What is the document?
Approved Document O: Overheating mitigation provides requirements for overheating mitigation as part of the Building Regulations 2010. The document came into effect on 15 June 2022 and applies to all new residential buildings after that date, Buildings already with permission are excluded if building works start before 15 June 2023.
Approved Document O differs from Approved Document F: Means of ventilation in that Approved Document F deals solely with ventilation which includes
Background ventilation
Extract ventilation
Whole-dwelling ventilation
Purge ventilation
Approved Document O focuses on limiting solar gains but also includes ventilation requirements from the perspective of removing excess heat.
Why has the regulations been introduced?
In 2022, the United Kingdom experienced three heatwaves. The highest-ever temperature recorded was over 40 °C in Lincolnshire on 19 July 2022.
Taking care conducted an analysis of government data from 2017 to 2021 and found that 6,723 people died in that period due to heat-related illness. It can be seen from their graph that in the last 2 years (and now 3 if you include 2022 - official statistics were released on 7 October 2022.), the number of heat-related deaths has shot up. In 2022, 3,271 excess deaths were recorded during heat periods (registration delays mean that figure could still rise).
Mean temperatures have been steadily on the rise in the last 100 years with 16 of the 17 hottest years on record have occurred since 2000 as can be seen from the below chart published by the Met Office. A strategy to deal with rising temperatures appears to be needed more than ever.
What are the requirements?
Simply put, new residential buildings must provide overheating mitigation.
There are two methods of assessing overheating::
Simplified method or
Dynamic thermal modelling
The simplified method is a prescriptive way of dealing with the regulations. The method focuses on limiting solar gains by prescribing a maximum glazing area and calculating what amount of ventilation there should be to remove excess heat.
Dynamic thermal modelling refers to the use of CIBSE's TM59 methodology.
Developers may choose to undertake dynamic thermal modelling instead of the simplified method. However, there are cases where there is no choice but to undertake dynamic thermal modelling.
How does noise come into the building regulations?
Approved Document E: Resistance to Sound deals with sound insulation between dwellings and within the same dwelling. There is also mention of noise in Approved Document F; however, this is mainly to address noise from ventilation systems as well as outside noise when considering ventilation to the outside, either through openable windows for purge ventilation or using noise-attenuating background ventilators in areas of sustained and loud noise (e.g. a moan road). Traditionally, dealing with external noise happens during the planning stage.
Now with Approved Document O, noise is relevant to determine whether dynamic thermal modelling must be carried out. Noise mitigation strategies determined during planning often presume that residents will have to shut their windows to achieve internal noise guidelines, and this is then at odds with the assumption that windows can be used for adequate ventilation or for removing excess heat. This is extensively addressed in Acoustics, Ventilation, and Overheating Guide developed by the ANC and the IoA. For this reason, Approved Document O recognises that the simplified method would not be appropriate where external noise is an issue.
What are the external noise level conditions?
This is where things begin to get a little contentious. The document itself does not directly state what external noise levels are an issue. There aren't hard and fast criteria which leaves it somewhat open to interpretation. What the document does say is that:
And this is what is causing some murmurings in the acoustics community. The commonly accepted guidance for good internal noise levels is 10 dB less than those quoted in Approved Document O. I.e. ProPG recommends internal noise levels of 30 dB LAeq,T at night and no more than 10 events exceeding 45 dB LAFmax. ProPG itself bases those levels on the guidance contained in BS 8233:2014 as well as WHO guidelines.
On the face of it, it appears that paragraph 3.3 in Approved Document O is simply wrong or at the very least not supported by the literature. However, these higher levels appear to have been advised for pragmatic reasons. I.e. if we used the WHO/BS 8233/ProPG internal noise levels and calculated the equivalent external noise levels, virtually all new houses would be subject to dynamic thermal modelling and this might be unnecessary. So to solve that problem they appear to have added 10 dB to them. Essentially, it appears to be fudging the numbers.
The counterargument in support of Paragraph 3.3 is that the internal noise levels referred to by WHO/BS 8233/ProPG are based on health effects arising from prolonged exposure as noted in a footnote to the internal ambient noise level table in BS 8233. The argument is that for a few nights a year, residents will choose to expose themselves to higher internal noise levels to remove excess heat.
The argument goes that the purpose of Approved Document O is to firstly limit heat gains and then remove excess heat. Essentially, the idea is that reducing heat gains would mean fewer occasions to remove excess heat and the exposure to noise by opening windows would not be prolonged. Short-term exposure to noise levels exceeding those linked to long-term adverse health effects should not lead to those adverse health effects. So, it doesn't make sense to set the internal noise level criteria on those health effects.
Still, that does not deal with the issue that Paragraph 3.3 in Approved Document O flatly states that residents are likely to keep their windows closed at those noise levels which is a claim that doesn't appear to be supported - hence a number of acoustics professionals raising objections.
My view is that it would have been better to link the requirement for dynamic thermal modelling to already existing external noise level risk assessments, such as the one provided in ProPG. There doesn't appear to be any prospect that this will be changed in Approved Document O in the near future so all eyes are on how industry guidance will straighten things out.
What is the industry guidance?
The ANC and IoA are hurrying to write guidance so that noise consultants approach this in a consistent way. Some favour calculating the external noise level criteria from the internal noise levels quoted in paragraph 3.3 of Approved Document O. What's going in favour of that approach is that external noise levels based on those calculations don't seem that unreasonable. Others feel that because paragraph 3.3 is not supported by research, whether windows are considered closed should be consistent with existing guidance such as ProPG or AVO.
The draft guidance is currently out for consultation and can be found here, It's expected that the final guidance will be published in January 2023 with the current draft favouring the calculating external noise levels from the internal noise levels quote in paragraph 3.3.
Any other issues?
It's not just noise that has caused a bit of confusion. The document has other ambiguities or potentially even contradictions with other building regulations. For instance, the document refers to "minimum free area" in the simplified method - except what is actually meant is "minimum equivalent area". Or that's what we think it means because the regulations continually state that the "equivalent area" of the windows must be more than the "minimum free area" given in tables 1.3 and 1.4. It would make much more sense to substitute "minimum equivalent area" every time you read "minimum free area" and this apparent misuse of terms has the potential to cause confusion.
The Department for Levelling Up, Housing and Communities have released an FAQ that does clarify a lot of issues and those should be read alongside the document.
What should new housebuilders do?
It seems likely that building control officers are going to struggle at first with the new requirements along with developers. Keep an eye on the final guidance expected to be released by the ANC and IoA and if in doubt get advice from an acoustics expert.
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