Compared to external thermal insulation, internal insulation is always a worse solution from the point of view of building physics.
With it, we can practically not escape the formation of condensation on the inside of the wall, between it and the insulation. During the heating period, the water vapor contained in the warm air passes (diffuses) through the wall to the outside. Encountering a cold barrier on their way out, they condense on it. This is exactly what happens with the internal thermal insulation - after overcoming the thermal insulation layer, water vapor meets the cold wall and condenses on it (with this method of thermal insulation, the wall temperature is constantly low because it cools outside and thermal insulation on the inside prevents it from warming). ).
For this reason - the requirements for the thermal insulation material are increasing - it must be able to absorb the formed condensation without damaging itself. At the same time, it must be able to remove the condensed moisture to the surface of the wall - so that the structure remains as dry as possible and the maximum amount of moisture is evaporated back through the inner surface during the summer.
Another major disadvantage of internal thermal insulation is that all connections of the thermally insulated wall with the ceiling and the adjacent walls constitute a thermal bridge, as they interrupt the thermal insulation layer.
The building structure in this situation remains exposed unprotected to the weather. Because much of the temperature difference between "inside" and "outside" occurs in the thermal insulation layer, the wall remains very slightly warmer than the outside air. For this reason, it dries much more slowly than if it is uninsulated and, in general, the humidity in the wall remains higher.
A large water load for the outer walls is the strong side rains, in which under the influence of the strong wind a large amount of rainwater is pumped into the wall. Normally, this does not cause damage, as the heat energy that passes through the wall (in the form of heat loss) pushes the water back from the wall and dries it.
However, if a well-functioning wall structure is insulated from the inside, the water that has penetrated it can no longer be removed from the heat energy and remains in the wall. In combination with the constantly low temperature of the wall in winter at outdoor temperatures below 0 ° C, this water in the wall freezes and causes the destruction of the structure.
Frequent wetting of building structures leads not only to damage caused by moisture (salinization, frost, mold and mildew), but also to accelerated weathering and destruction of building materials due to the influence of atmospheric conditions. Wooden surfaces and unplastered masonry are especially endangered. In these cases, moisture enters the joints and surfaces very easily and dries very hard. The situation is further complicated by the fact that the internally insulated walls are subject to greater thermal expansion and have a high tendency to form cracks, through which even larger amounts of water enter in side rains.
In cases where internal thermal insulation is made, priority should be given to the problem of preserving the building structure and removing moisture from it, before minimizing heat loss and deliberately using smaller thicknesses of thermal insulation material (and not to chase at any cost the achievement of the heat transfer coefficients recommended in the regulations).
Theoretically, applying a vapor barrier on the inside (for example: gluing aluminum foil) to the wall can solve the problem of condensation and reduce the accumulation of moisture to zero. However, practice shows that this is by no means to be done, as this barrier never remains dense over time (material problems, cracks due to working building elements and their movement). The problem is not only in the difficult sealing of all connections and openings in the wall (openings for contacts, switches and cables, doors, etc.), but also especially in the so-called flank diffusion (laterally through the uninsulated adjacent walls), where even in perfect laid and sealed vapor barrier in the structure always penetrates moisture. Through these openings and violations in the vapor barrier in the structure can enter large amounts of moisture, which during the warm period precisely because of this vapor barrier can not evaporate and remains accumulated in the wall.
Assuming that the accumulation of moisture in the structure during internal thermal insulation is inevitable - more attention should be paid not so much to preventing its formation, but to how this moisture is easily absorbed by the structure and how it will be in the summer. evaporated as quickly as possible. This can be done by moisture-sensitive and capillary-active construction products and materials. Capillary causes the distribution of moisture and its removal to the surface of the building element - where it can evaporate freely. In the name of this, the evaporation process should be accelerated as much as possible, where possible it is necessary to avoid the installation of a vapor barrier or to put a moisture adaptive one (intelligent vapor barriers, which are diffusion closed in winter and open in summer).
Ideally, the condensate formed should be absorbed directly from the material from which the insulated wall is made and from there evaporate outside during the summer. For this purpose, however, it must be constructed of a capillary conductive material (preferably soft-baked bricks). Before laying the thermal insulation, all vapor barriers should be removed from its inner side (cement plaster, waterproof paints, etc.)
In case the wall cannot or for various reasons should not absorb the formed condensation, this should be done by the thermal insulation layer. This can best be done by wood fiber thermal insulation boards or calcium silicate thermal insulation boards, which can be mounted so as to have the greatest possible contact with the wall.