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 noise in bedrooms during night time periods. 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.
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 be taken of the safety of any occupant 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.
Paragraph 3.3 of ADO states that windows are likely to be closed during sleeping hours if noise within bedrooms exceeds:
• 40 dB LAeq,8hours
• 55 dB LAmax,f more than 10 times per night.
If open windows are proposed as mitigation for controlling overheating, the internal sound level predictions should be within the above limits with windows open. If an open window will result in these limits being exceeded, one of the alternative methods should be adopted. ADO notes that it may be possible to increase the above values and still satisfy the requirements of the simplified method if acoustically specified balconies are used.
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).
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 above which dynamic thermal modelling should be used to demonstrate compliance:
• 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.
Dynamic Thermal 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.
Understanding Noise at Work Regulations