Well-built high-performance buildings need mechanical ventilation. Times have changed, nobody’s really arguing about this any more.
But all the other homes built in the last 50 years need mechanical ventilation also—those that only just met the Building Code minimum standards of their day. They’re not airtight (many of them are horribly draughty) but they still need proper ventilation. How do I know? Because most New Zealand homes have mould.
Here’s a handy rule of thumb: is your building occupied by people? If yes, mechanical ventilation is required and it needs to run all the time. (Extract fans in the toilet or bathroom that can be turned on or off don’t count.)
In the past, I’ve made the “build it tight, ventilate right” argument. It makes for a snappy explanation! I’ve been arguing for mechanical ventilation for years (preferably with heat-recovery, because efficiency and lowering carbon emissions is good, right?). Some of the most spirited arguments have been with BRANZ staff. So you bet I was happy when BRANZ came out last year with a revised opinion, recommending mechanical ventilation in new builds. It paired this with a recommendation for setting air-tightness targets.
But: I am now of the view that it’s wrong to conflate the two (high-performance buildings are very air-tight, so they need constant ventilation at calculated levels) and the conversation needs to move on.
New Zealand is world-leading in lots of fields. Very unfortunately, the prevalence of cold, damp mouldy homes is one of them. The causal factors are many and varied, which I explore in Section I of my book. In brief, it’s a combination of our temperate climate and how we live in our homes. New Zealand has high relative humidity and entrenched cultural resistance to adequately heating our homes. The housing crisis is increasing over-crowding. Fuel poverty means many people can’t heat their homes (and don’t ventilate, because the air outside is even colder). Significant social changes mean many houses are locked up tight and unoccupied for long periods each day, trapping moisture inside.
There are basic messages that we need to be sharing with clients, colleagues and the general public. People produce moisture: when cooking, washing, showering and most of all—breathing. Mechanical ventilation removes stale, damp air and replaces it with fresh, filtered air. That’s good for the people breathing it and good for the durability of building structures.
Mechanical ventilation reduces relative humidity levels, which is crucial because mould follows damp. It is pretty much impossible to avoid mould when indoor RH > 70%. (Some New Zealanders sleep in a room that is over that level 90% of the time.)
Our industry—and policy makers—need to be having a much more sophisticated conversation about mechanical ventilation. And without delay, because meantime people’s lives are being ruined, even ended, by disease caused by their cold, damp housing.