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.

Find below relevant NHBC standards/reports for further reading:

NHBC NF44 Understanding Overheating Good Homes Alliance - Overheating in New Homes
Climate Risk Mapping