Breathable structures

The hygroscopicity and moisture buffering of materials should not be confused with breathable structures. The term breathable structure refers to a building with an outer structure that allows the partial pressure equalisation of the gases in the air by diffusion through the structure. The term breathing does not mean the free flow of air through the structure. That is always considered to be a construction error. Even a breathable structure needs to be air-tight. 

The health impact of breathable structures is based on the idea that the carbon dioxide inside is not only removed through ventilation but that it also exits through the structure while oxygen enters correspondingly. The challenge for structures in the Finnish climate is that water vapour is one of the gases in the air. Because the movement of water vapour plays a key role in a building’s physiology, vapour barriers are part of building design and water vapour permeability is part of the classifications of structures. 

The diffusion of water vapour in single-material structures is not a problem because these structures do not have surfaces where moisture could condense in certain circumstances. Nearly all contemporary structures have layers, which means that the condensation of water vapour must be prevented. The water vapour resistance of the inner surface must always be about five times that of the exterior wind protection plate, even if no actual vapour barrier is used. The good thermal insulation of the wind protection plate is also an advantage to a breathable structure. 

Advantages of breathable structures:

• allows better drying of the structure inside
• reduces indoor humidity in the autumn
• removes the carbon dioxide generated by people breathing from air inside
• permeable structures together with hygroscopic, moistureabsorbing organic insulation (pulp fibre, linen, sawdust) stabilise humidity fluctuations in the indoor air
• water damage to the structure, such as from a leaking roof, is easier to detect
• structures operating “the wrong way” (for example, unheated summer cabins where the indoor air is cooler sometimes than the outdoor air) do not cause problems

Potential risks of breathable structures:

• risk of mould and moisture damage if too much moisture accumulates in the structure
• possible mould spores on the outside can get inside if there is significantly less pressure inside the house than outside
• when the thermal insulation lining is wet enough, its insulating properties are reduced (the effect is a few per cent)
• wet wind protection plates deform more, which reduces the seal on the structure
• excessively dry indoor air in the winter 
• paper-based vapour barrier membranes are more susceptible to tearing during installation or during the building’s lifetime
• gaseous impurities (for example, radon) pass through the structure
• potential corrosion of metal fittings near the outer surface
• internal moisture can leak into structures

In practice, gases very seldom pass through the entire structure. “Breathable” interior and exterior surfaces, which is another way of describing the ability of the structure’s surface structures to stabilise the humidity, have far more relevance. For example, the breathable surface (= wettable, moisture absorbing) absorbs the elevated humidity in the residence at night and releases it during the day, which makes the indoor air more pleasant to breathe all day round. This ability is by no means restricted to the external walls: in fact, it is also advantageous to add intervening walls to balance out heat and humidity.