PassivHaus ventilation questions

Picked up this interesting paper off a thread on the Green Building Forum. It hails from Canada and dates from 1969, but it has an air of authenticity about it. It answers a question I have long wondered about – just how much fresh air do we need to survive and prosper?

Every time we breathe in, we draw in just under 1 litre of air. If it’s fresh air, we draw it in at atmospheric conditions, which equate to 21% oxygen and 0.03% CO2 (or 300ppm – since 1969 this has increased to 380ppm, but that’s another story and it doesn’t effect the calculations here). Our lungs absorb oxygen and give back CO2, so that what we breathe out consists of 16% oxygen and 4% CO2. In the course of an hour, while at rest, we breathe in and out around 500 litres or 0.5m3.

If you are asleep in an enclosed space of, say, 16m3 (typical size for a small bedroom), then, over 8 hours, you will have added 4m3 of exhaled air so that, when you get up in the morning, around 25% of the air volume in the room will have passed through your lungs. That means that CO2 levels should have reached (4 x 0.25) 0.5%, which equates to 5,000ppm.

My little experiment last week, sleeping in a bedroom of around 30m3 on which I shut the door and closed the windows, got the CO2 detector to register just over 2,000ppm, which falls fairly accurately within these parameters. So far so good. The paper suggests that 4,000ppm CO2 is unpleasant and represents really poor IAQ. I would concur. I thought 2,000ppm was stale and stuffy.

Now what is interesting to me about this paper is that it concludes that a safe level of ventilation is around ten times what we breathe in and out. At rest, we breathe around 8lts per minute: the recommended ventilation rate is 80lts per minute. That’s enough to ensure that indoor CO2 should never get above 500ppm, which is unnoticeable. That’s an incredible safety margin. In fact, you can control indoor CO2 levels with much lower rates of ventilation, as people rarely if ever stay in one room for more than 8 hours at a time. So bear in mind that the minimum accepted ventilation levels have a huge margin of safety built into them.

How do these figures compare with an average leaky house, which experiences around a complete air change every four hours. Say the house is 300m3. That means that 75m3 of air is changing every hour. Which works out at 1.25m3 or 1250lts per minute. Enough to keep 15 people breathing easily, at the recommended ventilation levels. And that’s with the windows closed.

What about an air tight, PassivHaus style structure? Assume something like 0.6 ach at 50 Pascals – that in fact is the PassivHaus standard for air tightness. It is estimated that at normal atmospheric pressure, the actual air changes are 20 times less than they are at 50 Pascals, so we could assume a real air change of 0.6/20 = 0.03 ach. Again, lets assume a 300m3 house. 300x0.03 equates to 9m3 per hour or 150 lts per minute. That’s the air leakiness that you get with a 300m3 PassivHaus. It is still surprisingly high. In fact, it’s still fine for two people (at the very safe 80 lts per minute each). So rumours that you would suffocate if you lived in a PassivHaus with no ventilation are perhaps just a little exaggerated. In fact, you would survive just fine.

These calcs do call into question the insistence (from PassivHaus practitioners) that you have to have MVHR installed in an airtight house if indoor air quality isn’t to suffer. The problem with air quality is actually more localised. You can induce it over a prolonged period (like 8 hours sleep) in an enclosed space (like a bedroom), but you would struggle to notice it in a whole house. MVHR is very much a whole house solution to a problem that affects individual rooms. Why ventilate the living spaces at night? Why ventilate the bedrooms during the day?

All good questions. The debate will run and run.