Kappa value - thermal mass value
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)
where:
- 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.
NB see k-value for alternative meaning.
[edit] Related articles on Designing Buildings Wiki
Featured articles and news
RTPI leader to become new CIOB Chief Executive Officer
Dr Victoria Hills MRTPI, FICE to take over after Caroline Gumble’s departure.
Social and affordable housing, a long term plan for delivery
The “Delivering a Decade of Renewal for Social and Affordable Housing” strategy sets out future path.
A change to adoptive architecture
Effects of global weather warming on architectural detailing, material choice and human interaction.
The proposed publicly owned and backed subsidiary of Homes England, to facilitate new homes.
How big is the problem and what can we do to mitigate the effects?
Overheating guidance and tools for building designers
A number of cool guides to help with the heat.
The UK's Modern Industrial Strategy: A 10 year plan
Previous consultation criticism, current key elements and general support with some persisting reservations.
Building Safety Regulator reforms
New roles, new staff and a new fast track service pave the way for a single construction regulator.
Architectural Technologist CPDs and Communications
CIAT CPD… and how you can do it!
Cooling centres and cool spaces
Managing extreme heat in cities by directing the public to places for heat stress relief and water sources.
Winter gardens: A brief history and warm variations
Extending the season with glass in different forms and terms.
Restoring Great Yarmouth's Winter Gardens
Transforming one of the least sustainable constructions imaginable.
Construction Skills Mission Board launch sector drive
Newly formed government and industry collaboration set strategy for recruiting an additional 100,000 construction workers a year.
New Architects Code comes into effect in September 2025
ARB Architects Code of Conduct and Practice available with ongoing consultation regarding guidance.
Welsh Skills Body (Medr) launches ambitious plan
The new skills body brings together funding and regulation of tertiary education and research for the devolved nation.
Paul Gandy FCIOB announced as next CIOB President
Former Tilbury Douglas CEO takes helm.
UK Infrastructure: A 10 Year Strategy. In brief with reactions
With the National Infrastructure and Service Transformation Authority (NISTA).