Natural Air Flow
The Challenge:
Adequate ventilation is an essential priority for all NZ homes, but for some Kiwi homeowners a mechanical heat recovery ventilation system is not a preferred option – either because of budget constraints or because they prefer to have environmentally friendly systems in their home.
Passive ventilation, cross ventilation, or “the Stack Effect”
A practical and effective means of ensuring good healthy air flow and quality in the home uses the structure of the building itself to create natural air flow across the internal spaces, remove stale air and draw in fresh air.
In many cases the building is designed with high-level opening clerestory windows to utilise the physics of warmer air rising to the highest point – known as the “Stack Effect”. Hot fusty air flows naturally up and out of the upper level windows or skylights, creating a pressure difference that draws in cooler clean air through lower level windows or doors.
This method of providing good air flow is an effective way to keep homes cool in our increasingly hot summers. It has been effective for many centuries in Moorish, Mediterranean and African architecture as a simple way to deliver natural ventilation for health and comfort.
Cross ventilation is a similar principle, functioning across from one side of the home to the other, creating a gentle flow of air to maintain comfortable temperatures, reduce humidity, and ensure continual fresh air and removal of dust, pollens and pollutants such as VOCs (volatile organic compounds harmful to health).
These ventilation methods need careful design to integrate with:
Eaves and shading to protect against too much solar gain through windows
Site orientation to maximise prevailing winds and sun – over seasonal variations
How the home will be used and the positioning of different living, sleeping, working and service spaces
Weathertightness especially for high level windows
Window sizes, placement and relative heights
High performance windows (double or triple glazed) with easily managed opening and closing systems
Reduction of internal moisture and suitable extraction of moisture from kitchen and bathroom areas
“The stack effect is how we deal with natural air flow. Cool air is always more heavy, on the ground – it’s designing houses where that cool air is dragged into the house and it heats up, and it rises and exits via top light windows.”
Duncan Firth: Solarei Architecture
“Although we built a 2-storey house, we do use the principles of opening a window. We have windows on both sides of the house so there can be a natural draught, and that works.”
Victoria Mitchell: Statement Homes
“What the house does, it naturally heats, with its northern orientation, and then designing in specifically cross ventilation and stack effect – it’s designed either (1) you open it up and have cross ventilation which is a good Kiwi way of doing things in Northland, and (2) looking at stack effect, hot air rising and how we design that in throughout the house .”
Duncan Firth: Solarei Architecture
“We were really worried about overheating but it hasn’t been a problem. We have windows that slide across so airflow can really come through. The design of it really impacts, so if you can partner with a good architect who thinks about these things it doesn’t have to be a problem. It didn’t cost much to do, it was just the design of the window and where the air was pulling from – low windows on the bottom floor that open out to the ground, and skylights that funnel it up like a chimney – it comes down to design, if you get the right people involved early on then you’re winning.”
Victoria Mitchell: Statement Homes
Whare Paetara in Prebbleton, a Bob Burnett Architecture design that is centred around a large stone spine wall:
“The wall also allows for high-level windows to provide natural cross ventilation and bring an uplifting quality of morning sunlight.”
River Drive House in Kerikeri by Solarei Architecture – Highly Commended in the 2024 Superhome of the Year Awards:
“The cross-sectional design of the house utilises stack effect and cross ventilation techniques for natural ventilation, keeping the house passively cool during summer and well ventilated throughout the year.”
