Oversimplifying the overheating issue

7 Apr 2021

Tom De Saulles, The Concrete Centre's advisor on Building Physics expresses concerns that elements of new MHCLG proposals represent a backward step on addressing overheating in new homes.

Proposed revisions to  the Building Regulations seek to address the growing risk of overheating by introducing new requirements for tackling the issue. A proposed option to use dynamic thermal modelling to assess risk is welcome. However, a new ‘simplified method’ is also presented as an alternative option. This essentially entails a set of fixed overheating related design requirements for all new homes, such as maximum window size and shading requirements.

The option to use the simplified method, sounds good, but unfortunately it is oversimplified, not fully accounting for all the factors that influence  overheating risk and could have unintended consequences for other housing needs e.g. daylighting and building context.  

A better alternative could be a performance-based overheating standard. This would give greater design flexibility, making it easier to address overheating risk alongside other requirements, using solutions best suited to the site conditions and client needs of each project.

This could be achieved by using the dynamic thermal modelling option for high risk and more complex situations, with performance assessed against the CIBSE TM 59 standard. Since this may be overkill for lower risk, more straightforward housing designs, these could be addressed using an updated version of the current SAP overheating assessment, which is simpler and quicker to use. This approach is along the lines of that proposed by the Zero Carbon Hub following extensive work on overheating in homes.

This view broadly aligns with that of the Greater London Authority, which says: "Tackling overheating in new buildings and especially homes is a key priority in the London Plan and therefore we strongly support the proposal to address overheating risk through a new requirement in Building Regulations. However, the simplified method does not offer the level of rigour necessary to demonstrate overheating mitigation and could lead to daylight and ventilation issues leading to poor internal air quality levels". As such, the dynamic thermal analysis proposal should, at least, be set as a requirement for London and a similar analysis tool proposed for other parts of the country. This would align Building Regulations with the London Plan which already requires this for all major development. This would also help support our approach in London, recognising the significant overheating risk that exists."

This view broadly aligns with that of the Royal Institute of British Architects, whose letter to the Ministry of Housing Communities and Local Government (MHCLG) included concerns about: "The Building Regulations suggest a reduction in performance relative to a prescribed notional building. The notional building does not reward efficient building form and orientation." and the importance of "Net zero carbon buildings seek to balance operational energy consumption with the UK grid renewable energy capacity. This means they should minimise their energy demand, including all energy uses".

The deadline for consultation responses to Part L was 13 April 2021.

Written by Tom De Saulles, The Concrete Centre's advisor on Building Physics