Approved Document O — Overheating and Acoustics Explained

The new Part O of the Building Regulations, also known as Approved Document O, deals with overheating in new build residential properties, setting requirements to limit solar gains while maintaining acceptable acoustic conditions.

This new piece of legislation is now in force and so all new houses, flats and other applicable types of residential development must comply.

This note explains how the acoustic aspects of Part O are applied in practice, including typical criteria, compliance routes and when specialist assessment is required.

Understanding Part O

Part O of the Building Regulations for England and Wales focuses on mitigating the risk of overheating within residential properties. It outlines specific criteria and performance standards aimed at controlling internal temperatures to ensure the comfort and well-being of occupants, particularly during warmer months.

The most suitable method for controlling overheating must take account of several factors, one of which is night-time noise in bedrooms. The intention of the acoustic requirement is to avoid residents having to choose between thermal comfort or adequate control of outdoor sound in bedrooms at night.

Other factors that must be accounted for are security, pollution, protection from falling and protection from entrapment.

Compliance Requirements

Paragraph 2.0 of Approved Document O lists four example methods for removing excess heat:

• Opening windows,
• Ventilation Louvres in external walls,
• A mechanical ventilation system
• A mechanical cooling system

The document states that reasonable provision must be made to limit unwanted solar gains in summer and provide an adequate means to remove heat from the indoor environment.

In meeting these obligations, account must also be taken of occupant safety and reasonable enjoyment of a residential property. ADO states that mechanical cooling may only be used where sufficient heat cannot be removed from the indoor environment without it.

ADO gives two methods of compliance; a simplified method and a dynamic thermal modelling method.

The Simplified Method

Paragraph 3.3 of ADO states that residents are likely to close windows during sleeping hours if noise within bedrooms exceeds:

• 40 dB LAeq,8hours – this is a time averaged mean sound level throughout the night time period 23:00 – 07:00

• 55 dB LAmax,f more than 10 times per night. These are peak levels from individual events.

So if open windows are proposed as mitigation for controlling overheating, the internal sound level predictions should be within these limits with windows open.

If an open window will result in these limits being exceeded, one of the alternative methods should be adopted. For compliance with the simplified method, the open area required from windows must be at least 4% of the floor area of the room in moderate risk locations (most of the country), and at least 13% of the floor area in high risk locations (some parts of London).

Outdoor Sound Level Requirements for the Simplified Method

A useful approximation is that an outside to inside level difference for a window open to the extent that satisfies the simplified method is 9 dBA for medium risk areas. It follows that the following external sound levels are the limit at which the simplified method can be used and this enables assessment of a site to be undertaken before development is completed.

• 49 dB LAeq,8hour, this is a time averaged sound level at dwelling elevations throughout the night

• Short duration maximum sound levels to not exceed 64 dB LAmax,f more than 10 times per night.

Thermal Dynamic Modelling Method

If external sound levels are too high for the simplified method to be used, or the developer chooses to adopt an alternative more flexible route, then dynamic thermal modelling is required.

The thermal modeller will predict overheating risk using CIBSE’s TM59 methodology with some parameters dictated by Approved Document O.

The modelling takes account of shading provided by the built environment or external shading devices, low g-value glazing, mechanical systems and airflow provided by noise mitigating devices such as acoustic louvres. It is the only option for demonstrating compliance in areas of poor air quality or where sound levels are too high for the simplified method to be used.

This dynamic thermal modelling method requires work from acoustic consultants and thermal modellers to ensure that the final solution allows adequate control of overheating while maintaining sound levels compliant with the requirements of ADO.

We can provide indicative maximum open equivalent areas (EA) for bedroom windows to thermal modelling engineers for them to use in the overheating calculations.

When is an acoustic consultant required for Part O?

An acoustic survey is often undertaken to establish whether the simplified method can be applied with confidence, even on subjectively quieter sites.

Approved Document O does not mandate acoustic assessment in every case. However, measured external sound levels are frequently required to demonstrate that night-time internal noise criteria can be met with windows open. Where this can be clearly shown, the simplified method can be adopted without the need for additional mitigation.

Where external sound levels are higher, or where there is uncertainty, a more detailed assessment may be required to determine whether alternative strategies such as enhanced façade performance, acoustically treated ventilation or mechanical systems are appropriate.

In this context, the role of the acoustic consultant is not only to help resolve issues on challenging sites and work with thermal modelling engineers, but also to provide proportionate evidence where required, ensuring that the acoustic aspects of overheating strategies are compliant and clearly understood by the wider design team.

If you require advice or an assessment to demonstrate compliance with the acoustic requirements of Part O, see our Building Regulations Part O consultancy services or contact us directly and we will be delighted to assist.

Last reviewed: 2026