Is a drier home a healthier home?

Many New Zealand households face problems with condensation, drafts, and mould. But is a drier home necessarily a healthier home? A new study has shed light on how we understand indoor humidity and its link to ‘drier’ homes. It urges green building rating tools to clarify how humidity and dampness are measured and understood.

Researchers from the University of Auckland and Massey University looked at winter moisture levels in a 40-unit apartment building in Auckland for residents aged 65 and older. Researchers found that relative humidity within the 7-Homestar certified building often exceeded the recommended 40-60% range, which is commonly used to minimise condensation and mould risk.

University of Auckland senior property lecturer Dr Mike Rehm, researcher Dr Rochelle Ade, and Dr V. Vishnupriya (Massey University) stress that elevated relative humidity doesn’t necessarily mean the building is ‘damp’ or unhealthy. In fact, when the same apartments were assessed using absolute humidity – a measure of the total moisture in the air – they fell consistently within the epidemiologically acceptable range for health and comfort.

“Humidity and dampness are often used interchangeably, but they are not the same,” says Rehm. “A building can have high relative humidity without being damp. That nuance is often lost, including in how certification tools like Homestar communicate ‘drier’ living conditions.”

The findings highlight that using relative humidity alone may not give a full picture of indoor air quality or health risk — especially in naturally ventilated homes in humid regions like coastal Auckland. Despite readings above the usual comfort range, most residents said they felt comfortable, suggesting relative humidity isn’t the only factor influencing occupant wellbeing.

Despite relative humidity being outside the ideal range, most residents in the study reported feeling comfortable in their homes. The researchers say this suggests relative humidity alone may not reliably reflect health risks or occupant satisfaction, especially for older people.

The researchers recommend that green building ratings consider both relative and absolute humidity metrics. One idea is to use relative humidity as a primary indicator for general performance, but also include absolute humidity when high outdoor moisture or specific climate conditions might skew results.

“In cities like Auckland, where outdoor air is often humid, natural ventilation may not reduce relative humidity. But that doesn’t automatically mean a home is unhealthy or inefficient,” says Rehm. He proposes that relative humidity be used as a primary performance metric, with absolute humidity included in some cases, to better reflect actual indoor conditions.

“Certification systems like Homestar already play an important role in improving New Zealand’s housing stock. Clarifying how terms like ‘drier’ are defined and measured could enhance their effectiveness and help align performance goals with occupant wellbeing,” he says.

Moving Towards More Accurate Humidity Guidelines

In places where outdoor air tends to be humid, natural ventilation might not lower relative humidity as much as expected. That doesn’t automatically mean a house is unhealthy. Factoring in absolute humidity can give a more balanced view of actual air moisture levels.

For homeowners and builders, understanding this difference helps set realistic expectations around condensation and mould prevention. Focusing solely on relative humidity targets may not always signal a need for costly interventions if absolute humidity remains healthy.

The paper, What is Drier? Understanding Humidity in Green-Certified Dwellings is published in the Journal of the Royal Society of New Zealand. It is part of a growing body of work calling for more precise, performance-based humidity guidelines in building standards.

This study is part of a growing call to revise building standards with clearer, performance-based humidity measures that reflect New Zealand’s unique climates and housing styles. Improving how we define and measure ‘dryness’ indoors could lead to better policies, healthier homes, and more satisfied residents.

By combining relative humidity, absolute humidity, and occupant experiences, future green ratings and building codes could offer smarter guidelines. This shift would support homes that stay comfortable, safe, and energy-efficient across New Zealand’s diverse weather conditions.