On 15 June 2022, Approved Document O (ADO) came into effect. This document presents the requirements of Part O of Schedule 1 of the building regulations, which aims to protect the health and welfare of occupants of new residential buildings by reducing the occurrence of high indoor temperatures. Contained in Approved Document O are two requirements relating to night-time noise levels. But how are these relevant to overheating? And what impact will they have on the thermal assessments and design of new residential premises going forward? Let’s find out.
Where does Approved Document O apply?
Approved Document O (ADO) applies to new residential buildings. According to section 0.3, these are defined as:
· Residential (dwellings), meaning dwellinghouses and flats;
· Residential (institutional), which includes homes, schools or other establishments where people sleep on the premises, including homes for caring for older or disabled people, and those under the age of 5;
· Residential (other), including residential colleges, halls of residences, student accommodation and accommodation for children aged 5 and over.
The requirement does not apply to hotel rooms. The assumed logic here being that stays in hotels are typically short-term and limited exposure to higher internal temperatures may be acceptable.
What is the requirement?
Requirement O1 states,
“Requirement O1 Overheating mitigation
1) Reasonable provision must be made in respect of a dwelling, institution or any other building containing one or more rooms for residential purposes, other than a room in a hotel (“residences”) to-
a. Limit unwanted solar gains in summer;
b. Provide an adequate means to remove heat from the indoor environment.
2) In meeting the obligations in paragraph (1)-
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.
On the face of it, this requirement seems clear; reduce solar gain and provide adequate cooling – only using mechanical cooling where the requirements can’t be achieved naturally.
However, there are some subtleties and further discussion in the statutory guidance which elaborate on this requirement.
So what’s the acoustic requirement?
While Requirement O1 itself doesn’t specifically state that noise must be taken into consideration, it does mention that account must be taken of the reasonable enjoyment of the residence. Quite rightly, this includes consideration of noise levels during mitigation of cooling.
As an example, if you live on a busy road and need to leave your windows open at night for cooling, the building needs to be designed so that the noise impact doesn’t outweigh the benefit of cooling.
Paragraphs 3.2 to 3.4 of the statutory guidance go on to say that,
“In locations where external noise may be an issue … the overheating mitigation strategy should take account of the likelihood that windows will be closed during sleeping hours (11pm to 7am).
Windows are likely to be closed during sleeping hours if noise within bedrooms exceeds 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)."
So in effect, what this tells us is that building designers should not rely on openable windows to provide cooling, if the internal noise levels are going to exceed these thresholds as a result.
For reference, LAeq noise levels can be thought of as an average noise level over the course of the night, while LAFmax noise levels represent individual events (cars backfiring or sirens).
What does this mean for the planning process?
Until now, the issue of internal ambient noise levels from external sources has been devolved to Local Authorities, and not part of the Building Regulations. Often, Local Authorities will condition the internal noise levels of new residential development, especially in areas they consider at high risk of noise exposure. However, these conditions have historically related to noise levels with the windows closed, during the ventilation condition (most of the year). Therefore, most of the existing housing stock in the UK has been designed and built with no consideration of noise levels with windows open during the overheating condition (the hottest days of the year), meaning that occupants have to make a decision between cooling and lower noise levels.
Even since 15 June 2022, Local Authorities are continuing to condition noise levels with windows closed, leaving the issue of noise during open windows to Building Regulations. This means that, hypothetically, acoustic assessments used to discharge planning conditions could come to different conclusions at the detailed design stage if the overheating strategy identifies that a different window type or method of cooling is required to that assumed at Stage 3.
It is therefore imperative that the acoustic consultant on the project encourage the client and design team to consider the issue of overheating and cooling strategy at the earliest stage possible, so mitigation assumptions during that the planning and detailed design are aligned.
What does this mean for the design process?
At the detailed design stage, the team will need to consider overheating and the cooling strategy, and co-ordinate this with the acoustic requirements. As mentioned above, there is a risk that the assumptions used to discharge noise conditions during planning are no longer relevant once the cooling strategy has been decided on, and it’s important to have these discussions as early as possible.
Approved Document O permits two types of overheating assessment, one being the ‘Simplified method’, which is more of a desktop assessment, and one being ‘Dynamic Thermal Modelling’, which is a detailed building model and calculations of solar gains and internal ambient temperatures.
The Simplified Method gives maximum glazing areas and recommendations for shading, as well as minimum free areas for buildings with and without cross-ventilation.
Dynamic Thermal Modelling involves a standardised approach to predicting overheating risk for residential buildings on the basis of CIBSE’s TM59 methodology for predicting overheating risk.
On the basis of the minimum free area requirements of the Simplified Method, the ANC and IOA’s Guide to Demonstrating Compliance with the Noise Requirements of Approved Document O (Draft July 2022) suggests that, in areas at a ‘high risk’ of overheating, the Simplified Method can only be used where external noise levels are up to 44 dB LAeq,8hr or 59 dB LAF,max, while these figures are slightly higher for ‘medium risk’ areas at 49 dB and 64 dB respectively. In our experience, this would put the majority of development in zones 1-3 London (which are classed as ‘high risk’ and are generally exposed to higher noise levels than those stated above) into the category which would require dynamic thermal modelling.
On the basis of the above, we can expect to see more dynamic thermal modelling required for developments in London, as well as medium risk areas with higher noise levels. Acoustic designers and thermal modellers will need to work closely together to identify a cooling strategy which meets the requirements of both disciplines. For acousticians, the question we can pose to thermal modellers is, “The maximum open area which works for noise is X – does this work thermally?” Whereas thermal modellers could pose the opposite question, “The minimum open area which works for the thermal requirements is X – does this work for noise?”
We have seen that the new Building Regulations requirements for Overheating also contain requirements for night-time noise levels, and that these need to be balanced with relevant planning conditions.
We have also seen that the route to compliance with Approved Document O may depend on the outside noise levels. Above a certain noise level, the minimum free areas for cooling using the Simplified Method will not provide adequate protection from outside noise.
Acousticians and thermal designers will need to liaise with architects and the developers at an early stage to determine the appropriate route to designing an overheating strategy which works for thermal and acoustic comfort.
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