Internal insulation of exterior walls is always used when outside insulation of the facade is not possible. It is necessary, for example, in the energetic renovation of listed buildings, where the external appearance may not be changed. In addition, she also plays a role in other refurbishment projects - for example, for as economical as possible refurbishment of old buildings or for partial renovations.
Disadvantages of interior insulation: risk of condensation damage and thermal bridges
In terms of building physics, interior insulation of exterior walls has some disadvantages. On the one hand, so-called thermal bridges can not be eliminated by this method. Thermal bridges are areas of the building envelope that have a significantly lower thermal resistance than their neighbors on walls or ceilings, resulting in a stronger local heat flow. Moist room air can also condense on these cooler spots, resulting in damp spots or mold growth. In addition, the risk of condensation damage is also within the entire wall structure: The internal insulation prevents room-side heating of the outer walls, on the insides of which very low temperatures can exist. At a relative humidity of over 50 percent and surface temperatures below 10 degrees Celsius, condensation can form between the inner insulating layer and the outer wall. For the sake of completeness, it should be mentioned that the usable area of the so refurbished rooms is reduced slightly by internal insulation.
Incidentally, the danger of condensation damage is not only present in the cold season, but also in the summer months. In physics, this process is referred to as reverse diffusion: It occurs when moist-warm air from the outside contains significantly more water vapor than the cooler indoor air. The moisture then diffuses from the outside to the inside - compared to winter in the opposite direction - through the walls and condenses on inner colder layers, ie between the outer wall and Innendämmschicht.
Effective interior insulation with capillary-active material
Modern interior insulation systems today ensure that the disadvantages of the process can be largely eliminated if the renovation is professionally carried out. Experts now assume that an airtight condensation barrier ("vapor barrier") in the long run is not available. Material defects, still "working" wall areas or difficult to seal areas - for example in the area of supply lines, windows or doors - limit their effectiveness. The alternative is to store the consensual water in harmless form and to dry in the warm season. The building substance or the internal insulation itself must have certain material properties.
Moisture-resistant and capillary-active building materials distribute the condensation and direct the moisture to the surface of the insulation layer, where it can dry. A completely air-tight vapor barrier proves to be counterproductive, whereas a moisture-variable internal insulation layer is optimal. If the outer wall to be insulated is made of a capillary-conductive material, such as soft-fired bricks, its capillarity must be preserved. Inner barrier layers through water-impermeable paints or cement plaster must not be created in such cases. If the exterior wall to be insulated from the inside is not able to absorb the condensation in innocuous form, this task must be taken over by the insulating materials. Among other things, wood or calcium-silicate plates are well suited for this, which, in the case of direct and large-area contact with the outer wall, cause the diffusion behavior of the areas to be insulated to change.
In any case, professional interior insulation of exterior walls belongs to experts who are familiar with the latest developments in this construction segment and who are able to put them into practice. In this case, interior insulation can be an economical as well as energetically effective method for sustainable renovation.