Building Regulations Approved Document O (Overheating) – Acoustic Compliance
Approved Document O of the Building Regulations was introduced to reduce the risk of overheating in new residential development.
While the document is primarily concerned with thermal comfort, it has direct and significant acoustic implications, particularly for bedrooms at night where increased ventilation is often required to control heat.
In many locations, outdoor sound levels mean that reliance on open windows for cooling can lead to unacceptable indoor noise conditions. As a result, compliance with Part O frequently requires early coordination between acoustics, ventilation strategy and thermal modelling, rather than being left as a late stage check.
Why acoustics matters for Part O
Part O specifically limits indoor sound levels in bedrooms during sleeping hours where open windows are proposed as a means of removing excess heat.
The intention is to avoid residents having to choose between:
- Thermal comfort, and
- Reasonable protection from environmental noise at night.
In practice, this creates a design constraint on many sites close to roads, railways, or other noise sources, where open windows can result in:
- Excessive night-time LAeq levels, or
- Frequent peak events exceeding acceptable limits.
Where this occurs, alternative ventilation or façade strategies must be considered.
Simplified method and dynamic thermal modelling
Approved Document O provides two routes to compliance:
Simplified method
If sound levels are low enough to comply with the simplified method, then, subject to other non-acoustic factors, overheating control may be achieved using open windows, provided the dimensional criteria set out in Approved Document O are met.
On many urban and suburban sites, the threshold for using the simplified method is exceeded at plots facing main roads, meaning compliance must be demonstrated using the alternative route.
Dynamic thermal modelling
Where the simplified method is not suitable, compliance is demonstrated using dynamic thermal modelling (CIBSE TM59).
In these cases, acoustic input becomes a critical part of the modelling process, as ventilation openings, louvres and façade strategies must be designed to allow heat to be removed without exceeding the noise limits set out in Approved Document O.
This requires coordination between the acoustic consultant and the thermal modeller to ensure that both overheating and noise criteria are satisfied.
Our role in supporting Part O compliance
We provide acoustic assessment and design input to support Part O compliance, often alongside acoustic survey work for residential development. This allows Part O implications to be considered before layouts and specifications are fixed and mitigation solutions found at the earliest opportunity.
Our work typically includes:
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Measurement and assessment of night-time sound levels, including peak event analysis and comparison to open-window night-time noise criteria
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Identification of plots or façades where the simplified method can be used for compliance
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Information on required sound level reductions and limits on allowable equivalent area (EA) for open windows on all parts of a development site for use by thermal modelling engineers.
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Liaison with thermal modellers as needed
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Early-stage identification of Part O risks to avoid late design changes
Part O and Planning Applications
It is a common misconception that compliance with planning acoustic guidance (such as BS 8233 or ProPG) automatically results in Part O compliance. In reality:
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Planning acoustics often focuses on closed-window conditions, with some commentary on the effect of open windows, while
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Part O explicitly considers open-window night-time conditions in bedrooms to control overheating.
As a result, developments that are entirely acceptable from a planning perspective can still face Part O challenges later in the design process if acoustic implications are not considered early.
Further guidance and industry discussion
David Garritt co-authored an industry article for Construction Management on the acoustic implications of Approved Document O, exploring the interaction between acoustics, ventilation and overheating control in more detail:
Noise requirements in Approved Document O – Construction Management Magazine
If you have a new residential development that is subject to Part O of The Building Regulations, please call or email us for discussion or advice, whatever stage the project is at.
David Garritt BEng MIOA
Director and Acoustic Consultant
Speak to an Acoustic Consultant
If you would like to talk to us about any aspect of acoustics and how we can help, please do not hesitate to contact us:
e-mail: contact@sdgarritt.co.uk
Phone: Doncaster main office, 01302 854 303
David Garritt’s mobile, 07894 073 973
Elizabeth Garritt BSc AMIOA
Acoustic Consultant