The floors in a building are normally insulated for two reasons – heat retention and soundproofing.
Noise in buildings can be divided into impact sound and footfall sound. Footfall sound is sound that can be carried by air caused by stepping onto and walking on the floor. Footfall sound is not usually perceptible outside the room where the sound has originated.
Impact sound, on the other hand, is the transmission of the same sound into the structural components, so for example walking on bonded parquet is transmitted by the screed throughout the entire building unless the screed is not a sound bridge to the other rooms but provides effective insulation against the transmission of impact sound.
Insulation using for example STYROPOR® boards meets both requirements for heat and impact sound insulation, but on its own it is too soft and therefore not able to absorb point loads from furniture. Which is why a rigid layer that “shares the load” and also helps to absorb impact sound is laid on top of this insulation — screed.