Posted: 27th May 2022
Approved Document O (“ADO”) was published on 15th December 2021. It aims to improve the health and wellbeing of occupants in dwellings and other rooms for residential purposes (other than a hotel) by controlling solar gains and improving the removal of excess heat. This will reduce the likelihood of overheating and is part of the government’s plans to achieve net zero. As well as reducing overheating, the successful mitigation strategy will need to consider the potential noise impact of having bedroom windows open overnight.
ADO is one of the The Approved Documents. These provide guidance on how to meet the requirements of the Building Regulations 2010.
The Building Regulations set minimum standards to ensure that new buildings, conversions, and renovations, are safely constructed and will provide a reasonable standard of amenity, health, and performance for future residents or users of the building.
The regulations do not apply to work subject to a building notice, full plans application, or initial notice submitted before 15th June 2022, provided the work starts before that date. They are only applicable for use on sites in England.
Requirement O1 of the Building Regulations requires that:
a. Limit unwanted solar gains in summer,
b. Provide an adequate means to remove heat from the indoor environment.
a. Account must be taken of the safety of any occupant, and their reasonable enjoyment of the residence, and
b. Mechanical cooling may only be used where insufficient heat is capable of being removed from the indoor environment without it.
Compliance with Requirement O1 can be demonstrated by using either the simplified method described in ADO or using dynamic thermal modelling.
Using the simplified method, the overheating risk category is determined as either high risk (central London and parts of central Manchester), or moderate risk (all other areas in England). To restrict solar gains, limits are set on the maximum glazing areas. For properties in high-risk areas, shading is also required to certain facades.
The second part of the simplified method considers removing excess heat. Residential buildings should have a minimum free area in windows or other openings as shown in Tables 1.3 and 1.4 (shown below).
However, requirement O1(2) means that other aspects need to be considered when developing the overheating mitigation strategy, including,
These other considerations can affect the use of open windows in an effective overheating mitigation strategy.
As noted above, if external noise levels are too high then residents are unlikely to open their windows. This means the minimum free area requirements wouldn’t be met and the simplified method cannot be used. The more detailed dynamic thermal modelling method would have to be carried out, potentially with an alternative overheating mitigation strategy.
Paragraph 3.3 of ADO sets the maximum night-time noise criteria to inside bedrooms:
3.3 Windows are likely to be closed during sleeping hours if noise within bedroom exceed the following limits.
a) 40dB LAeq, T averaged over 8 hours (between 11pm and 7am).
b) 55dB LAfmax more than 10 times a night (between 11pm and 7am).
It’s important to note that this only refers to noise levels inside bedrooms at night. This means that overheating mitigation strategies during the daytime and to other rooms within dwellings do not need to take noise levels into account.
There are two methods that could be used to prove that a development complies with ADO. The first method would be to measure background sound levels and calculate noise intrusion to inside bedrooms. This stage should be followed for all developments, to make sure that the selected overheating mitigation method will work when the development is completed.
The second method is to measure sound levels inside bedrooms overnight in completed dwellings. This method might be requested by Building Control Officers if there is a concern that the noise limits can be met with open windows, for example.
Paragraph 3.4 of ADO confirms that in-situ measurements to demonstrate compliance with the criteria should be conducted using the methodology in the Association of Noise Consultants’ Measurement of Sound Levels in Buildings with the overheating mitigation strategy in use – i.e., with windows open.
The Association of Noise Consultants (ANC) have subsequently published draft guidance on how to demonstrate compliance with Approved Document O.
The sound level meter and monitoring equipment should be a Class 1 instrument in accordance with BS EN 61672-1:2013. The sound level meter should have been calibrated in an accredited laboratory within the last two years.
A Class 1 acoustic field calibrator should be used to provide an on-site calibration check prior to and after the survey to verify the results achieved.
The requirements of ADO only apply to bedrooms.
The ANC draft guidance recommends that one bedroom in every ten is tested, using the likely ‘worst-case’ for overheating and noise. Until this guidance becomes approved, we recommend that any test strategy is agreed with your Building Control Officer.
The noise criteria require the survey to be conducted overnight, between 23:00 and 07:00 hours.
The ANC guide suggests that for most sites testing over a single night should be sufficient. But, as with the rooms to be tested, until this method becomes formalised is best to agree a test strategy with your Building Control Officer prior to undertaking any survey.
The ANC guide recommends that tests are carried out during a normal weekday night and under dry ground conditions.
Heavy rainfall or wet roads would increase noise levels. Staying on meteorological impacts, for marginal sites the direction of the wind towards or away from a busy road could make the difference between sound levels meeting the noise limits of Approved Document O or not.
Confirming if noise levels can achieve the criteria in paragraph 3.3 determines whether open windows can be used as the overheating mitigation strategy.
If an alternative mitigation strategy is required, then this needs to be known at the initial stages of the building design so that it can be fully integrated. That might include attenuated louvre ventilation apertures, mechanical ventilation systems (such as MVHR or MEV), or a mechanical cooling system (e.g., air conditioning or reverse cycle heat pumps).
For many sites, a sound level survey will have been carried for the BS 8233 and ProPG: Planning and Noise assessments, as part of the environmental noise impact assessment to support the planning application. Your acoustic consultant may be able to use this survey to confirm if open windows can be used to mitigate overheating in bedrooms overnight, or if alternative schemes are required.
However, for some sites this survey may not have been carried out or a single measurement position only used to the ‘worst-case’ façade. Whilst that would provide an assessment of the suitability of the site for residential development at planning stage, it wouldn’t necessarily enable a detailed assessment of whether bedroom windows can be opened to other facades or other buildings on the site. Similarly, buildings being converted into residential flats under Prior Approval permitted development legislation only need to consider noise from nearby commercial sources. They would not necessarily have measured noise levels from roads, trains, or other general sources.
In these cases, we would strongly recommend that you commission a sound level survey as soon as planning consent has been granted. We can use this survey – and if needed 3D computer modelling – to calculate sound intrusion into each of the proposed bedrooms in your development and verify your available options for the overheating mitigation strategy. Getting this confirmed at the outset means you have clarity as the design develops, and don’t have the potential that a full redesign is required if the strategy needs to be changed at a later stage, or worse still, makes sure that you don’t get to the end of the project and realise that commissioning tests have failed.
We have extensive experience in undertaking environmental noise impact assessments to support planning applications on projects throughout the country. This includes using specialist 3D computer noise modelling to calculate sound levels at each window of a development where needed, and façade sound insulation software to calculate internal sound levels through the open windows. This process will allow us to verify at the design stage if you can use open windows as part of your overheating mitigation strategy for some, or all, of the bedrooms on your development.
Getting this right at the outset could save you £000’s, either by confirming that mechanical systems are not needed on some or all the bedrooms, or, by confirming where mechanical systems are necessary at the very earliest stage, so they don’t need to be retrofitted later.
Commissioning noise tests can often be fitted in at short notice. With our three offices, including in London and Manchester – the two high-risk areas in Approved Document O – we are perfectly placed to assist with projects throughout the country.
To discuss your project in more detail with one of our experienced acoustic consultants, and to find out how we can assist in making sure your development meets the requirement of Approved Document O, please call us at your local office number, or use the contact form.