THE BUILDING’s SKIN
A well-designed building skin is like a down jacket: there is an outer and an inner layer and in between is the insulation.
Every building needs an outer and an inner skin in order to stay tight and keep wind and weather out and comfort in. The outer layer is known as the wind-tight level and the inner layer as the air-tight level.
The wind-tightness of the wall is achieved by the external rendering or by boards or wind and rain-tight adhesive membranes on ventilated constructions. Façades with open gaps and glass façades are furnished with permanently UV-resistant façade linings.
Wind puts the house under constant pressure. The proverbial tornado from the socket outlet impacts the comfort of a room considerably. Air drawn in through leaking gaps moves, as it is heavier, to the deepest point in the room, that is the floor. Permanent cold feet are the result – a very unpleasant feeling. In this case not even good insulation values to the cellar help if air from outside can enter through cracks in the building’s skin. In addition, the ingress of moisture into the construction can result in structural damage and even the sound protection is reduced.
Wind-tightness tested to standards
The wind-tightness of a building is stipulated in directive 6 of the OIB (Austrian Institute of Construction Engineering). In Germany the design of the underroof is regulated in the ZVDH (Central Association of the Roofing Trade) directive.
On the outside the construction must be protected from influence of the elements. On the inside the aim is to prevent the room’s moisture from entering the construction and the insulation at random. The air-tight layer is usually on the so-called warm side of the exterior construction elements. In solid constructions it is usually the indoor plastering that adopts this function but in wooden constructions vapour-retarding sheets are used, for example. The air-tight design of the building’s skin is set out in standards and directives, and for a very good reason. Besides draughts and inferior air quality, poorly designed air-tight levels can result in damage to the construction. When air from the interior passes uncontrolled through gaps into cooler regions, moisture condenses there and mould and rot can easily form.
Where does water vapour come from and where does it go?
The house occupants produce moisture in the rooms every day: through breathing, sleeping, cooking, showering… A household of 4 people can emit up to 10 litres of moisture into the room’s atmosphere in the course of a day. Part of this is removed through airing but the water vapour left in the air, similarly to the warmth, tends to seek its way via the exterior construction components to the outside. Warm air can retain more moisture than cold air (e.g. at a temperature of 20°C 17.3g/m³ and only 2.14g/m³ at -10°C.) If warm air cools too quickly it produces moisture and condensation occurs. By the way: anyone who has ever taken a bottle of beer out of the refrigerator is familiar with this principle.
Diffusion – Convection
The warmer and moister the air indoors, the higher the vapour pressure. When a building component or a layer of a component separates two rooms with different temperatures and humidity, there is varying water-vapour partial pressure on both sides. Water vapour moves through building material with this difference in pressure. This is known as water vapour diffusion. The path and direction are always from warm to cold. A much greater part of humidity escapes through leaks due to air currents. Whereas only 0.3 litres of water escape through 1m² of wall or ceiling during the heating period, up to 30 litres of water pass into the construction through a 20 cm long and 2m wide crack.