Indoor air quality is an important factor for human health and well-being, however, not all buildings provide the same level of indoor air quality, and some may even expose the occupants to harmful pollutants. In this post, we will explore how Passivhaus homes can offer superior indoor air quality compared to conventional homes.
Why is indoor air quality important?
According to the World Health Organization, the indoor air quality affects the health, comfort and well-being of the people who live or work in indoor environments. Poor indoor air quality is associated with a variety of symptoms, such as fatigue, nausea, dizziness, irritation, headaches and increased risk of respiratory and chronic diseases. It is estimated that, on average, we spend 87% of our time indoors, of which 69% is spent in our homes. This means that the quality of the air we breathe inside our buildings has a significant impact on our health and quality of life.
How does a Passivhaus home compare to a conventional house?
In a study published on the International Journal of Environmental Research and Public Health, researchers compared the indoor air quality of highly energy efficient houses (including Passivhaus projects), equipped with mechanical ventilation systems, against the air quality in conventional houses built to the Austrian building standards, not equipped with a controlled ventilation system.
The researchers measured the concentration levels of varied indoor air pollutants, including VOC (Volatile Organic Compounds), Aldehydes, Carbon Dioxide, Radon and Mould Spores in a total of 123 homes in Austria. Two rounds of measurements were taken: the first one took place three months after the residents moved into the house, and the second one took place one year after the initial test.
In the conclusions of the study, the most extensive investigation of this type, the researchers stated: "This study shows that indoor air quality in energy-efficient new houses (private homes, with mechanical ventilation) was higher than in conventional new buildings. This was true for almost all investigated parameters like, inter alia, TVOC, aldehydes, CO2, radon, and mould spores”.
Here is a summary of some of their results:
Graph 1: Concentration of indoor air pollutants in standard and energy efficient homes in Austria
While there are limitations of applying the results of this Austrian study to the Australian reality - considering the differences in climate and the requirements demanded by the building standard - some case studies done in Australia seem to confirm the superior air quality provided by Passivhaus homes.
Cameron Munro (Passive Analytics) and Joel Seagren (Fantech), monitored the carbon dioxide concentration in the master bedrooms of two Melbourne homes for a month. One conventional leaky house with no mechanical ventilation system and an airtight home with a mechanical ventilation system. Graph 2 shows the results.
Graph 2: Typical carbon dioxide concentrations in master bedrooms of Melbourne homes (winter)
Source: Cameron Munro (Passive Analytics) and Joel Seagren (Fantech)
They found a greater disparity between the air quality levels of a conventional leaky house and an energy efficient home than what was observed in the Austrian study. While in the leaky house (left) the carbon dioxide concentration peaks above 2000 ppm multiple times, in the energy efficient home the levels of carbon dioxide consistently stay under the 1000 ppm mark.
Even though carbon dioxide is not particularly hazardous, it is commonly used as a marker for Indoor Air Quality. A high concentration of carbon dioxide will likely correspond to a high concentration of other pollutants, like the ones presented by the Austrian study.
So why do Passivhaus homes have superior air quality levels than conventional homes?
In a conventional house, built according to the minimum building standards and unequipped with heat recovery ventilation systems, opening the windows is the only solution for ventilation. When the weather conditions are favourable, windows can be left open, the internal air is refreshed and internal temperature is kept within a comfortable range.
When the weather conditions are not favourable, however, there will be a conflict between conditioning the internal space and refreshing the internal air. You can open the windows and refresh the stale air, but you will be losing the internal conditioned (heated/cooled) air to the external environment. Alternatively, you can shut the windows, keep the conditioned air inside, but the air quality will be compromised. In practice, because the discomfort from cold or hot temperatures is more readily perceived than the discomfort caused by poor air quality, most people will prioritize internal temperature and sacrifice the air quality.
In Passivhaus homes that conflict does not exist. When external weather conditions are favourable, you can open the windows and let the fresh air come from the outside, just like you would in a conventional house. When external conditions are not favourable (too cold or too hot), you can shut the windows and let the ventilation system work its magic. The system will remove stale air from the inside, draw fresh air from the outside, filter the external air and only a fraction of the heat will be lost in the process. Your house will remain within a comfortable temperature range, maintain the internal air healthy and use very little energy.
Passivhaus and Bushfire Smoke
With the above learnings, we understand how Passivhaus homes can manage indoor air pollution. And indeed, this is a priority. According to the American Environmental Protection Agency (EPA), the concentration of pollutants found indoors are usually two to five times higher than the ones found outdoors. However, in exceptional situations, like bushfires, the outdoor air quality can also become a threat to human health and present additional challenges to indoor air quality. And in those situations, Passivhaus homes have also been demonstrated to offer much better protection against the outdoor pollution than conventional houses.
In another study done by Munro and Seagren, published in Renew Magazine (Keeping the smoke out - Renew), the authors compared the concentration of PM2.5 (particles of less than 2.5 microns in diameter) in two adjacent Melbourne homes for 36 hours during the 2019/2020 Australian bushfires, and found that in the airtight home equipped with mechanical ventilation system, the peak concentration levels of these particles were about 25% lower than the ones found in the conventional leaky house. The results are presented below (Graph 3):
Graph 3: Performance of airtight and standard buildings during 2019/2020 smoke events in Melbourne
Source: Keeping the smoke out - Renew (Cameron Munro and Joel Seagren)
When outdoor PM2.5 levels were 600 µg/m3 (blue line), the conventional leaky building without mechanical ventilation (red line) reached PM2.5 concentrations of just under 500 µg/m3 and the airtight home reached peaks of 320 to 380 µg/m3 (yellow line).
A conventional home is not designed to keep the smoke out. If you shut your windows and turn the air conditioning on, you might be able to keep your house cool, but eventually the internal air will become stale, and you will be forced to open the windows. Moreover, even if you refuse to do so, the smoke will still make its way inside the leaky envelope and add up to the internal air pollution. On the other hand, Passivhaus homes are much better sealed than conventional houses and the level of smoke inside can be better managed by adding a higher grade filter to the mechanical ventilation system.
The mechanical ventilation system of a Passivhaus home is conventionally equipped with an F7 filter, which already provides some protection to external pollution, as it was demonstrated in the comparison above. However, if an additional HEPA filter is installed downstream from the ventilation unit, the concentration of particle mass indoor can be further reduced to levels below the recommended threshold of 25 µg/m3.
To demonstrate this, Munro and Seagren monitored the levels of PM2.5 in two identical Passivhaus homes in Canberra for four days during the 2019/2020 bushfires (Graph 4). One home was equipped with a standard F7 filter (red line) and the other with an F7 filter and an additional HEPA filter (yellow line). Both homes had much lower concentration of particle mass than the exterior environment. However, the home equipped with additional filtration had substantially lower levels than the one equipped exclusively with an F7 filter.
Graph 4: Performance of two identical Passive homes in Canberra during 2019/2020 smoke events, one with a standard F7 filter, and the other with additional HEPA filter
Source: Keeping the smoke out - Renew (Cameron Munro and Joel Seagren)
Conclusions
Because of its high level of airtightness and the use of heat recovery mechanical ventilation systems, Passivhaus homes have better levels of indoor air quality than conventional homes. They can be ventilated without compromising thermal comfort, reduce the penetration of external pollutants, and further protect the occupants from external pollutions by using additional filters in the ventilation system.
References
Indoor Environmental Quality in Mechanically Ventilated, Energy-Efficient Buildings vs. Conventional Buildings. By Peter Wallner 1, Ute Munoz 2, Peter Tappler 2, Anna Wanka 3, Michael Kundi 1, Janie F. Shelton 1 and Hans-Peter Hutter 1;
Keeping the smoke out, by Cameron Munro & Joel Seagren. (Keeping the smoke out - Renew)
The National Human Activity Pattern Survey (NHAPS): A Resource for Assessing Exposure to Environmental Pollutants, by Neil E. Klepeis and others, and published by the Lawrence Berkeley National Laboratory in 2001;
Wallace, Lance A., et al. Total Exposure Assessment Methodology (TEAM) Study: Personal exposures, indoor-outdoor relationships, and breath levels of volatile organic compounds in New Jersey. Environ. Int. 1986, 12, 369-387.