7. Overheating

Many of the instances of overheating in new homes identified by the Building Research Establishment (BRE) and documented in NHBC NF44 Understanding Overheating occurred because of a lack of ventilation. 

Good design will ensure that there is cross ventilation. Single aspect apartments should be avoided and adequate ventilation (large volumes of air to remove unwanted heat) must be achieved through correct window design. The new Approved Document Part O (ADO) requires measures to limit solar gains and to ensure there is enough ventilation to remove heat. The movement of air across a home, by transfer grilles or undercut doors, also has to be preserved and not obstructed by thick floor finishes, for instance carpets.

Strategies to reduce solar gains apply to both naturally and mechanically ventilated homes. There are also more onerous requirements for ventilation to prevent overheating in high-risk homes, as described in ADO. Designers need to be aware of all interacting parts of the building regulations that cover ventilation, energy conservation and overheating.

There are often misunderstandings, for residents and building managers, about the purpose and operation of ventilation systems particularly mechanical ventilation with heat recovery (MVHR). Many MVHR systems have low energy fans designed to run very efficiently with normal background ventilation levels, but often they are not designed to operate at the same efficiency to supply the air changes that may be needed to mitigate overheating. For systems that are not designed to counter overheating, continuous running in boost mode will cause unnecessary noise and increased expense for the householder.

It is critical for the home’s overheating strategy to be designed in tandem with the general ventilation (Approved Document Part F) and specifically for the kind of mechanical ventilation unit installed. Heat recovery systems should also be designed to avoid recirculating warm air in summer, with a summer by-pass, so as not to exacerbate overheating issues.

Things that can go wrong:

• overheating
• noisy internal environment
• very high running costs
• homeowner misunderstanding.

Future-proofing recommendations:

• DESIGN: Understand the requirements of Approved Document Part O and design to reduce solar gains and achieve adequate cross ventilation where applicable to remove excess heat
• DESIGN: Consider external environment (noise and pollution) in ventilation design
• DESIGN: Where noise is an issue, apply dynamic thermal modelling (TM59) for Approved Document Part O assessment
• MAINTAIN: Maintain the ventilation pathways as intended by the design.

Further Reading

Met Office, web article

The chance of a summer as hot as 2018 is around 50% by 2050. Projections show maximum summer temperatures could rise by 6-9°C by the end of the century compared to 1981-2000 average. (Met Office, web article, August 2020)

Gov.uk, Ministry of Housing, Communities & Local Government, English Housing Survey Headline Report

In 2019, 11% of residents in homes built from 2003 onwards reported that at least on part of their home got uncomfortably hot. (Gov.uk, Ministry of Housing, Communities & Local Government, English Housing Survey Headline Report 2019-20, p42)

Gov.uk Ministry of Housing, Communities & Local Government, Research into overheating in new homes, Phase 1 report

A 2019 study by Aecom for DCLG showed that all house types in all locations will be vulnerable to overheating when future climate change predictions are considered. (Gov.uk Ministry of Housing, Communities & Local Government, Research into overheating in new homes, Phase 1 report, September 2019, p21)

Find below relevant NHBC standards/reports for further reading:

NHBC NF44 Understanding Overheating 

Good Homes Alliance - Overheating in New Homes 

Climate Risk Mapping