Build tight and ventilate right

Extreme summer heat and cold winters highlight the importance of effective ventilation. While opening windows can help, mechanical ventilation offers a more consistent solution

At a glance:

• Heating and cooling are key design considerations in residential buildings.
• New builds are more airtight than older homes.
• Relying on occupants to open windows is often ineffective, so alternative ventilation methods need to be considered.
• Mechanical ventilation continually renews indoor air, ensuring a healthy and comfortable living environment.

Indoor overheating is a major concern over summer, with reports of uncomfortable and unhealthy conditions. This is usually more of an issue for attached dwellings, which often have only two external walls and can suffer from overexposure from the sun and limited ventilation options.

In winter, common complaints include cold interiors and high moisture levels – and ‘crying’ windows. In some cases, mould and mildew develop, leading to unhealthy living conditions.

Inadequate heating, low levels of insulation and high levels of air leakage in older dwellings are well reported. Less talked about is indoor air quality.

Here, we outline how ventilation supports indoor air quality and moisture control by reducing humidity and removing pollutants that negatively impact health and comfort in homes.

Building Code requirements

Building Code clause G4 Ventilation requires that buildings are adequately ventilated with outdoor air – specifically that odours, gases, moisture and contaminants such as bacteria are effectively managed and removed or reduced to maintain a healthy indoor environment.

Acceptable Solution G4/AS1 calls for occupied spaces to be ventilated by natural or mechanical ventilation or a combination of both.

Dwellings can comply with these ventilation requirements simply by having a total openable window area or other external openings of a minimum of 5% of the building’s floor area. There are additional requirements for apartments or other accommodation types where there is only one external wall for openable windows.

While this can provide passive ventilation, it relies on occupants actively opening and closing windows and doors.

Air exchanges in dwellings

There are three primary ways that air exchange occurs in dwellings:

• Occupants opening windows, skylights and doors.
• Leakage (infiltration) through gaps in the building exterior envelope. This should not be relied upon for fresh air requirements.
• Manual/automated mechanical ventilation systems.

In most new builds, opening windows and doors remains the primary method of ventilation.

Currently, mechanical ventilation is largely used for source control in kitchens, bathrooms and laundries – areas that experience high levels of moisture by daily living. This is usually in the form of rangehoods for kitchens and extract fans for bathrooms and laundries.

Research shows that regularly opening windows and doors can be a simple and effective way to ventilate (see Build 156 New homes, old habits). However, it should be stressed that this does carry a comfort disincentive, which means it is less likely to be done in the colder months.

While opening windows fully for just 15 minutes a day can significantly reduce indoor moisture levels, many households neglect this simple habit – often due to comfort concerns or a lack of awareness about the importance of ventilation.

Ventilation and airtightness

Guidelines recommend that, for a healthy indoor environment, ventilation rates in dwellings should be between 0.35 and 0.5 air changes per hour (ach) – this equates to one-third to one-half of the interior air being replaced with fresh outside air on an hourly basis.

Older houses are usually highly air leaky and will tend to have air exchange rates that are difficult to control and likely to be greater than what is needed. This has a significant impact on comfort and ability to retain heat.

With newer homes being significantly more airtight, this has exposed the underlying fact that ventilation has historically not been adequate. In fact, air infiltration measurements on real houses are well below those inferred by the results of blower door testing. While this is good news from an energy efficiency perspective, it places even more emphasis on the need for better ventilation.

Because infiltration should not be relied upon for ventilation, occupants need to open windows and doors more regularly or incorporate mechanical ventilation in the build.

There are several reasons that opening windows and doors is not practicable, so greater consideration needs to be given to effective, efficient forms of inbuilt mechanical ventilation.

The phrase ‘build tight and ventilate right’ is well known. While modern construction achieves airtightness, ventilation still needs improvement.

Mechanical ventilation

To ensure adequate overall ventilation for good indoor air quality, mechanical ventilation needs to be considered. However, overventilation should be avoided as it can lead to unnecessary heat loss in winter.

Ideally in all new-build dwellings, moisture is being removed at source using localised air extraction systems that vent to the outside.

In addition to the above, incorporating mechanical ventilation systems that either run continuously or in response to moisture and CO2 levels is best practice. Outdoor air needs to be filtered, and options that incorporate a heat recovery system that pre-warms the incoming air are available.

These are some key findings from Levy-funded research on heat recovery ventilation system performance:

• Modern heat recovery systems can capture approximately 90% of heat from outgoing air. The research highlighted that, if the ductwork and heat exchanger are outside the thermal envelope, there is a significant negative impact on performance.
• Planning your ducting layout at the design stage will ensure you keep the ducting inside the thermal envelope.
• Efficient performance depends heavily on correct system design and installation, including carefully commissioning.
• Balancing extract and intake airflow is essential for optimal efficiency.

While heating and cooling a house are important aspects for a comfortable internal environment, sufficient ventilation to create good indoor air quality is equally important. A combination of heating and efficient ventilation is critical for a healthy indoor environment.

 

This article was first published in BRANZ’s Build December 2025 magazine. www.buildmagazine.co.nz. Words supplied by BRANZ.