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Last edited 12 Jun 2019
NB see k-value for alternative meaning.
In the Standard Assessment Procedure (SAP) and Simplified Building Energy Model (SBEM), used to demonstrate compliance with Part L of the building regulations, k-value (short for Kappa value or thermal mass value) refers to the heat capacity per square metre of a material, measured in kJ/m2K. This is used to quantify the thermal mass of building elements such as walls and floors. The higher the k-value, the greater the thermal mass, that is the more heat the element is able to store.
Thermal mass can be used to even out variations in internal and external conditions, absorbing heat as temperatures rise and releasing it as they fall. In building design, this can useful for evening out and delaying extremes in thermal conditions, stabilising the internal environment and so reducing the demand for building services systems.
Typical K-values might be (ref the Concrete Centre SAP 2009 – Fabric energy efficiency & thermal mass):
- Timber frame wall: 9 kJ/m2K.
- Dense concrete block wall with a plaster finish: 190 kJ/m2K.
- Timber floor/ceiling: 9/18 kJ/m2K.
- Concrete floor/celing: 120/160 kJ/m2K.
The kappa value (k) of a material is calculated as:
k = 10-6 × Σ (dj rj cj) (ref BuildDesk)
- dj is the thickness of a layer (mm)
- rj is density of a layer (kg/m³)
- cj is specific heat capacity of a layer (J/kg·K)
The limiting value for thickness is where:
- The total thickness of the layers exceeds 100mm.
- Or, the mid-point of the construction is reached.
- Or an insulation layer is reached.
Following the Standard Assessment Procedure, k-values are used to determine a Thermal Mass Parameter (TMP, expressed in J/m2K), which is used to calculate the contribution of thermal mass to the efficiency of the dwelling. TMP is calculated from the sum of the area x heat capacity of all construction elements, divided by total floor area of the dwelling (TFA).
NB Rather confusingly, the term k-value (sometimes referred to as a k-factor or lambda value λ) is also used to refer to the thermal conductivity of a material, that is, how easily heat passes across it. This is a fundamental property, independent of the quantity of material. It represents the steady-state heat flow through a unit area of a material resulting from a temperature gradient perpendicular to that unit area. It is expressed in W/mK.
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