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 the occupational and environmental hygiene sciences as we know them today. He was instrumental in stopping the cholera epidemics in Munich, 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 carbon dioxide measurement 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.3
He seems to have forgotten 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 во pascals is many times lower than the pressure required in his candle-extinguishing experiment (between 700 and 10,000 pascals).
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. b 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 Ecology Neubeuern 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.0
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 re
wall made of unprocessed materials through which the flow of vapor is unhindered is called a breathing wall. The concept is central to the Building Biology goal of creating a healthy dwelling. (Tire term “breathing wall” is really a misnomer because the walls of course are not the primary ventilation source for the building. The term “vapor-permeable wall” would perhaps be more accurate.) Another Building Biology concept is that of the building envelope being our third skin (clothing being our second skin). This analogy is a more useful one in describing how a breathing wall works. Our skin is the organ of contact with the outer environment and regulates the balance of moisture and temperature of the body in relation to the environment. Skin must remain permeable to facilitate a healthy interaction between the natural environment and the human organism. So too, according to Building Biology,
ferred 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 rolein maintaininga healthy inner 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.
A constant supply of oxygen-rich air and the reduction of carbondioxideareessentialtoa healthy indoor climate, but it is a misconception that walls can "breathe" air, especially massive walls built from earth, masonry, or solid wood, 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 such a massive wall. 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 the Building Biology recommended rate of about one complete change of air per hour, either mechanical ventilation with a heat recovery system or cross-ventilation through open windows several times a day is necessary. Massive wall systems are especially well suited for natural ventilation methods because their extraordinary heat storage capacity keeps heat loss at a minimum during brief opening ofthe 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. d
It is true that wall structures without vapor barriers allow for the free flow of moisture or water vapor. Moisture always moves from a warmer area to a colder one, from a higher vapor pressure concentration to a lower 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.
must our third skin, the walls of our dwellings, remain permeable in order to achieve an optimal environment for health.
There is an intimate connection between the health of an individual and the health of the environment. All building processes involve the extraction of raw materials from nature and the disruption of the natural ecosystem. The alternative materials and methods described below use these materials in a minimally processed state with far less environmental impact than the highly refined and processed materials prevalent in conventional construction. When one considers that 40 percent of the material resources entering the global economy are related to the building industry,2 it becomes clear that the building material choices we make have a global impact on the health of the ecosystem, the ultimate determinant of our own health.
In recent years, with renewed interest in environmental concerns and energy
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 in the wall. This, of course, works only as long as all wall finishes are also highly permeable to water vapor.
The actual amount of water vapor an exterior wall can shuttle to the outside of a building is rather low. For example, 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. e Again, it is obvious that the majority of the moisture that is usually generated inside a home needs to be removed through active ventilation, using windows and/or mechanical ventilation systems.
Building and finishing materials with a high moisture buffer or hygric capacity improve indoor air quality tremendously because they help mitigate temporary humidity highs. Nearly all natural building materials are highly hygroscopic, especially wood, earth, lime, and cellulose. Lime plaster (13 grams per square meter) or clay plaster (30 grams per square meter) can absorb large amounts of water vapor. But as soon as you finish a lime plaster with a standard latex paint, water vapor absorption drops (to below 9 grams per square meter).f Therefore it is important to choose surface treatments that are highly permeable to water vapor, such as lime wash, silicate, or casein paint. Note that this moisture buffering effect relies on only the first 1 to 1.5 centimeters of the interior wall surface. Thus almost any wall structure can benefit from the moisture buffering effect of adding a material such as a clay plaster.
It is unclear why the breathing wall concept persists when it is riddled with 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, which often makes no sense from a building science point of view, Building Biology favors the so-called flowthrough design, which allows water vapor to pass through the wall assembly’s components without
efficiency, several alternative methods of building have enjoyed a limited renaissance among environmentally concerned homeowners, designers, and builders. Since the last edition of this book in 2001, the negative impact of human activity on the global environment has become increasingly evident, and in our efforts to lessen this impact the green building movement has experienced exponential growth. The Bau-Biologie or Building Biology study course states that “there is almost always a direct correlation between the biological compatibility and ecological performance of a given building material.” This statement is exemplified in the proper use of natural, minimally processed, and locally found and crafted building materials.
