Cheapest functional ventilation option for NZ homes

21 January 2019 by Jason Quinn

When folks ask me what they should do first to their home, should they insulate the walls or the roof, or put in new windows, they’re surprised when the first thing I say is put ventilation in the building.

Ventilation is extraordinarily important to your health, good sleep, and productivity. It removes excess moisture and CO2, and dilutes volatile organic compounds (VOCs) from your furniture, plus carbon monoxide and nitric oxides and all sorts of toxins in your homes. You need it all the time, not just when you remember to open the windows for a few minutes, or if it’s a nice day outside. That’s why I say the solution is mechanical ventilation.

People trying to build homes slightly better than New Zealand building code, or even just current code construction, are aiming to do better than we’ve done in the past. They try to build better houses, houses that are warmer and healthier and everything else. But you can create real health hazards in buildings if you don’t understand the physics. You can make the home LESS healthy, without adequate ventilation by improving its construction.

We shouldn’t need to teach folks how to live in a house to prevent mould. Normal, modern living in a home should not produce a mouldy house. The house should be able to manage itself. BRANZ states “around 50% of New Zealand homes are affected by mould” and go on to attribute this to house condition and “occupant behaviour, such as a lack of heating and ventilation”.  The Australians have more than 40% of new buildings with mould. Not sure who is worse off overall.

The normal of how we live in homes has changed over the last 40 years so our building needs to change. It’s now normal for a professional couple to get up in the morning, take a shower, and go to work. Because of security risks, they tend to shut their home up while at work. Their brand-new house should not be wrecked by simply taking a shower and going to work.

I always (well almost) recommend a balanced ventilation system and in most places mechanical ventilation with heat recovery (MVHR). At some point we’ll discuss balanced and MVHR ventilation, how the system should be designed, lessons learned that are available in New Zealand on how to make sure they perform and so on. But, for now, I just wanted to answer my friend’s question, which is What if I’ve already got a house, and I didn’t design it for that, and I just need something to keep my family healthier? And, I’m willing to pay a little bit more. Well, if you ventilate and heat the home in the winter, you can get most of the health benefit with a simpler & cheaper solution. You can do this with a small amount of initial cost by installing a continuous extract fan this is barely adequate, but works in most cases.

Ventilation similar to this is actually built into the building code in several places overseas. What you want is an extract fan in your bathroom that’s continuously running at a very low rate, designed to roughly produce about a third of air change in the whole building every hour, all the time. The fan needs to have certain features to actually be practical, right? It needs to be low power, so you’re not paying a lot of money for the fan to run. It needs to be very, very quiet, so you don’t know it’s running. It should be not too expensive, and easy to install.

There are several options available in New Zealand to do that. I’ll talk about those in a future article. The ideal exhaust fan would: be able to exhaust both the toilet, if you have a separate toilet, and the bathroom (if they’re next to each other); it would be powered directly back to the fuse board so folks wouldn’t turn it off; it would run all the time, in a very low powered setting, providing continuous ventilation at the right rate; and it would have a boost switch. So you can hit the switch next to the door, and it goes on high and boosts and makes some noise. To everybody else in the home, and the people who come into your home, it looks like just a little bathroom fan. But you know you’re getting adequate ventilation all the time.

However, this system means you’re missing out on any energy recovery from the exhaust you could have had so it’s more expensive to run than a MVHR system as you’re throwing away the heat that could be recovered. Plus you’re missing out on the ability to filter the air coming into your home. You really don’t know where the supply air is coming from. If you have a house on piles, it’s probably coming through the floor, or the gap around the floor. If it’s on a slab, it’s probably coming through the seal plate gap, where the seal sits on the slab, but it could be coming through almost anywhere. You’re also not guaranteed to have fresh air distributed everywhere in your home. If you leave all the doors inside the home open, it’s likely to be adequate though. But a more integrated, higher quality solution is the next step up – a discussion for another day.

Overseas, this has been used for decades now. A lot of homes and climates that are similar to New Zealand, and even colder, have shown this is adequate. Anything less than this, and you’re very likely to have problems in your home; unless you’re going to live like your grandparents did, opening doors and windows all winter long, for several minutes EVERY day, TWICE a day. Otherwise, continuous, low cost, mechanical extract ventilation is your answer.

Technical Footnotes:

Mouldy bathroom ceiling right next to window that opens and exhaust fan on a switch.

This solution is really only barely adequate for single family detached home. If the fan is powerful or the home is very airtight than it could backdraft combustion devices like gas water heaters but we are talking about lower power fans than typical. You would need to change to balanced or very well compartmentalized units with passive inlets to make this work in multifamily / apartments / commercial situations.  Terry Brennan from the USA has put together a best practice guide for exhaust only systems in multifamily that’s a good start if you are going to try this.

No. Passive ventilation and cutting holes in windows won’t work – you do need the fan. Scotland study ‘concluded that trickle ventilation as installed and used is ineffective in meeting desired ventilation rates, evidenced by high CO2 levels reported across the sampled dwellings.’ Reference: Sharpe, Tim, Paul Farren, Stirling Howieson, Paul Tuohy, and Jonathan Mcquillan. “Occupant Interactions and Effectiveness of Natural Ventilation Strategies in Contemporary New Housing in Scotland, UK.” International Journal of Environmental Research and Public Health 12, no. 7 (2015): 8480-497.

Australia ‘40% of new buildings have condensation and mould.’ Reference: Clarke, J and Maxwell, S “Improving Australian Housing Envelope Integrity A Net Benefit Case for Post Construction Fan Pressurisation Testing.” The Australian Institute for Refrigeration Air Conditioning and Heating (AIRAH), October 2016.

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