A diffusion-free wall structure aims at a sd value of less than 1.0 m of equivalent air layer thickness, according to the researches of building biology institutions (see Dümmer 2013 in References). The smaller the value, the lower the resistance which counteracts the moisture transport. This makes certain statements on the gas exchange:
Gases have the tendency to create a concentration equalization. Consumed air aspires to fresh air and vice versa. The gases meet in the wall and thus become the air and heat exchanger. It would be interesting at this point to scientifically verify how much gas and how much heat flow into which direction.
For which insulation material and insulation thickness are the most efficient results to be achieved?
This relationship has been established in experiments at the Institute of Building Biology in Neubeuern. It is thus possible, by means of the low air resistance of the construction, to effect the passage of oxygen into the interior of the building when an oxygen consumer is active. It comes to the gas exchange. A candle can not be extinguished if an SD value below 1.0 m illuminates a 1 cbm space. For a 24 cm thick wall of our HarKun design, the sd value is below 1.0 m.
Diffusion and capillarity are simultaneously and jointly effective for a dry wall.
An example: A damp sponge in a wooden crate dries badly, even if air flows around the crate. If the crate is opened on two opposing sides, a vacuum is produced by evaporation on the surface in the damp sponge, and the water reclines. At the same time, trailing air permeates the moist millieux, is saturated with moisture and transports it outwards, which leads to rapid drying. If the airflow changes direction, the water and the air can escape to the other side. However, if one side of the wooden box remains closed, like a steam brake on the inner side of the outer wall, this suction does not find its quick compensation. The drying of the outer side by the wind causes the capillary flow to depart and the water lying further inside evaporates even more slowly. Another steam brake is created on the outside. The same happens in the opposite direction when the temperature and humidity conditions are reversed. Warm outside air pushes the moisture into the cooler interior and there it meets the steam braking layer. For these reasons, the conventional designs with the steam and wind braking layers are very sensitive.
Blowerdoor tests – a vacuum is generated in the building – can show any defects or leaks. These must absolutely be eliminated, since building damage must be avoided. The wall, floor, roof, window and door connections require particular attention during the processing of the adhesive sheets and films.
The variant of diffusion and capillarity on the one hand and of “open wooden box” made of natural building materials, on the other hand, has proved its worth for centuries. It is less sensitive, more robust and allows certain leaks.