Most of the world's energy consumption is used for heating, and a large proportion of this is used for heating buildings. To minimize this energy consumption for the heating, the Energy Saving Ordinance stipulates that heat losses via walls, ceilings, and floors must be limited.
This can also be achieved in existing buildings by installing additional thermal insulation on the outside, e.g. on the exterior walls (see diagram). This makes a significant contribution to climate protection in terms of reducing carbon dioxide and other exhaust gases.
Limitation of heat losses due to additional thermal insulation
In the following, the example of a heated building is used to qualitatively illustrate what must be considered when planning a wall construction with regard to moisture development. This can be illustrated by means of a glazier's diagram.
Moisture in parts of the building significantly reduces thermal insulation. Who has not already experienced the freezing effects of drenched clothing. This effect also occurs, for example, when a rainproof coat prevents body moisture from evaporating. Condensation causes the clothes to soak through, and moisture condenses on the inside of the rainproof coat.
Similar effects also occur in an improperly constructed wall structure, for example, when vapor-impermeable layers are placed on the outside.
Persistent moisture in building constructions leads to a reduction of thermal insulation, persistent moisture penetration of walls leads to uninhabitability. The formation of mold and spores is toxic and hazardous to health.
Section through a wall with external thermal insulation -
External Thermal Insulation Composite System
A proper structural design (see graphic) can prevent persistent moisture and thus structural damage in walls, ceilings, and floors.
Thermal insulation ETICS of an external wall -
Transition from the ground floor to the basement
Basically, according to the Energy Saving Ordinance and DIN standard 4108, the calculation of the minimum thermal insulation is required.
On this basis, the moisture development, caused by the moisture flowing into the wall construction from the interior, needs to be verified by means of the Glaser method.
Glaser diagram: water vapor saturation pressure
Adapted from: projekt-baudenkmal.de
The Glaser diagram shown schematically represents the section of a wall from inside to outside.
The water vapor pressure is shown on the vertical axis. For this example, an interior temperature of 20°C and an exterior temperature of -10°C are assumed.
At a relative humidity of 100%, a vapor pressure of approx. 2300 Pa is generated on the inside and approx. 260 Pa on the outside. Due to this vapor pressure gradient, the water vapor diffuses into the wall and condenses here to some extent, especially in the cold season.
The horizontal axis in the graph indicates the Sd values. They correspond to the equivalent diffusion resistance of the air. It is a measure of the resistance of the building component to the passage of water vapor.
Adapted from: projekt-baudenkmal.de
The Glaser method assumes an average relative humidity:
inside 50%, outside 80%.
Example Inside 50% of 2338 Pa equals 1169Pa.
These water vapor pressure values are re-determined by calculation and entered in the coordinate system.
At the layer boundary between the insulation and the synthetic resin plaster, the red and blue lines (see graphic) come into contact and cause condensation to form. According to a calculation method, mT = 0.4055 kg/m2 develops.
During the evaporation period in summer, this construction could evaporate mV = 0.934 kg/m2. This construction is permissible according to DIN 4408. The high vapor diffusion resistance of the exterior plaster has a retarding effect on the drying out of the wall.
Ventilated facade
A structural-physical optimization of this wall construction can be achieved by a rear-ventilated facade, see figure "Rear-ventilated facade".
The verification shown by the glazier diagram only takes into account the penetration of moisture by diffusion of water vapor from the inside into the wall construction under normal conditions of room humidity. Extremely occurring room humidity over a longer period of time or wetness, e.g. due to splash water, is not considered.
It should be noted that the glazier diagram refers only to surfaces with the specified building construction. Structures that have other building material layers must be analyzed separately using the Glazier method.
Computer programs can be used to perform the evaluation with little effort and at low cost. Architects and civil engineers offer the necessary service here.