Ventilation and airtightness - a love affair?

Have we created a problem when we plugged the holes in the envelope of our buildings? Instinctively, we credit cracks in the fabric of our houses with the provision of much-needed fresh air. But, let’s think of a rubber dinghy. If we perforate the tubes, we know they will deflate as very little air will flow back. Instead of letting air in, the leaks will make the boat unfit for its intended purpose. Gaps in the building envelope should be perceived similarly to punctures in the inflatable.?

In the following, I will locate indoor air quality (IAQ) and humidity issues in a historic context before discussing ventilation needs, and eventually arriving at empirical research data for a reality check.?

Up until the late modern period, breathing was seen as necessary only to cool the heart. The causal link between indoor air and poor health emerged in the 19th century, culminating in inquiries and scientific initiatives in many countries. At a House of Commons Select Committee in 1835 England, George Birkbeck testified that he never encountered a building that was well-warmed and ventilated. Asked whether he attributed the lack of performance to the “want of practical knowledge on the subject”, he replied:

“I do; heating and ventilation, especially the latter, seldom entering into the mind of the builder when he projects his building; he begins as if he did not know that ventilation could be necessary; he trusts to the doors and windows, to neither of which belongs the business of ventilation. The doors admit the occupants to the chambers; the windows the light; and apertures ought to be introduced to admit air for ventilation as regularly as the other openings.” 1

A few years later in Germany, eminent academic Max von Pettenkofer suggested that natural ventilation may not be suited to deal with the apparent problem of poor IAQ:

“A further reason to insist on clean air in apartments is the knowledge that bad air is the source of many chronic ills, and that it certainly plays a part in the evils that plague the nation: scrofula, tubercles, etc. Thus, where the natural ventilation is insufficient to prevent an increase of the carbonic acid content of the air in our living and bedrooms below 1 per thousand, artificial ventilation has to be employed.” 2 (Translation K. Rosemeier)

Long before airtightness surfaced as a design task, German engineer Haase lamented the IAQ of residential premises:

“At every turn, one still encounters rooms and entire buildings in conditions in which no one is able to feel comfortable. Numerous tenement blocks, whose poor ventilation already makes itself noticeable to the olfactory nerves of the passers-by, are not among the healthiest facilities by far.” 3 (Translation K. Rosemeier)

Leaping into 20th century New Zealand, a study focused on homes with gas appliances?. 45 houses, mostly built before 1970, in Auckland, Taupo, and Rotorua were investigated for one night. Nitrogen oxides were found elevated in all houses with unflued gas heaters, and concerning levels of carbon monoxide were detected in some dwellings. High carbon dioxide levels were ubiquitous throughout the sample. Formaldehyde concentrations exceeded the referenced World Health Organization's (WHO) recommendation at the time (0.1 ppm) in two cases, and the referenced level of concern (0.06 ppm) was breached by a further three measurements. Respirable particle thresholds were crossed in five houses.

My research found incidences of inadequate IAQ in all 15 houses without whole-house ventilation built after the year 2000. These houses were not particularly airtight, with a median n?? value of 6.5/h ?.

Considering the dampness of houses, a similar picture arises. Abel ? reported from a survey in the Swiss city of Bern in 1896, where 5% of all rooms were found to be damp. New Zealand research from the 1940s and 1970s ?'? attests that dampness and mould occurred in a large number of New Zealand houses. Bastings, in 1947, already explicitly identified inadequate ventilation as a contributing factor, and in a later publication? blames the absence of chimneys in all rooms of the “modern” house, and the resulting reliance on leaks, as one cause of the malaise.

In 1957 in Germany, Schüle1? listed 8% of living rooms, 33% of bedrooms, and 28% of kitchens out of 733 researched rooms as damp. 46 years later, Brasche et al 11 examined 5,530 homes and found 1,827 units to some degree damaged by moisture. Roughly 30% of the sampled dwellings employed some form of mechanical ventilation and were found to be significantly less damp. But more strikingly, homes that had properly sealed windows were fairing a good deal better than their counterparts with no seals in the windows.?

While the presence of window seals does not determine the airtightness of a building, it is often alleged that their addition changes a building’s ability to manage indoor moisture. Clausnitzer12 inspected 920 older apartments in Germany. 26% of these had perceptible mould. For kitchens, he found that the occurrence of seals in windows was indeed in line with a moisture build-up there, but for windows in the other rooms of the apartments, the link did not hold.

While none of the before-mentioned studies measured the airtightness of the sample buildings, it is a fair assumption that all of them were rather air-leaky. New Zealand houses, for example, were in the 1950s on average 2.5 times leakier than in the early 1990s 13.

If an airtight building envelope was responsible for poor moisture management of dwellings, historic reports of dampness were hard to explain. While the airtightness of buildings and their use, for example, the integration of laundries and bathrooms, has changed over time, the disregard for the proper ventilation of houses remains a constant.

Recommendations for a health-based per-person air change rate are in the range of 7-8 L/s. Only a small fraction of this air will be inhaled. We need about 60 times more air than the amount we breathe for a sufficient dilution of air pollutants.?

If we were to dimension a hole in our building envelope large enough to meet our ventilation requirements on a calm day, we would be looking at an opening the size of a small window. But in contrast to a window that seals well, we cannot regulate the airflow through leaks, adapt to changing wind speeds, and exclude nuisances such as noise and insects from the indoor environment. Moreover, as wind is the main driver for air movements in low-rise buildings in milder climates, our hole would have to move in the facade with the prevailing wind for good effect.

Calling the airflow through leaks “background ventilation” is, therefore, a misnomer, as it does not deliver a reliable baseline for air exchange. And whole-house air change rates are only half the story. I can have a very well-ventilated laundry, and therefore a high overall air change rate for my dwelling, and still have insufficient fresh air delivered to my bedroom. Fresh air in adequate amounts needs to get to the breathing zone where people dwell – leaks cannot guarantee this. Lastly, leaks may induce exfiltration of air, depending on their distribution, height in the building, and the prevailing wind, adding the potential of damaging the structure via offloading moisture on cold surfaces on its way to the ambient.?

When air leaks through the building envelope, not only the quantity of it but also its quality is questionable. It is likely that air that trickles into our homes picks up pollutants and fibres en route. In addition, the outdoor air may not be pristine either, and filtration of air coming in through leaks is imperfect at best.

To dilute pollutants, fresh air needs to be of significantly better quality than the stale air in the enclosure it replaces, which is not guaranteed for air of uncertain origin.

It has to be noted here that many of the issues associated with leaks in the building envelope pertain to intentional holes such as windows, doors, and trickle vents in the same way. Noise, smells, outdoor pollutants, insects – not to mention larger animals, can enter the indoor environment with every ventilation event, and without heat recovery, comfort is compromised. Many people will trade warmth for poor IAQ in winter1?.?

To test the relationship between air leaks, air change rates, and pollutant concentration, 15 homes built after the year 2000 without whole-house mechanical ventilation were investigated in three locations in New Zealand. This research was supported by BRANZ from the Building Research Levy.

Looking at Figure 3, the columns for air leakage and air change rates are ill-correlated. Knowing a value for one does not allow us to predict the other.

A range of indoor air contaminant concentrations was measured over the course of a week. While recommended thresholds for all contaminants were exceeded occasionally, most prominently, CO? concentrations were crossing the 1,000 ppm threshold regularly, and reached an almost tenfold peak (Figures 4 and 5).

Results for house AKA were contrasted with an airtight building of similar size, same occupancy rate, and location. The latter employs a balanced ventilation system with heat recovery. Figure 5 clearly illustrates that the ventilation mode determines IAQ outcomes, rather than air leaks. This is further corroborated by research on airtight (n?? from 0.29-3.33 h?1) new houses with mechanical heat recovery ventilation systems in France, where indoor air contaminant concentrations were lower than the national average, apart from volatile organic compounds related to new construction, which however declined over the time of the testing 1?.

To assess the impact of air leaks on the moisture content in the sample houses, temperature, and relative humidity were recorded over one week in winter, and the water content in indoor air was calculated from averages.

In the air-leakiest home, AKD, a slightly higher water content in air was recorded than for the three times tighter home DUA. Even when locational factors are excluded (houses with identical first letters are in the same location): house DUA is drier than DUD with more than twice the amount of air leaks.?

For ventilation effectiveness, neither air leakage nor air change rates are helpful indicators, as success will be established by the degree of mixing in the enclosure, and the extent to which fresh air provision and occupancy are matched.

Conclusion

Airtightness should rather be viewed as a supporting factor in our quest for good IAQ? than a reason to worry.

The history of indoor air quality and dampness of dwellings teaches us that conditions in our homes were dire long before houses were anywhere near airtight. Negative side effects of an increased level of airtightness fail to manifest in field research, where factors other than airtightness are far more important in determining outcomes. Changing the airtightness of a building is of no consequence to the ventilation requirements. Buildings need a ventilation concept, already requested by Birkbeck in 1835. Leaving the provision of fresh air to chance is not acceptable!

With a ventilation concept in place, airtightness of the building fabric gains importance, as air entering or exiting through leaks may well lead to short-circuiting, poor mixing of air and suboptimal acoustic outcomes. Moreover, if mechanical ventilation with heat recovery is used - which is desirable for energy efficiency and comfort - the recovery rate will suffer from air bypassing the heat exchanger. While the need for ventilation is unrelated to the level of airtightness of the building envelope, the effectiveness of ventilation hinges on it. An airtight building envelope is an ally when good indoor environmental quality is the goal. But regardless of whether we live in a draughty old villa or a hermetically sealed box: we need to design for adequate ventilation!

References

1. Report from the select committee (of the House of Commons) on the ventilation of the Houses of Parliament; with minutes of evidence. (House of Lords, 1835).
2. Pettenkofer, M. J. über den Luftwechsel in Wohngeb?uden. (Cotta, 1858).
3. Haase, F. H. Lüftungsanlagen im Anschluss an die gebr?uchlichen Heizungssysteme. Polytech. J. 277, 597–612 (1890).
4. Bettany, B. L., Chauvel, R. B. & Edmunds, C. J. Domestic indoor air quality : a survey of the impact of gas appliances. (Domestic indoor air quality : a survey of the impact of gas appliances, 1993).
5. Rosemeier, K. Healthy and affordable housing in New Zealand: the role of ventilation. (The University of Auckland, 2014).
6. Abel, R. Die Entwicklung der Gesundheitstechnik w?hrend der letzten 50 Jahre und ihre Einwirkungen auf die Gesundheitsverh?ltnisse in Deutschland. 50, (1927).
7. Bastings, L. Dampness and mould in modern houses. N. Z. J. Sci. Technol. B Gen. Sect. 28, 195–227 (1947).
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10. Schüle, W. Feuchtigkeitssch?den in Wohnungen. (Forschungsgemeinschaft Bauen und Wohnen, 1957).
11. Brasche, S., Heinz, E., Hartmann, T., Richter, W. & Bischof, W. Vorkommen, Ursachen und gesundheitliche Aspekte von Feuchtesch?den in Wohnungen. Bundesgesundheitsblatt - Gesundheitsforschung - Gesundheitsschutz 46, 683–693 (2003).
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13. McNeil, S. et al. Trends in Airtightness of New Zealand Homes. in (2011).
14. Bruce-Konuah, A. Window use in single person offices: do occupants control personal ventilation to provide adequate IAQ? in Proceedings - Healthy Buildings 2012 (Queensland University of Technology, 2012).
15. Derbez, M. et al. Indoor air quality and comfort in seven newly built, energy-efficient houses in France. Build. Environ. 72, 173–187 (2014).