8 Apr 2009

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.


  1. That's a very interesting train of thought.

    We also exhale water vapour & sweat. What impact would this have in an unvented passive haus?

    Would it contribute to condensation / mould growth?

    Perhaps the ventilation is there for reasons beyond just CO2 levels?

  2. I have to agree with Chris. Living in a leaky caravan, and with our belongings mouldering (literally) in a leaky old farmhouse I can say that ensuring fresh air reaches throughout your home is vital.

    Still corners of stale air breed mould, damp and insects. Even when the rest of your dwelling feels fresh, unventilated regions can go down hill rapidly.

    Having originally wanted to avoid MHRV, I'm a convert to the idea that it ensures air changes reach throughout a house. Occasionally used rooms, and rooms that are shut up for much of the day (should) stay fresh.

    Ask me again in a year's time when our house is - hopefully - up and running.

  3. I agree with the two previous commenters, it's water vapour that's the problem.

    However, your figures are useful to show that reasonably good airtightness is not a safety issue with respect to CO₂ therefore strategies to buffer water vapour and carry out ventilation to dry the house out at suitable times (when relatively warm, dry outside air is available) are worth consideration whether or not heat recover is used.

  4. Great analysis Mark.

    It certainly appears that the CO2 concentration should not cause a problem.

    But like the others who have commented, I think that water vapour is real enemy.

  5. I am not so sure water vapour is such a big problem in a bedroom. Sure, RH will rise in an unvented room with people sleeping in it, but condensation and black mould will not appear unless the RH rises to 100%, which is unlikely without a water source like a shower or temperatures are falling below 12°C. And 80lts of fresh air per person per hour is plenty enough to keep RH levels similar to outside, just as it will keep a lid on CO2 levels.

  6. Water vapour causes mold and other problems only where it condensates. And for condensation to happen you need a surface colder than the dew point of the air (which depends on temperature and RH): badly insulated windows and badly insulated walls, something a PassivHaus hopefully lacks.

    I suspect the legally required ventilation levels in the UK are largely inspired by experience with uninsulated Victorian houses, and are far above what is actually needed for comfort and health in a modern house.

    They also seem based on the assumption that users never manually open the windows when the need arises.

  7. Ed, Mark & Markus, you are probably right.

    In a highly insulated house, I would expect condensation to be largely absent, but the article does provide food for thought on what really constitutes "acceptable air quality", and how both CO2 & RH levels can be appropriately maintained without mechanical intervention

  8. Alan ClarkeApril 12, 2009

    I'm not going to try and grapple with imperial units at this hour, but I think you've lost a power of ten there somewhere Mark. 80 l/min/p is only going to give you around 4000 ppm CO2 which is generally understood to be too much - and certainly not maintain 500ppm (you'd need around 2500 l/m/p to get that).
    Having read this paper I'd carry on designing ventilation systems to give around 300 l/m/p (5 l/s/p) to maintain 1400 ppm, and 500 l/m/p (8l/s/p) to maintain 1000 ppm.

  9. How about the need for ventilation of other potential problems ,such as VOC's
    unsure , but would this come into it.
    i never leave windows open in the winter , though i know many people that feel its to stuffy if they dont, probably more to do with personal habits , than any real need for so called fresh air

  10. Also regarding MHRV , doesn't the HR bit also have benefits in reducing energy consumption of the passive house design?

  11. The Times today carries a feature about the new roof cover at Wimbledon Centre Court. It comes with a ventilation system that is capable of delvering 143,000lts/s for the 15,000 crowd. That works out at 572lts/m each.

  12. It's not necessarily true that PassivHaus implies whole house ventilation. There is a one room ventilator:

    You could pretty easily wire it on a timers or CO2 detectors if you didn't want to ventilate your entire house.

  13. Sorry, but some misunderstandings here:
    1. Mould growth occurs at 80% r.h. if maintained over a couple of hours when substrate is available, which it usually is, as fungi feed on almost anything. Condensation is definitely not required for this.
    2. Yes, you can approximate average atmospheric air exchange from pressurisation test. Yet: as atmospheric ventilation levels depend on a great deal on such unreliable things as wind and weather, I would never rely on that.
    3. An n50 of 0.6 h^-1 does not tell anything about the distribution of leakages. Could be just one major leak in one room, or evenly spread mini leaks. Rely on this at your peril!

  14. CatherineMay 30, 2009

    Hi, I've really been enjoying reading "The Housebuilder's Bible." Personally I always turn back the bed and open the window in the mornings to help the mattress breathe out the moisture accumulated during the night and help keep down dust mite levels, and of course mould is horribly bad for your health too...

  15. My understanding that CO2 is only a marker of IAQ and sick building syndrome. High levels reflect large quantities of unmeasured gases such a VOCs that are being offgassed from the building components.

  16. 1/10th of the 50Pa air infiltration would be much closer to the natural air change rate.
    For my house (in eastern Canada) the natural air change rate in winter is about 1/5th of the 50Pa rate. During my 2nd winter I never ran my HRV after the end of November, and the humidity stayed around 40% for most of the winter. The house volume is 1530m^2 (54,000cf), tested 1.1ACH@50Pa, and is occupied with 2 adults & 4 children (and often 2 extra adult guests).

  17. p.s. CBD110 has moved from the link I posted on Green Building Forum.