The breathing wall concept: misconceptions
By Katharina Gustavs
Published in Prescriptions for a Healthy House by Paula Baker-Laporte
The breathing wall concept goes back to Max von Pettenkofer (1818–1901), one of the most accomplished hygienists of his time and the pioneering founder of occupational and environmental health sciences as we know them today. He was instrumental in stopping the cholera epidemics in one of the largest cities in Germany during the second half of the 19th century. By initiating the construction of a central water supply and sewage treatment system, he greatly improved public health and achieved celebrity status.
In his dedicated search for better living conditions, von Pettenkofer introduced the measurement of carbon dioxide as an important indicator of overall indoor air quality. His measurements of air exchange rates in a room with brick walls, a masonry heater, and sealed windows led him to hypothesize that the brick walls must let air pass through. Even with the keyhole and other cracks sealed, the air exchange rate dropped only about a quarter compared to the rate prior to sealing. (1)
Somehow he forgot to consider the effect the masonry heater would have on the ventilation rate. Thus he proceeded to demonstrate that when air is pumped through a brick cylinder, sealed on the outside except for both ends, a candle flame at the other end could be extinguished. In his eagerness to prove his hypothesis, he overlooked the fact that the maximum natural air pressure across a wall of about 30 Pa (pascals) is many times lower than the pressure required in his candle-extinguishing experiment (between 700 and 10,000 Pa).
Von Pettenkofer’s celebrity status may have been one of the reasons his hypothesis of natural ventilation through walls was not scientifically debunked until the 1920s. (2) Though he never used the term “breathing wall,” this concept took on a life of its own that continues to this day. The Institute of Building Biology and Sustainability IBN in Germany recommends avoiding the use of the term because it does not reflect the reality of the complex processes occurring in a wall and usually leads to misconceptions. (3)
In Building Biology, a natural home is considered to be a living organism in the sense that it should be — as much as possible — self-sufficient, energy-efficient, and built from materials that are part of the natural cycle and do not contribute to toxic waste. The roof and wall systems are often referred to as our third skin, implying that, just like human skin, the building envelope is in constant contact with the environment and plays a crucial role in maintaining a healthy indoor climate despite unfavorable weather conditions outside. Let us have a closer look at what does or does not permeate a wall with regard to air and moisture.
Air exchange
It is true that a constant supply of oxygen-rich air and the reduction of carbon dioxide – mainly generated by breathing occupants and pets – are essential to a healthy indoor climate. As discussed above, it is a misconception to assume that walls can “breathe” air. Despite their varying degrees of porosity, the air pressure difference between outdoor and indoor air is never high enough to promote an air exchange through exterior walls, especially massive walls built from earth, masonry, or solid wood. If air does get through a wall, it is not through the wall itself but through poorly sealed joints and cracks. This, however, is the least desirable way to supply fresh air because it promotes high heat loss in winter, makes for very unpleasant drafts, and invites moisture problems.
To ensure that the total volume of air in a given room is replaced completely with fresh outside air once per hour (4) – as recommended by Building Biology – either mechanical ventilation (preferably with heat recovery to save energy) or cross ventilation through open windows several times a day is necessary. Massive wall systems, including mass timber or solid wood wall systems, are especially well suited for natural ventilation methods because their extraordinary heat storage capacity keeps heat loss at a minimum during any brief opening of the windows in winter.
It is interesting to note here that human skin does not breathe air either. All oxygen for our inner organs is supplied by the air inhaled through the nose and mouth, which in keeping with the analogy of the third skin would be comparable to the windows and doors in a house. Though the outermost layer of our skin (up to 0.4 millimeters) can extract oxygen from the ambient air, the oxygen does not cross into the body. (5)
Moisture transport
It is true that wall assemblies without conventional vapor barriers allow for the free flow of moisture or water vapor. Since warm air can hold more moisture than cold air, moisture always moves from a warmer area (higher vapor pressure concentration) to a colder one (lower vapor pressure concentration). As a result, water vapor tends to flow from the inside out in the north and from the outside in down south. In mixed and moderate climate zones, it has a tendency to flow from the inside out during the winter and from the outside in during the summer. Smart vapor retarders (e.g. from Pro Clima) can change the rate at which water vapor passes through them, depending on ambient humidity levels. Their so-called permeance is low in winter, when it is dry, and high in summer, when it is humid.
Massive wall systems made from earth, clay, or solid wood also have a high capillary activity that is capable of wicking away liquid water. Though any wall system should be designed to prevent vapor condensation from occurring, the wicking capacity of natural building materials provides additional insurance that liquid water will not get trapped inside the wall. This, of course, works only as long as all wall finishes are also highly permeable to water vapor such as lime, silicate and clay paints, as well as natural oil finishes.
The actual amount of water vapor an exterior wall can shuttle to the outside of a building is rather low. In winter, when outside temperatures are low in northern and moderate climates, only about 1 to 2 percent of the indoor moisture can make it through a brick wall. (6) Again, it is obvious that the majority of the moisture, which is usually generated inside a home, needs to be removed through active ventilation, using windows and/or mechanical ventilation systems.
Moisture buffering
It is true that building and finishing materials with a high moisture-buffering capacity improve indoor air quality tremendously because they help mitigate temporary humidity highs. Many natural building materials are highly hygroscopic and can absorb large amounts of water vapor, especially wood, clay, and lime.
Untreated wood, for example, can absorb 60 grams of water vapor per square meter, clay plaster between 50 and 70 g/m2, and lime plaster about 30 g/m2. As soon as a surface is covered with standard latex paint, however, water vapor absorption drops to below 10–20 g/m2. (7) It is therefore important to choose surface treatments that are highly permeable to water vapor, such as lime, silicate and clay paints, as well as natural oil finishes. Using no finish at all is another option.
Note that this moisture-buffering effect relies on only the first 1 to 1.5 centimeters (about half an inch) of the interior wall surface. Thus almost any wall assembly can benefit from the moisture-buffering effect of solid wood paneling or lime or clay plaster.
It is unclear why the breathing wall concept persists when it is riddled with so many misconceptions. What is clear, however, is that any building envelope has to meet two major challenges: first, not to let any water in, and second, if water does get in, to let it out again. In contrast to the widespread use of polyethylene vapor barriers, Building Biology favors the so-called flow-through design, which is transparent to water vapor, but keeps the wind out and protects against condensation.
Last updated 11 November 2019
References
Max von Pettenkofer. Über den Luftwechsel in Wohngebäuden. Literarisch-Artistische Anstalt der J. G. Cotta’schen Buchhandlung, 1858.
Erwin Raisch. “Die Luftdurchlässigkeit von Baustoffen und Baukonstruktionsteilen.” Gesundheitsingenieur. Issue 30 (1928).
Winfried Schneider. “40 Jahre Baubiologie – Klischees, Innovationen, Trends.” Wohnung und Gesundheit. Vol. 120 (2006), pp. 12–14.
This recommendation for healthy indoor air should not be confused with blower door measurements, which are performed at an artificial pressure differential to determine air leakage (ACH50).
M. Stücker et al. “The Cutaneous Uptake of Oxygen Contributes Significantly to the Oxygen Supply of Human Dermis and Epidermis.” Journal of Physiology. Vol. 538 (2002), pp. 985–994.
W. Schneider and A. Schneider. Building Biology Assessment of Building Materials and Building Science. Course Module 7 of Building Biology Online Course IBN, 2014.
Moisture uptake of building materials with an equilibrium moisture content of 50% within twelve hours when exposed to an ambient relative air humidity of 80%. Ulrich Röhlen and Christof Ziegert. Lehmbau-Praxis: Planung und Ausführung. 2014, p. 40–41.
