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Kappa Values

Heat waves are known as “silent killers” in some circles. Although they may not have the same level of visibility as other catastrophes such as storms or floods, or be as well publicised as a virus pandemic, Public Health England has connected 3441 “excess deaths” to the summer heatwaves of 201620172018 and 2019.

  • 908 excess deaths over the summer 2016 period.

  • 778 excess deaths over the summer 2017 period.

  • 863 excess deaths over the summer 2018 period.

  • 892 excess deaths over the summer 2019 period.

Hot weather raises the risk of sunburn and skin cancer, but there is less awareness of the harm to our thermoregulatory system in hot weather. Thermoregulation is the method through which your body keeps its internal temperature constant.


In the Summer of 2019, the Committee on Climate Change told the United Kingdom Government that little progress was being made on addressing the risks posed by rising temperatures. The Committee’s annual report to Parliament included in its conclusion: “Homes are not adapted for current or future high temperatures, there is a lack of awareness of the risks to health from high indoor temperatures” It also noted that “around 20% of existing homes currently overheat even in cool summers”, and that “overheating risks are not adequately addressed in the current Building Regulations”.


The Future Homes Standard: 2019 included “Consultation on changes to Part L and Part F of the Building Regulations for new dwellings” and requested the inclusion into BR 443:2019 “Conventions for U-value calculations”, a method to calculate Heat Capacity.


Consequently, in BR 443:2019 two methods for calculating Heat Capacity are now named.

One method will enable Heat Capacity Values of building element to be calculated at the same time as U value calculations.


So, what is Heat Capacity? (often called kappa value)

It’s the ability of a material to take in heat and store it.


It’s the reason the jam in a sponge roll stays hotter for longer than the sponge around it.

The jam can store heat for longer than the sponge. If a sponge roll is put in a fridge overnight the jam takes longer the next day to get to room temperature than does the sponge, as the jam can also store ‘coldness’ for longer than the sponge.


So, we need buildings fabrics that have both insulation properties (the sponge) and ability to absorb and release heat slowly (the jam).

Jam Sponge Insulation Diagram

What information do we need to calculate the kappa value of a building element (wall/floor/roof) can help prevent overheating?


  • We need to know the specific heat capacity of the materials used.

This is a measure of the amount of energy in Joules required to raise the temperature of 1kg of a material by one degree Kelvin. The units are J/kgK

  • The density of the material used
  • The thickness of the material used


Typical examples below


Material Specific Heat Capacity Density
(J/kgK) (kg/m3)
Expanded polystyrene 1200 50
Fiberglass 700 150
Acoustic Tile 1340 290
Softwoods (pine) 1350 510
Particle board (low density) 1300 590
Hardwoods 1250 720
Paper 1300 930
Particle board (high density) 1300 1000
Concrete – cast light 1000 1200
PVC 1250 1400
Brick 840 1600
Concrete – cast dense 840 2100


The heat capacity (kappa value) is then calculated by:

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 kappa value is taken over all elements of the building element but stops being calculated at which ever of these conditions occurs first, even if it occurs part way through a layer.


  • Halfway through the element
  • There is a layer with a lambda of less then 0.08 W/mK (often insulation)
  • A total thickness of 100mm


Once the kappa has been calculated for the building elements involved, it needs to be used in SAP and SBEM as a Thermal Mass Parameter (TMP). This is the total heat capacity of a dwelling divided by its floor area, and is expressed in kJ/ m²K.


For dwellings the Thermal Mass Parameter can vary significantly from 55 kJ/m²K for a lightweight timber frame construction to 500 kJ/m²K for a solid masonry construction.


In SAP the Thermal Mass Parameter (TMP) is required for heating and cooling calculations.

Currently there are high, medium or low default values available



  • Low 100 kJ/m²K
  • Medium 250 kJ/m²K
  • High 450 kJ/m²K


However, SAP 10 calculations will require a detailed calculation of a building’s actual TMP based on build material, construction and calculated kappa values.


Mayplas, a market-leading manufacturer of thermal materials for the building industry with over 45 years of experience in insulation conversion, has started integrating kappa values in their U value estimates upon request. Take a look at the sample below.














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