms and bedrooms exceeding 24°C. More than one-third of the monitored bedrooms were classed as overheating.
Overheating is not just “too much insulation”
This is one of the biggest misconceptions.
MBIE’s recent H1 consultation material, drawing on BRANZ analysis, is explicit on this point: overheating is not simply caused by insulation. It is a combination of design factors such as solar gains during the day, window shading, heat absorption of building materials, ventilation and orientation. BRANZ has also said that the current H1 settings are not, on their own, creating overheating.
That matters, because it changes the conversation.
The issue is not, “Should we back off insulation?”
The real question is, “Have we designed the building to control summer heat as deliberately as we design it to retain winter heat?”
Where overheating usually starts
BRANZ’s passive solar guidance is useful here because it points out that overheating is often a room-specific problem, not necessarily a whole-house one. The rooms most at risk are typically the ones that receive strong direct sun, have limited external shading, are enclosed during the day, only open on one side, and cannot take advantage of cooling breezes.
That description fits a lot of modern layouts surprisingly well:
- large west-facing glazing
- bedrooms with only one opening side
- upper-floor rooms
- apartments with limited cross-ventilation
- open-plan areas with lots of glass but little external shading
This is one reason apartments can be particularly difficult. A room or apartment with only single-sided ventilation generally has fewer passive cooling options than one that can purge heat across the building. BRANZ’s passive design material and related NZ overheating guidance both point to ventilation rate as a major variable, and older BRANZ overheating research found outdoor temperature had the greatest impact on indoor temperature, followed by ventilation rate.
What good summer design actually looks like
The best-performing homes do not rely on one fix. They use a set of design decisions that work together.
A good NZ summer-comfort strategy usually includes:
- Sensible glazing placement, not just lots of glazing
West glazing is often the troublemaker because it captures lower-angle afternoon sun when outdoor temperatures are already high. BRANZ’s guidance for better-performing homes specifically recommends limiting west-facing windows and being deliberate about orientation. - External shading, not just internal blinds
External shading stops solar radiation before it gets through the glass. That is fundamentally different from trying to manage heat after it is already inside. BRANZ and EECA both point to shading as a core design tool, and BRANZ’s climate and passive-design material notes that eaves or shading devices are part of what lower-carbon, better-performing homes will increasingly need. - Ventilation that can actually purge heat
It is not enough to say “the windows open.” The real question is whether the building can get rid of accumulated heat. BRANZ has highlighted ventilation as a key overheating variable, and MBIE’s proposed H1 modelling changes even moved toward more aggressive passive-cooling assumptions, including reducing the natural-ventilation setpoint for housing from 24°C to 22°C. - Some thermal mass, used properly
Thermal mass is not a magic answer, but BRANZ research has shown that exterior-insulated heavy walls can reduce overheating, maintain warmer night-time temperatures and reduce auxiliary heating energy requirements. In the right design, mass can help flatten temperature swings, especially when paired with night purging and controlled solar access. - Modelling early, not relying on assumptions
EECA notes that modelling can guide decisions on insulation, glazing, orientation and ventilation to optimise comfort and energy performance. That matters because overheating usually comes from the interaction of several “reasonable” decisions, not one obviously bad one.
What a well-performing building tends to have in common
If you look at higher-performing NZ homes, including Passive House projects and similar envelope-led designs, the pattern is not “more stuff.” It is more discipline.
They tend to have:
- controlled glazing ratios
- a clear summer shading strategy
- fewer exposed west-facing problem rooms
- better airtightness
- a ventilation strategy that is intentional rather than accidental
- documented summer-comfort thinking, not just winter heat-loss thinking
That last point is telling. Passive House criteria require a documented strategy for summer comfort, and specifically warn that overheating can still occur in parts of a building even if whole-building averages look acceptable, which is why deeper simulation may be needed where risk is suspected.
Why this matters more now
BRANZ says the risk of summertime overheating is likely to increase with a warming climate, and its recent data shows indoor temperatures are already rising. The buildings we design now will likely face hotter summers over their lifespan than they do today.
So the issue is no longer whether overheating is “a real NZ problem.”
It is whether we are still designing as though winter is the only season that matters.
The takeaway
The best-performing homes in summer are usually not the ones with the fanciest systems. They are the ones where the envelope, glazing, shading, orientation and ventilation were designed as one system.
Overheating is not a failure of insulation.
It is usually a failure of balance.
