Air Quality in an Airtight House

This post is part of my Sustainable House Day submission.

For background also read my previous post All Uptight.

To recap, to build a home suited tor Tasmanian winter conditions using minimal active heating requires:

  • high levels of insulation
  • thermal mass
  • an appropriate amount of north facing glazing
  • air tightness

However once your house is really airtight you run into issues of condensation/ humidity/ mold, high levels of Carbon Dioxide (CO2), other noxious gases (eg. VOCs) etc. unless there is adequate ventilation.

To ventilate without loosing our precious heat we installed a Heat Recovery & Ventilation (HRV) system, also known as a Mechanical Ventilation & Heat Recovery (MVHR) system.

clothes basket & breadmaker are not essential to HRV operation 😉

A ventilation specialist specified the ducting layout for us:

blue = supply air, red = extract air

Because of Covid-19 we did the whole installation ourselves. It’s wasn’t that difficult and the results discussed below indicate the system is working very well.

In our case the commissioning flow rate/ air exchange rate was initially based on the rule-of-thumb 1/3 total volume per hour. So 110m2 (floor area) x 2.4m (wall height) = 268m3 volume / 3 = 90m2/hour.

After a few weeks of monitoring & testing we settled on a flow of 100m2/hour. At this rate the unit is just ticking along, nearly off the chart for power consumption (15 watts) and noise (barely audible). I think in hindsight we should have bought the next smaller model, but this is what was recommended.

red line is the operating point, 40Pa pressure
our Qv is 100, so noise level is better than top row
status from app running on phone

The heat recovery is supposed to be about 92% efficient. Below shows an example of the unit’s temperature measurement – air is being extracted from the bathroom, kitchen & laundry @ 21.6C, and after recovering the heat, the exhaust air delivered to the outside is 14.1C. That’s an impressive recovery.

status from app running on phone
status from app running on phone

I’m not sure how “avoided year-to-date” is calculated, but if that were a electric radiant bar heater, 1393kWh @ $0.27 would be $376. Whereas at 15W the actual running cost for the year has been 73kWh or $20.

TIP: The 90mm diameter ducting is quite expensive. We just managed to get by with a 50 metre roll. Be especially careful to allow for the extra length of bends when estimating how much you will need.

Air Quality – CO2

We have a number of air quality monitoring devices running 24/7. More details in a later post. Recall the CO2 in Hawaii and Cape Grim at present is around 411 Parts Per Million (PPM) (409PPM last year, 387PPM 10 years ago). As shown in a typical graph below our outside CO2 PPM is 470PPM and the inside concentration varies wildly depending on the time of day, number of occupants, and HRV scheduled setting.

the hump corresponds to lunch time

According to this reference a good indoor level should be between 400 & 1000PPM. You can buy a CO2 sensor for our HRV and it will automatically bump up the exchange rate to compensate, but we just manually boost the rate if we are having friends around. We’ve seen the concentration get up to 1500PPM. I’ve set a schedule so that during the middle of the day the exchange rate is lower; this reduces power consumption a little.

last 7 days – highest in the evening

Air Quality – Condensation/ Humidity

Keeping condensation/ water vapor low is very important to avoid mold, especially in the bathroom, and on cold south facing windows.

There are reports of new houses being demolished after 6 months and/or needing extensive modifications because they were made airtight in order to meet the minimum 6-star energy rating but sufficient ventilation wasn’t designed.

Simple observation – we’ve never seen much condensation, and definitely no mold in the bathroom. When measuring humidity in the bathroom after a shower, the concentration reduces to normal levels within two hours.

blue=outside, green=bathroom humidity
last 7 days – green=living room, orange=bathroom

Air Quality – Particulate Matter

This aspect of air quality is the most subjective one to measure. We have a single sensor that can measure PM1 through PM10 size particles to a reasonable degree of accuracy. According to Wikipedia particles smaller than 2.5 microns (that’s 0.0025 millimetres) can penetrate deeper into the lungs.

last 7 days – mostly less than 5ug/m3
a bit more legible – last 3 hours

Our HRV has two G4 rated bag filters. According to Australian Standard
AS1324.1-2001 & AS4260-1997 this means better than 90% arrestance for No. 4 Test Dust. What ever that means! These bags are replaced every 180 days. According to a paper co-authored by Cameron of Passive Analytics our total PM level of 5ug/m3 is well below the average household of 15 to 20.

We have no way to measure Volatile Organic Compounds (VOCs). Our Melamine joinery was supposedly low VOC, but I’m sure our cheap & nasty lounge suite was anything but. Below you can see the huge spike in PM (10x the background rate!) when the sofa is sat upon each evening:

new sofa – full of dust? or worse?

I couldn’t find any specific information regards our interior paint.


3 thoughts on “Air Quality in an Airtight House

  1. Hi folks, congrats on the house, looks great. Are you able to supply the info of the ‘ventilation specialist’ if possible? I’m having trouble finding the right people to speak to….
    Many thanks, Andrew


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