Why NZ Homes Overheat

Recent reports out of Auckland highlight a worrying trend: many new townhouses are sweltering in the summer heat. Homeowners are finding that some modern, million-dollar builds cost twice as much to cool over summer as they do to heat in winter, with bedrooms reported at 30 °C at 9 pm while it’s only 24 °C outside. These aren’t just isolated cases—they signal a broader design problem that could leave tens of thousands of Kiwis living in uncomfortably hot houses if nothing changes.

Design Flaws Are the Culprit, Not Insulation

Industry experts and building researchers are clear: it’s poor passive design—not insulation or advanced panels—that’s driving the overheating. Large unshaded windows, minimal ventilation, and bad orientation let solar gains flood in and get trapped. In one Auckland townhouse example, a west-facing façade with huge glazing and tiny opening areas baked upper floors in the afternoon sun.

Blaming high insulation or SIPs for hot homes is a myth. Building scientists explain that insulation helps prevent overheating—just like a chilly bin keeps contents cool by keeping heat out. BRANZ has called it a misconception that insulation causes overheating; in reality, the opposite is usually true. A well-insulated home slows outdoor heat transfer in summer while preserving warmth in winter. The real problem is designs that invite too much sun and don’t allow effective airflow or shading.

SIPs + Good Design = Year-Round Comfort

High-performance construction like Structural Insulated Panels (SIPs), paired with smart design, can keep homes cool in summer and warm in winter. One Northland case study in a Formance SIP home—with dark metal cladding that gets scorching in February—still reports interior surfaces staying pleasantly cool, highlighting the envelope’s performance. By contrast, older uninsulated villas often soak up heat all day and radiate it into bedrooms at night.

Fixing overheating doesn’t mean ripping out insulation; it means designing smarter: shade big windows, orient for the climate, enable cross-flow or use mechanical ventilation with summer bypass, and control glazing ratios. Combine these with an airtight, well-insulated envelope and you avoid turning homes into ovens.

References

1. RNZ: The big bill to cool new Auckland townhouses
2. EXPOL: Overheating homes? The real problem is design
3. RNZ: Insulation’s role in overheating a ‘myth’

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