Dynamic thermal properties

A free tool for calculating the thermal properties of construction elements has been launched by The Concrete Centre following development by Arup.

Part L of the Building Regulations, which deals with the conservation of energy, is currently being revised along with the compliance tool for housing known as SAP (Standard Assessment Procedure). Included in the changes to SAP is a more rigorous treatment of the effects of thermal mass in housing to take account of the role it can play in passive design. When used appropriately, thermal mass can improve energy efficiency during the heating season by providing a means of capturing and slowly releasing free heat gains from the sun and internal appliance.

This cuts the load on the heating system and reduces CO₂ emissions, particularly in well insulated, airtight homes. The revisions take account of this passive heating effect, and to help designers take advantage of this the Concrete Centre has commissioned Arup to develop a free Excel based tool for calculating the thermal mass in floors and walls.

The methodology is based on BS EN ISO 13786, and is fully aligned with SAP, allowing thermal mass values (also know as Kappa values) to be easily calculated and directly inputted to SAP software. If required, admittance values are also produced, providing an alternative means of assessing thermal mass.

Another feature of the tool is its ability to calculate decrement. This is a property of thermal mass which describes the way in which the density, heat capacity and thermal conductivity of a wall for example, can slow the passage of heat from one side to the other (decrement delay), and also attenuate gains as they pass through (decrement factor). Designing for a long decrement delay of around nine hours or more, and a low decrement factor can help reduce overheating problems in summer.

The motivation for producing this tool is a growing need among architects and engineers for more information about the thermal properties of construction elements other than just their U-value. This is needed to help optimise the passive performance of buildings and ensure a high level of inherent energy efficiency. Going forward, it is likely that far more attention will be paid to getting this right given the forthcoming changes to Part L and SAP. Another driver is the issue of climate change adaptation, which is starting to result in greater scrutiny construction materials and their thermal properties.