What is a kappa value?
We’ve had a few questions recently about kappa values. This blog post will explain what they are and why they are used.
A kappa value is a measure of the effective heat capacity per unit of area, i.e. a measure of the thermal response characteristics of the construction. In basic terms it shows how well a specific construction will store heat for later use.
To understand why this is important we need to talk about thermal mass. The thermal mass of a building is key when constructing low energy buildings such as Passivhaus. The thermal mass is a building’s ability to absorb, store and release heat. You can think of it like a battery – take a stone floor for example, during the day this will absorb heat, through radiation from the sun or from warm air next to it. When the air is cooler, then the heat will be released from the floor. This can help to even out the temperature of a building.
The kappa value is a measure of how much heat will be stored per metre squared of a specific construction. The higher the number, the more heat will be stored for later release.
How is a Kappa Value calculated?
The heat capacity or kappa value per unit area, k in kJ/m²K, for a construction element can be calculated from:
k = 10-6 × Σ (dj rj cj)
- dj is the thickness of layer (mm)
- rj is density of layer (kg/m³)
- cj is specific heat capacity of layer (J/kg•K)
The calculation is over all layers in the element, starting at the inside surface and stopping at whichever of the following conditions is encountered first (which may mean part way through a layer):
- The total thickness of the layers exceeds 100mm
- The mid point of the construction is reached
- An insulation layer is reached (defined as thermal conductivity <= 0.08 W/mK);
How this affects the location of insulation
As an example, let’s look at a concrete floor with an underfloor heating system. Concrete has a high thermal mass and will therefore store more heat. So if you want to continuously heat the building – you would have the concrete between the insulation and your building i.e. a high kappa value so the concrete can store heat, and release it when required. However if you want a quick response from a heating system then the insulation should come before the concrete, giving a low kappa value to that construction element, to prevent the heating system having to heat the concrete. (More information about underfloor heating and flooring can be found on here.)
The specific requirements for the building and heating system will affect the choice of a construction element with a high or low kappa value.
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