This concept is critical for occupant and building health, so we must make sure to understand it properly
We were exhibiting at an annual architect’s conference recently when I was politely but firmly confronted by an architect who had a bee in his bonnet about airtightness. I get this quite a lot, actually, so I thought it would be good to address the issue.
The word ‘airtight’ seems to provoke fear in people. If you tell someone you’re going to draught-proof their house, they love you, but if you tell them you’re going to make it airtight, they suddenly get worried. I’ve seen people start to pull at their neck collar, as if struggling for air. It makes me wonder if we need a new word to describe the concept — but more about that in a bit.
Of course, nobody wants to live in a house that’s tight on air. But that’s only one side of the story in a Passive House or any well-designed, net zero building — the other is the introduction of fresh air via a mechanical ventilation system.
You never hear people express concern about the airtightness of their cars, because everyone understands that you can open the windows or switch on the ventilation. We accept, indeed expect, our cars to be airtight, but we don’t apply the same logic to our homes.
In Passive House design, what we mean when we talk about airtightness is minimising uncontrolled air leakage from a building — draughts, essentially — and optimising the delivery of tempered fresh air. Here’s the irony: we’re aiming to deliver more fresh air for occupants, not less.
Often when I make this argument, about controlling fresh air using mechanical means, the dissenter has their next rebuttal at the ready. ‘So what happens when there’s a power cut?’ Well, if you do experience a prolonged power cut and you’re worried about indoor air quality, simply open some windows — problem solved.
Equally, sometimes the case is made that it’s expensive to run a ventilation system. But let’s have a look at the facts. A mechanical ventilation system with heat recovery, which is ventilating a 100 m2 home, would typically use just 5 kWh of electricity per week, costing about €2.20/£1.70 at current prices. This doesn’t even include the energy savings from these systems recovering about 90% of heat in exhaust air. Add that into the equation and the net energy use is miniscule.
Then there are the benefits to indoor air quality and occupant health from using a good mechanical ventilation system, such as:
1. Lower levels of radon. A friend of mine, Dr Barry McCarron, of South West College in Enniskillen, earned his PhD studying the relationship between airtightness and radon. Barry’s study discovered that certified Passive Houses, the tightest of airtight buildings, have lower concentrations of dangerous radon gas than non-passive dwellings. That’s because they are so airtight and well ventilated, at the same time.
2. So-called ‘natural ventilation’ won’t work effectively if there’s no wind, or if there isn’t enough of a temperature difference to drive the movement of air. Mechanical ventilation works as long as you have power.
3.People tend to close windows and ‘hole-in-the-wall’ vents in winter, because they just bring in cold air, and as a consequence indoor air quality deteriorates. But in a Passive House the mechanical ventilation system recovers heat from exhaust and air and uses it to pre-heat incoming fresh air, so this isn’t an issue.
4.What’s more, you can install filters in your mechanical ventilation system, to remove dangerous forms of air pollution.
There’s one more thing we need to consider – and it’s our good old friend building science. The number one reason we should aim for airtight building enclosures is to protect the structural integrity of our building fabric. If your external envelope allows warm moist air to leak out, there’s a very high risk that water vapour in that air will condense within the cooler building structure, and potentially cause damp and rot. It’s for this reason that we typically use the term ‘airtightness and vapour control’ – the two go hand-in-hand.
So, if you ever have concerns about airtightness, recall the mantra ‘build tight, ventilate right’. If you follow good building science in designing your building enclosure and mechanical systems, you’ll end up with no draughts, less noise pollution, excellent indoor air quality and a structure that is well protected from water vapour.
But all the negative reaction to the term ‘airtightness’ does make me wonder if this is the right word, or if we need new language to describe this vital concept. Perhaps the term ‘draught proof’ would resonate better with designers and the general public? Or how about air permeability — this is how the concept is described in the building regulations, after all. Or what if instead of talking about making a building air tight, we talked about cutting ‘air leakage’? I’m sure none of us would want to live in a leaky home.
Or do you have any ideas of your own? If you have a suggestion for a one or two-word punchy term that fits the bill, lets us know at email@example.com.