When designing a rendered facade for old wall structures, consider the following issues:
Load-bearing capacity
Check the total weight of the structure and take into account the thickness and weight of the chosen insulation, the mortar and other materials used in the system. The thin rendering system weighs around 20 kg/m
2 and the thick rendering system around 60 kg/m
2. Design the insulation so that its mechanical strength can cope with the weight of the structure it must support as well as any other possible mechanical stresses.
Mechanical strength and cleanness of the supporting wall structure
Separately check that the supporting wall structure has sufficient strength for adhesives and fasteners. Sometimes you need to verify the strength with separate tensile strength tests. Select the fixing system according to the instructions of the rendering system owner. When using adhesive mortar, the old surface has to be clean and dust free, otherwise the mortar will not stick.
Smoothness of the supporting wall structure
Regardless of whether you dismantle the old facade or not, always check the smoothness of the supporting wall to ensure its suitability as the base of a new insulation structure. Smooth uneven supporting wall structures with, for example, plaster. The supporting wall structure can also be evened with separate framing and flexible stone wool insulation installed in between the frame studs. Such a structure, when correctly installed, ensures a smooth and straight facade. It is, however, more expensive than the previous alternatives.
Moisture in structures
External insulation warms up the old structures and dries them. The water vapour permeability of stone wool is very high, which ensures that moisture evaporates away from the structure. In buildings where internal air conditions are particularly demanding, with high humidity and/or high temperature (public baths, saunas, laundries, factories, etc.), the rendering structures need to be suitable for the physical functioning of the old structure as well as the new one. Rendering applications with stone wool and inorganic mortar can be used for such buildings.