What is a U-value?
What is a U-value?
Anyone who deals with energy-efficient construction and energy standards will inevitably be confronted with the U-value (formerly k-value). It represents the heat transfer coefficient, a measure of the heat flow through a single or multiple layer of material when different temperatures prevail on both sides. The U-value (W/m2K) indicates the amount of heat (in kWh) that is transported through a component surface of 1m² in one second if there is a temperature difference of one degree Celsius (= 1 Kelvin) between the inside and outside.
Put simply, the U-value indicates the heat loss of a building component. The lower the value, the better it is.
The U-value is calculated in accordance with ÖNorm EN ISO 6946 and is the reciprocal of the total thermal resistance Rtot.
𝑈 = 1
𝑅tot
R tot is made up of the sum of the thermal resistance of the individual layers and the internal (Rsi ) and external (Rse ) thermal transfer resistances. The thermal resistances of the individual layers R are obtained from the sum of the quotients of the layer thickness d and the thermal conductivity λ (lambda) of the individual layers. The thermal transfer resistances for the inside and outside are characteristic values that can be taken from a table in the Austrian standard EN ISO 6946.
The detailed formula for calculating the U-value is therefore as follows:
U =1/(Rsi + sum Rcomponent layers + Rse) W/m2K
The quality of the building envelope is regulated by law in all federal states. For components such as external walls, external ceilings, walls against the ground, walls against heated or unheated parts of the building, the maximum permissible U-values defined in OIB Guideline 6 apply.
For example, the limit value for an external wall in contact with the ground is 0.40 W/m2K according to OIB Guideline 6. A 30cm thick reinforced concrete wall with 10cm XPS has a calculated U-value of 0.32 W/m2K.
An exterior wall above ground must not exceed a value of 0.35 W/m2K. A plastered exterior wall complying with the current standard, consisting of 20cm reinforced concrete and an EPS insulation layer thickness of 20cm, has a U-value of 0.15 W/m2K. In comparison, a Porotherm brick wall with a thickness of 25cm and the same insulation has a U-value of 0.12 W/m2K.
As can be seen from both examples of above-ground exterior walls, the limit values of the OIB guideline are often undercut, which is not least due to additional requirements for buildings, such as the fulfilment of low-energy or passive house standards.
