Technical newsletters about emerging issues & our latest research

A review of the treatment of thermal mass under Building Regulations Part L1a suggests that, rather than relying on default assumptions, there is an advantage to be gained by undertaking an individual dwelling assessment.

The results of a thermal mass parameter assessment are similar for a range of house types on the same development. This gives comfort that the approach could be relied upon as a general strategy to meet CO2 targets.

Part L1a 2010 of the Building Regulations introduced a ‘thermal mass parameter’ into the SAP calculations. This is a single variable which changes the outcome of a Part L1a CO2 assessment to reflect the influence of lightweight or heavy construction methods (see our previous article 16th March 2012).

The default assessment option is a qualitative test that decides if a dwellings thermal mass parameter is Low ( 100kj/m2K), Medium ( 250kj/m2K) or High ( 450kj/m2K). This is the straightforward approach to completing Part L assessments.

A more detailed approach is available to SAP Assessors. It is possible to calculate a thermal mass parameter based on the dimensions of a particular dwelling. This method relies on conventions outlined in SAP Tables 1e and 1f.

As part of the assessment of a current scheme, we have calculated thermal mass parameters for a range of house types. The construction has the following characteristics:

  • FLOOR – Suspended concrete floor carpeted
  • HEAT LOSS WALLS – Cavity Wall, plasterboard on dabs, AAC block
  • PARTY WALLS – Plasterboard on Dabs, AAC blocks both sides
  • HEAT LOSS ROOFS – Plaster board insulated slope OR at ceiling
  • INTERNAL PARTITIONS – Plasterboard on timber frame

The simple SAP method would suggest that these houses have Medium thermal mass (250kj/m2K). For comparison, the chart below shows the results of an individual dwelling assessment.

Thermal Mass Parameter vs Internal Floor Area and House Type

  • Calculated thermal mass parameters are 160kj/m2K on average (This is closer to a Low thermal mass parameter than a Medium).
  • This reduction in thermal mass parameter resulted in a CO2 saving of circa 2%. (A similar CO2 reduction could be achieved by including weather compensators or 0.1 KWpeak of Photovoltaic Panel).
  • The range between results across this scheme is 30kj/m2K. This consistency of results suggests that thermal mass parameter assessment can be relied upon as part of a general strategy to secure Building Regulations Compliance and Code for Sustainable Homes CO2 credits.
  • Thermal mass is an important characteristic of buildings. Lightweight construction has the advantage of being more responsive to changes in heating set-temperatures. Lightweight construction is therefore more suited to heating systems that can quickly respond to changes in occupant demand. In theory, such a combination  could reduce energy use.

Posted on June 28th, 2012

Author: Richard Hodkinson