First of all, the basic materials here are described in various articles as adobe, loam, clay or mud.

The important thing is that any bricks made form the materials are not fired in a kiln, but are air-dried

The use of the materials can be in the form of bricks for building walls or as a plaster for rendering interior walls as an alternative to things like gypsum plaster.

The context seems to suggest that the latter is the case. (quote: "bearbeitete Wände")

Temperature regulation:

"In the case where unfired clay is used in the form of bricks this material forms a good body of thermal mass, which helps to even out temperature swings. However, even a 15mm plaster coat has significant thermal mass to store heat."

Humidity/moisture regulation:

"The clay plaster had such a strong ability to absorb peaks of air moisture after showers that it cleared the air without surface condensation. The effect of the extractor fan was of no statistical significance."

Here we come back to breathability, which does turn up now and again in articles on the subject, so it is not exactly wrong to use the expression:

Breathability and associated health benefits:

Healthy, durable, working buildings can only be brought about by designing with a full understanding of breathability; (1: May, Neil April 2005). The current focus on airtightness in design needs to also consider how vapour inside a building will be treated. Clay plasters, made from unfired clays and sands, are considered breathable (with excellent vapour permeability) and hygroscopic. Unfired clay can absorb and desorb indoor humidity faster than any other building material (2: Minke, G. 2006). Clay plasters regulate relative interior humidity between 40% and 70%. (3: Arundel, A. V. 1986) By keeping RH between 40% and 70% research has shown that the likelihood for airborne infectious bacteria and virus to survive is the lowest. Keeping RH between 40 and 60% also prevents building materials from off gassing toxins, such as formaldehyde. (3: Arundel, A. V. 1986)


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Note added at 2 days8 hrs (2013-12-06 22:57:36 GMT)
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If you want it to sound "prettier" and more saleable, I suggest "natural clay rendering" or "natural clay plaster" for "Lehm" (clay is natural anyway, but it sounds more like marketing speak.



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Note added at 2 days8 hrs (2013-12-06 23:09:01 GMT)
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If you wish to use the expression "climate-regulating" (as Jochen correctly suggests), then I suggest "room climate regulating natural clay plaster", as the context appears to concern a clay plaster/render for inside walls (bearbeitete Wände) - the outside walls are already described as being exposed natural stone (freigelegte Natursteinwände), probably previously rendered with concrete.