Building Code changes aim to reduce overheating in homes

Keeping new homes comfortable in summer is a key focus for the Ministry of Business, Innovation and Employment (MBIE), which aims to support homes that are warm, dry, healthy and comfortable throughout the year

MBIE will take a staged approach to strengthening how the Building Code and its supporting documents address overheating, giving designers a consistent way to assess and manage the risk before homes are built.

Subject to ministerial and Cabinet decisions, the changes would support new overheating compliance requirements and enable a technical assessment methodology to become a Verification Method in 2027.

Using that method would be voluntary. However, where designers used it to demonstrate compliance, building consent authorities would be required to accept it.

MBIE also plans a broader approach to overheating through its 2031 Building Code update, examining how relevant performance requirements work together. It said it would work with the sector to make the changes practical and proportionate for designers, builders and councils.

Guidance due this year

Ahead of the proposed regulatory changes, MBIE will release the technical methodology as voluntary guidance by the end of 2026, encouraging designers to consider overheating early in design.

MBIE is also working with Consumer NZ on information for homeowners this summer, covering heat build-up, ventilation and cooling options.

Homes retaining heat overnight

Separately, BRANZ research illustrates the challenges of summer overheating, showing how homes can retain heat well beyond sunset.

In its article ‘Too hot to sleep?’, BRANZ reported findings from the Household Energy End-use Project 2 (HEEP2), which monitored temperatures in more than 260 New Zealand homes.

Seventy percent of householders reported their home was warmer than they wanted at least some of the time in summer; 22% experienced this always or often.

Living rooms spent more than half of summer evening hours above 24°C and more than one-fifth above 26°C. A quarter of bedrooms remained above 24°C at 2am.

BRANZ identified warmer outdoor conditions, larger glazed areas and homes retaining daytime heat as contributing factors. Average overnight bedroom temperatures were about 2°C higher than in the original HEEP study roughly two decades earlier.

Designing for summer and winter

BRANZ’s ‘Designing to beat the heat’ article outlines measures builders and designers can consider, including window size and orientation, external shading, glazing selection and ventilation.

Appropriately sized eaves can shade north-facing windows from high summer sun, while admitting lower winter sun. East- and west-facing glazing may need vertical shading to control low-angle sunlight.

Building layouts and window openings can also support cross-ventilation, while high-level openings provide an escape route for rising warm air. Thermal modelling can help compare glazing and shading options before construction.

For existing homes, BRANZ identifies retrofit options including external shades, ceiling fans and insulation improvements. It cautions that shades fixed to a building must be securely attached without compromising cladding weathertightness.