Building physics, the U-value myth and thermal activation of the screed mass

One of the most serious errors in building physics measures is the assumption that insulation materials prevent heat loss. This is due to the overestimation of the U-value. This applies to the calculation of transmission heat losses for the steady state that never exists: As a result of the constant temperature changes over the course of the day, this steady state, i.e. thermally static conditions, never exists. In the thermodynamics of a house, we are dealing with a dynamic system. There are studies whose results prove the inefficiency of insulation thicknesses over 80 mm. This effect is mathematical and is due to the hyperbolic form of the U-value function. In the real world, insulation that is twice as thick does not halve heat loss.

Appropriate importance should therefore be attached to storage capacity when planning air conditioning systems for buildings. It is old wisdom that the storage capacity of the building mass has a major influence on temperature stability in the rooms. These building physics facts have led to the increased use of “building component activation” in recent years. Concrete ceilings or concrete walls are fitted with water-bearing pipe registers. The pipe registers ensure active thermal utilization (activation) of the concrete mass and equalization of the temperature differences between inside and outside as well as day and night. The disadvantage of component activation is the relatively low heating or cooling capacity per square meter of active surface. Depending on the position of the pipe registers, this is only approx. 20 W per square meter. The ClimaLevel Multiboden HKL does not have this performance deficit of component activation. This is because the system uses the screed as a storage and at the same time highly active mass: the screed is actively and dynamically used by pipe registers and also by the supply air flowing underneath. This is therefore referred to as thermal screed activation.

This system makes sense in terms of building physics and has the performance potential for a 365-day feel-good climate in office or residential buildings.