A new Building Regulation for overheating: practical considerations for the acoustic constraints Jack Harvie-Clark 1 , Nick Conlan 2 , Nick Dobinson 3 Apex Acoustics Ltd, Design Works, William St, Gateshead, NE10 0JP, UK James Healey 4 AESG, 309 Mermaid House, 2 Puddle Dock, London, EC4V 3DS, UK David Trew 5 Bickerdike Allen Partners LLP, 121 Salusbury Road, London, NW6 6RG, UK R Kazlauciunas 6 Zehnder Group UK Ltd, Concept House, Watchmoor Point, Camberley, GU15 3AD, UK

ABSTRACT

A new Building Regulation to mitigate overheating applies in England from June 2022. A requirement of the regulation is that the strategy to mitigate overheating must be usable, considering occupants’ safety and comfort. If internal noise levels exceed 40 dBA L Aeq, 8 hr , or 55 dB L Amax more than 10 times a night, then windows cannot be assumed to be open during the night time period. This paper discusses a triage process to consider the noise risk, the practical methods and uncertainties in the noise survey, environmental noise modelling, calculation of facade sound insulation with windows sufficiently open to mitigate overheating, and demonstrating compliance. Potential solutions for natural ventilation and mechanically assisted ventilation are outlined. Until now, the environmental noise impact on new dwellings has been controlled through the planning regime, where environmental health officers have acoustics expertise. The Building Regulations are enforced by building control officers, who may have little or no training in acoustics. More than 30 % of existing dwellings in the UK are exposed noise levels that may preclude reliance on opening windows to mitigate overheating in future dwellings.

1 jack.harvie-clark@apexacoustics.co.uk 2 nick.conlan@apexacoustics.co.uk 3 nick.dobinson@apexacoustics.co.uk 4 j.healey@aesg.com 5 dtrew@bickerdikeallen.com 6 Rupert.Kazlauciunas@zehndergroup.com

1. INTRODUCTION

The new Approved Document O – Overheating [1] (“AD-O”) – comes into force for new Building Regulation applications from 15 th June 2022. The environmental noise affecting new homes will have a significant impact on how they are designed to mitigate overheating; we estimate that around 30% of existing dwellings would not meet the noise level limits set using the Simplified Method for window opening requirements.

1.1 Why is noise a factor to take into account? Most people are unaware that noise presents the second biggest burden of environmental disease, after air pollution. The largest effect is sleep disturbance, mostly related to road traffic noise. Noise mitigation often relies on the sound insulation of closed windows. Mitigation of overheating – most commonly by means of opening windows – has traditionally been ignored by the industry. Conlan et al [2] reviewed 122 planning applications for major residential developments that were required, under the London Plan, to have both noise and overheating assessments. According to Conlan, in 85% of those applications, the windows were assumed to open to pass the overheating assessment, while the noise assessment assumed that windows were closed to achieve reasonable noise levels. It is disappointing to offer occupants a choice of one environmental comfort or another, but not a reasonable balance of both; however, this reflects the historical absence of a holistic, integrated assessment method to address thermal and acoustic comfort.

1.2 Where do noise level limits apply? Approved Document O (AD-O) is the first time that regulation of environmental noise falls under the Building Regulations, rather than Planning legislation. Requirement O1(1) of the Building Regulations indicates that reasonable provision must be made to:

1. limit unwanted solar gains in summer; 2. provide an adequate means to remove heat from the indoor environment. Requirement O1(2) says that in meeting the obligations above:

1. account must be taken of the safety of any occupant, and their reasonable enjoyment of the

residence; Noise at night is one of the five issues that must be addressed to comply with requirement O1(2), i.e. the “reasonable enjoyment of the residence”. AD-O has one paragraph, para. 3.3, that indicates the noise level limits, stating: Windows are likely to be closed during sleeping hours if noise within bedrooms exceeds the following limits.

1. 40 dB LAeq,T, averaged over 8 hours (between 11pm and 7am). 2. 55 dB LAFmax, more than 10 times a night (between 11pm and 7am). This statement is so brief that the reader is left to infer many things that may seem sensible, but are not actually indicated. This paper is our interpretation of what would be an appropriate way to interpret this brief specification. Given the very significant constraints of noise to the use of opening windows that we envisage, it is vital that practitioners adopt consistent approaches to the assessment.

1.3 Are the noise levels reasonable? The noise level limits set for inside bedrooms during the night time are 10 dB above the annual average guideline levels that are widely accepted as providing good protection for people against the adverse effects of noise. Those guideline levels are described in ProPG: Planning and Noise [3], and BS 8233 [4], which are both referenced from the Government’s Planning and Noise guidance [5]. However, those publications don’t take explicit account of a need to open windows to achieve thermal comfort. The annual average guideline levels have been derived from many large epidemiological studies into the adverse effects of environmental noise on people in their homes. The most comprehensive data gathering and review in recent years was the World Health Organisation’s Environmental Noise Guidelines for the European Region (2018) [6]. These guidelines consider the exposure of the property to environmental noise, rather than the internal conditions, and show the

wide range of responses to a given noise level between studies. They also omit guidelines for the L AF,max levels, on the basis of an insufficient evidence base. The authors of this paper are also major authors of the Acoustics, Ventilation, Overheating: Residential Design Guide [7] (“AVO Guide”). The AVO Guide proposes a more holistic but complex assessment than that in AD- O; it suggests a marginally higher limit of 42 dBA L Aeq, 8hr for bedrooms if opening windows are not required to mitigate overheating very often. The AD-O approach is simpler with an absolute threshold regardless of duration. Public health practitioners may suggest that the noise limits in AD-O are too relaxed, and that adverse impacts may still occur at these levels. While we would concur, it is perhaps necessary to take a pragmatic approach in this first instance of considering noise in mitigating overheating, to avoid over-designing in response.

1.4 How to determine if noise should be taken into account? AD-O notes in para 3.2 that: In locations where external noise may be an issue (for example, where the local planning authority considered external noise to be an issue at the planning stage), the overheating mitigation strategy should take account of the likelihood that windows will be closed during sleeping hours (11pm to 7am). Thus any planning requirement to consider noise in the environmental design clearly indicates that the response to demonstrate compliance with AD-O must consider noise. However, there are many sites for which the local planning authority may not require a noise impact assessment, but for which the development may not comply with the internal noise level limits. There are freely-available national noise maps [8], but these are based on calculated traffic flows for major roads only, and do not go down to the lower levels required to assess compliance with the AD-O limits. Therefore these maps also omit many sites that would not comply with the noise level limits required. It will be very difficult to rule out the need for a survey or comprehensive noise exposure predictions on almost any site. Environmental noise maps are usually prepared as a strategic noise management tool as a requirement of the Environmental Noise Directive. Information is available on the night time dB L night metric. No information is available from these maps on noise events using the LAF,max metric. As will be seen below noise from individual events at night can be the most significant noise constraint for most sites. Another source of information is previous planning applications for nearby sites – it can be possible to estimate potential noise levels, although it is typically more difficult to assess L AFmax levels from other consultants’ reports, as there is no consistency in the way these are measured, processed and reported. In all these sources of information, it is easier to determine that a survey should be undertaken, but much more difficult to rule out the requirement for a noise survey. For example, for average pass-by maximum levels for the quietest class of car with an internal combustion engine moving at 30 mph, a residential facade at 7 metres from the carriageway, it can take just 10 vehicles passing during the whole night time period (11 pm - 7 am) to cause an exceedance of the Building Regulations for the plots overlooking that road, for moderate risk locations. It is going to be very difficult to rule out such occurrences without a specific survey and detailed predictions as a form of evidence. AD-O also refers to in-situ measurements to demonstrate compliance, but most developers would rather know at the design stage if the development complies with the Building Regulations, so that an appropriate design response can be built in. However, this does provide some basis for retrospective judgement of compliance, albeit with further thought needed on a suitable procedure. 2. ACOUSTIC INDICATORS AND ASSESSMENT

2.1 Application of limits Clearly, human response to noise varies widely between individuals and circumstances, and does not change dramatically from 1 dB to the next; however, it is necessary to set thresholds so that designers have clear boundaries within which to work. Two criteria are identified for noise limits. The convention for these types of guidelines in noise assessments is that both limits apply. Therefore if

either criterion a) or criterion b) is exceeded, then windows should be assumed to be closed during the night time period, rather than requiring both criteria to be exceeded to invoke this outcome. In 2021 Conlan et al [9] undertook a review of 478 full eight-hour night time measurements, from 239 separate locations, and presented our conference paper. These data indicated that in 75 % of cases, the L AF,max criterion (ie. AD-O para. 3.3 b.) is more onerous than the average 8 hour criterion (AD-O para. 3.3 a.), at the measurement location (which is understandably the case unless receptors are at a reasonable distance from a continuously active source). Therefore in all cases, it is necessary to demonstrate compliance with both criteria, not simply one or the other.

2.2 Sources of sound There is no description in AD-O about the source of sound that may give rise to internal noise levels. As the outcome is that “windows are likely to be closed”, it is reasonable to consider external sources of sound only – although see discussion further below about noise from any internal mechanical services. Natural sources of sound are not mentioned; birds produce high sound levels, as can other animals, the wind, watercourses, and the sea, for example. It should be assumed that these and other natural (non-man-made) sources of sound are excluded from the assessment. The convention for this type of noise guideline is also that it applies to anonymous or “non- actionable” sources of man-made noise, as described in the ProPG and AVO Guide, for example. This means that noise from transportation sources (road vehicles, trains & planes) is included, but noise emanating from commercial / industrial premises or entertainment noise is usually treated a different way. This is because noise from commercial premises would fall under nuisance law and potentially the Environmental Protection Act – hence it is not appropriate for other guidance to indicate how that legislation may be applied. Usually, appropriate limits for actionable sources of noise would be lower than the limits in AD-O, therefore this should not be an issue. However, it is appropriate to include these sources of noise in this assessment, as if present, they are likely to be more intrusive and disruptive than transportation noise sources. Noise from mechanical services (e.g. ventilation or cooling) within the property are not covered under AD-O, which refers to AD-F1. However, noise from mechanical systems to mitigate overheating are not covered.

2.3 Noise parameters No references are given for the noise parameters (or “indicators”) used, but the descriptions in AD- O are insufficient by themselves. The Association of Noise Consultants’ Environmental Sound Measurement Guide [10] provides suitable guidance for scoping, survey methodology, preparation, site work, data handling and storage, and analysis. The L AF,max criterion actually requires further definition; it is not sufficient to simply state “L AF,max ”, because crucially, the reference time over which a single value is determined has a significant effect when considering the 10 th highest value. Indeed, the number of events recorded also depends on the reference time for evaluating this parameter. For example, you could choose to record the L AF,max every second, every minute, every hour, or every day. In Assessing L max for residential developments: the AVO Guide approach , Paxton et al [11] suggested that a time interval between one and three minutes would be suitable, based on the sleep study research into the effects of noise events. The uncertainty introduced of using a time period of one or five minutes was investigated in Empirical relationship between L night and L Amax [9] A one minute sample period reduces uncertainty, compared with a five or fifteen minute sample period. The average (mean) difference between the 10 th highest L AF,max and L night (i.e. L Aeq, 8 hr ) at the microphone position was 19 dB; in over 75% of cases, the L AF,max was more than 15 dB greater than the L night . This means that in over 75% of cases, the L AF,max will be the more onerous design constraint, and will dictate the design response. A smaller difference between the 10th highest L AF,max and L night occurs where there are higher volume traffic flows.

2.4 Survey equipment The Environmental Sound Measurement Guide indicates that sound level meters would normally be expected to comply with precision Class 1as defined in BS / IEC 61672-1 [12]. Calibration and

verification procedures should comply with either BS 7580 [13] or BS EN IEC 61672-3 [14]. This requires that meters and field calibrators are periodically checked by a suitable laboratory such that the measurement is traceable to national standards. There is no “Type Approval” requirement in the UK.

2.5 Survey duration A representative survey duration is essential - but how long is it necessary to measure to be reasonably confident that the measurements are representative? A good practice guide on the sources and magnitude of uncertainty arising in the practical measurement of environmental noise , by Craven & Kerry [15] suggests that short surveys may have a high level of uncertainty. The empirical analysis [9] suggested that adding 2 dB to the tenth highest L AF,max measured at one minute intervals from one full 8-hour measurement is often reasonable. Measurements for less than one complete night time period would not seem to be justifiable at all. Where there are lower traffic flows, results are less consistent from day to day, and hence a longer survey duration may be appropriate.

2.6 Modelling of L Aeq, T and L AF,max Modelling of the continuous equivalent sound from a sound source (eg motor vehicle) moving along a road is well established. And so is the modelling of a point source of sound, if you know where that point source is. As noted above, the representative event to be used in the modelling could have happened anywhere, and thus it should be considered for its highest impact on different aspects of the proposed residential development. An objective approach is to create noise models for new development sites using standardised source specific calculation algorithms (e.g. CRN, CRTN, AEDT, ANCON etc.). However these standard prediction models do not provide calculation algorithms for noise maxima using the dB L AF,max metric. Therefore a noise survey will always be required to have confidence that the appropriate source levels are used to evaluate the impact on the proposed development. If you simply use the same source model as for modelling the continuous equivalent levels, you will over-predict the L AF,max levels – because the event just happens at one place at a time, not everywhere along the road, for example – so propagation with distance follows the 20*Log (distance) relation rather than the 10*Log (distance), as for a line source. Therefore appropriate modelling of the measured source data is required to assess the propagation of a point source of sound that may occur anywhere along a road, for example. It is understood that some environmental noise software packages can carry this out without needing careful user operation, but others not.

2.7 Survey data analysis Now a new problem emerges – actually an old but unresolved problem. When interpreting the data for noise from events, characterised with the L AF,max indicator: unless the event was witnessed, you don’t know where the event occurred. Thus ten separate events may all be recorded at the same level on the microphone, but if one of those events happened close to the microphone, and others much further away, those events that happened much further away actually had a far greater sound power level. The distance that you assume between the source and the microphone has a direct relationship to the sound power calculated. It is tempting to assume that the event happened at the closest position (eg of the road) to the microphone, because that may seem like it would give the highest results, but actually the exact opposite is true. The further away the source was, the louder it must have been to make that sound level at the microphone. This adds uncertainty into the measurement and assessment of L AF,max levels. Suggestions to resolve this or links to relevant research gratefully received!

2.8 Free area requirement Once the noise levels incident at the facade are determined, it is necessary to calculate the reduction at the facade with the open areas necessary to mitigate overheating, to determine compliance with the internal noise level limits. The simplified method in AD-O indicates minimum free areas for

bedrooms in either Moderate risk or High risk locations – these free areas are the same whether the home has cross-ventilation, as shown in Table 1.3 of AD-O below (or without cross-ventilation, as shown in Table 1.4); an extract of Table 1.3 is shown in Figure 1 below .

Figure 1: Extract of Table 1.3 from AD-O indicating the minimum free area requirement However tempting it may be to jump to the conclusion that “free area” means the area within the window frame, Appendix D of AD-O explains that “free area” really means an equivalent area of 0.62 times the free area, as shown in Figure 2:

Figure 2: Extract of AD-O explaining how to interpret “free area” requirements Assistance (an online calculator or look-up tables) is given to calculate the equivalent area from a side hung window. Interestingly, the equivalent area of a window opening can be greater than the equivalent area of a “sharp-edged orifice” with the same free area. Thus the opening area of the window can in some circumstances be smaller than the free area requirement, which is likely to confuse many people. It’s a missed opportunity that the AD-O didn’t adopt the ventilation area terminology proposed by Jones et al [16]. Note also the discussion further below of the free area requirements to comply with the purge ventilation requirements of Approved Document F1.

Table 1.3 Minimum free areas for buildings or parts of buildings with cross-ventilation High risk location Moderate risk location Total minimum free area” The greater of the following The greater of the following: a. 6% of the floor area”! a. 9% of the floor area”! b. 70% of the glazing area? . 55% of the glazing areal? Bedroom minimum free area 13% of the floor area of the room! —_4% of the floor area of the room‘)

2.9 Calculating internal levels While there is an appropriate standard for calculating the sound insulation of facade elements and the global performance, BS EN ISO 12354-3, the sound insulation performance of a window wide open is not well known. A study published in 2018, Differences between Outdoor and Indoor Sound Levels for Open, Tilted, and Closed Windows, Locher et al [17] shows that the average difference between freefield external levels and internal levels for windows fully open is 10 dB on average, in the properties surveyed. A simple theoretical approach is to use the guidance in Annexe D of BS EN 12354-3 [18], as shown in Equation (1):

𝐷 𝑛,𝑒 = −10. 𝑙𝑔( 𝑆 𝑜𝑝𝑒𝑛

10 ) (1)

Where: S open is the area of the opening / m 2 .

Appendix D: Calculating equivalent area D1 The free areas in Section 1 of this approved document are geometric open areas that assume a clear sharp-edged orifice with a 0.62 coefficient of discharge (Cd). Different opening types will reduce the amount of air flow by both affecting the way air flows and reducing the physical area. Accounting for these factors gives the equivalent area.

This equation is equivalent to the performance of an element with area, S and zero sound insulation, R in Equation 1 as described by Harvie-Clark. [19]. The area of the opening can be calculated from the minimum free area requirement and equivalent area achieved by the window opening, which also depends on the opening angle. If we stick with the free area requirement as a function of the floor area for simplicity, and assume that the room height is 2.4 m, then Equation 1 can be simplified if all other noise ingress routes are disregarded as irrelevant compared with the opening. This leads to the following level differences between freefield external and internal noise levels:

● Where open area is 13 % of floor area, level difference = 4.7 dB ● Where open area is 4 % of floor area, level difference = 9.8 dB To do this, we have equated a ventilation free area with an acoustically open area. As the free areas are minimum values, these are the maximum level difference values that can be achieved, depending on the actual window dimensions. See further notes below about purge ventilation requirements exceeding 4 % of the floor area in any case. It can be seen that a value of 10 dB is consistent with the empirical evidence for windows being fully open. A value of 5 dB is consistent with the significantly larger opening indicated by a free area of 13 % of the floor area.

2.10 Definition of acoustic practice procedures The acoustic requirements are insufficiently defined in AD-O; appropriate interpretations and clarifications of the guidance are proposed, including:

● Indication that neither noise level limit should be exceeded; ● Sources of sound that should and should not be included; ● Use of appropriate, calibrated equipment; ● A reference time period for assessment of L AF,max of one minute; ● A minimum survey duration of one whole night time period (8 hours, 11 pm - 7 am). Hazards and additional uncertainty in the assessment can be due to

● Consideration of location of event causing L AF,max ; ● Modelling L AF,max A user guide detailing these requirements is necessary to ensure consistency of approach, for the reasons given further below. 3. UNDERSTANDING AD-O AND DESIGN RESPONSES

Environmental noise is only a constraint to the reliance on opening windows to mitigate overheating during the night time period in bedrooms. There are no noise constraints to the use of opening windows in other rooms at any time, and no noise constraints to using opening windows in bedrooms during the daytime. To understand potential design responses to a noise constraint, it is necessary firstly to consider how AD-O addresses overheating risk and mitigation, and how the noise constraint is described in AD-O.

3.1 Requirements of Approved Doc O There are two means of demonstrating compliance with AD-O. The first is described in Section 1: Simplified Method. This prescribes maximum glazed areas for dwellings depending on:

● Orientation of the largest glazed facade; ● Whether the building has cross-ventilation or not; ● Whether it is in a Moderate or High risk location. Along with a requirement for shading in High risk locations (which are most of Greater London, and Central Manchester is also suggested), these constraints meet the requirement to limit unwanted solar gains in summer. There are no real acoustic implications of this aspect to the requirements. It is the requirement to remove excess heat, however, that describes the minimum free area requirements for dwellings as a whole, and also for bedrooms in particular. As the noise limits only apply to bedrooms

at night, it is these requirements for bedrooms that have the acoustic implications. Tables 1.3 and 1.4 (for dwellings with and without cross-ventilation) both have the same minimum free area requirement:

Extract of Table 1.3 from Approved Doc O

This shows that for High risk locations the free area must be ≥ 13 % of the floor area, and elsewhere (Moderate risk locations) the free area must be ≥ 4 % of the floor area. Hence to be able to use the Simplified Method, it must be demonstrated that with these free areas, the internal noise level requirements can be achieved.

3.2 AD-O Section 1: Simplified Method The meaning of “free area” and the sound insulation of such a free area is discussed above. A free area of 4 % of the floor area may mean an outside (freefield) to inside level difference of only 10 dB, and in high risk locations where 13 % of the floor area is required as the minimum free area, a level difference of up to 5 dB results. If an assessment of the external noise levels indicates external noise below the internal limit plus the level difference, then noise can be demonstrated not to be a constraint to the use of the Simplified Method. Criteria for both the whole night average level, L Aeq,8hr and for the tenth highest L AF,max , must both be considered against their respective limits. Where external levels exceed the internal criteria by more than 5 dB (High risk locations) or 10 dB (Moderate risk locations), the Simplified Method cannot be used to demonstrate compliance with AD-O, and dynamic thermal modelling must be used. It is still possible to use opening windows to mitigate overheating using the dynamic thermal modelling method.

3.3 AD-O Section 2: Dynamic thermal modelling Dynamic thermal modelling takes greater account of the location of the dwelling in setting environmental factors such as external temperatures and exposure to wind. The simplified method is intended to be simple but robust, so a more detailed calculation of overheating risk may reveal that smaller open areas are sufficient, for example, to enable reliance on opening windows. However, arriving at a suitable design balance between reducing solar gains and mitigating overheating with smaller open areas is not a straightforward process. Commercial software for carrying out CIBSE TM59 [20] calculations provides an analysis of the architectural and thermal design – it cannot optimise between different inputs that may have different constraints, for example. Thus it requires judicious modelling to determine if an appropriate solution is even available for a given architectural design, that meets the noise constraints. There is conflicting information in AD-O about open windows – the section on Dynamic thermal modelling indicates how windows should be assumed to be open. Para 2.6 (b) states: (b) At night (11pm to 8am), openings should be modelled as fully open if both of the following apply.

1. The opening is on the first floor or above and not easily accessible. 2. The internal temperature exceeds 23°C at 11pm. The possibility of partially opening a window at night, such that it may sufficiently enable ventilative cooling but limit excessive noise ingress, is not explicitly mentioned. A larger open area may be

Table 1.3 Minimum free areas for buildings or parts of buildings with cross-ventilation High risk location Moderate risk location Total minimum free area” ‘The greater of the following: a. 6% of the floor area” 1.70% of the glazing area’? ‘The greater of the following: 2. 9% of the floor area”! b.$5% of the glazing area” Bedroom minimum free area 13% of the floor area of the room! 4% of the floor area of the room

required during the daytime, otherwise it could be an option to simply make the window opening areas smaller. There are many reasons to make the modelling of natural ventilative cooling as permissive as possible, as this is certainly desirable for sustainable and resilient buildings. However, the guidance as currently written precludes this type of approach, as windows must be modelled as wide open all night if the internal temperature is greater than 23 degrees at 11 pm. It is also noted that the free area requirement in Approved Document F1 for purge ventilation is at least 5 % of the floor area for hinged or pivot windows with an angle of opening at least 30 degrees. Thus it may be permissible to assume the 4 % floor area requirement applies for the simplified method, but when using dynamic thermal modelling all openings should be fully open if the criteria are met. At the time of writing, the regulation goes beyond the capabilities of some modelling software currently available, in requiring a differing pattern of excess heat removal depending on daytime and night-time.

3.4 Attenuated passive solutions If external noise ingress through open windows is excessive, one option is to use an attenuated opening to permit natural ventilation. Although that may sound simple – an attenuated louvre may come to mind, for example – embodiment of this idea is far from simple. When closed, the ventilator must maintain the air tightness and sufficient thermal insulation from outside. When open, it must allow the passage of air as freely as possible but reduce the noise. The practical possibilities of available products are explored in Ventilative cooling in noisy environments: practical options for the UK [21]. There is certainly an external noise range in which these can be effective, but early implementation is necessary – they may have a significant impact on the facade, and so need to be considered from a very early stage in the design. The implication in TM59 is that the adaptive thermal comfort model should be used, as this would constitute “primarily naturally ventilated” homes. There is limited data from manufacturers of acoustic louvres in respect of the effective area and discharge coefficient to enable practitioners to use these products in calculations.

3.5 AD-O compliance with closed windows If the criteria of TM59 cannot be met using openable windows, the overheating criteria that must be followed changes considerably. When using opening windows to mitigate overheating, TM59 uses an adaptive thermal comfort model to determine thermal comfort limits. This means that as the outside temperature increases, the allowable temperature inside also increases (but in a slightly different way). However, TM59 notes that: Homes that are predominantly mechanically ventilated because they have either no opportunity or extremely limited opportunities for opening windows (e.g. due to noise levels or air quality) should be assessed for overheating using the fixed temperature method based on CIBSE Guide A (2015) However, the fixed temperature method does not have separate limits for daytime and night time periods, but simply one criterion that 26 degrees should not be exceeded for more than 3 % of annual occupied hours. Thus new modelling guidance is required on how to assess a bedroom that uses a mechanical system at night but open windows during the daytime. Options to mitigate overheating with closed windows include mechanical ventilation (which AD-O classes as “passive means”, AD- O Appendix A), or mechanical cooling. To provide the ventilation aspect a whole dwelling MVHR system may be designed to run in a (sufficiently quiet!) boost mode to provide adequate ventilation to mitigate overheating during the night time period. It is more challenging to mitigate daytime overheating in this way, although that is not required under AD-O. An alternative to using a whole-dwelling system is to use a local MEV or PIV system. This is usually an entirely separate system from the ventilation system to meet Approved Doc F requirements, because it focuses on the bedrooms in question rather than on extracting from bathrooms or kitchens. In this case, external noise ingress through the inlet (or outlet, depending on the design) should also be considered.

3.6 Noise from mechanical systems Noise from mechanical systems for cooling is not explicitly covered under the 2021 Approved Documents O and F. Para 2.11 of AD-O discusses mechanical ventilation and mechanical cooling to mitigate overheating, and there is a note referring to compliance with the noise and maintenance clauses of AD-F. However, AD-F1 describes noise level limits from mechanical systems only for the provision of whole dwelling ventilation or extract ventilation (for water vapour). There is no description of noise level limits for mechanical ventilation to mitigate overheating. However, if mechanical systems (for either ventilation or cooling) are noisy, people may find the noise intolerable and turn them off. Noise limits are described in CIBSE Guide A, How loud is too loud?: noise from domestic mechanical ventilation systems [22] and the AVO Guide. 4. CONCLUSION AND FURTHER CONSIDERATIONS

4.1 Design considerations for noise The hierarchy of considerations for noise are likely to be:

1. Use the simplified method if noise levels permit; 2. Where exposure does not permit the use of the simplified method, identify the largest open

areas that are acoustically permissible – improve the thermal design (eg shading) to work with the acoustically-allowable open areas, using dynamic thermal modelling; 3. Consider mitigating noise through passive means – eg use of attenuating balconies and / or

attenuated vents; 4. If reliance on passive openings is not viable, design mechanical ventilation with either a

centralised or distributed system; 5. Use mechanical cooling.

4.2 Demonstration of compliance: competent persons scheme In view of these intricacies and complexities, it would seem mightily unfair to ask Building Control Officers to interpret anyone’s assessment of compliance with the noise guidance in Approved Document O. There are not currently any “approved methods” for compliance with any of the details discussed in this article. A competent person’s scheme or other form of certification for checked and audited practitioners would seem to be the minimum arrangements for effective implementation. Furthermore, a standardised form of reporting to accompany the Building Regulations submission should be provided. There are significant risks in moving regulations about environmental noise from Planning (where it currently sits), to Building Regulations. In the Planning regime, noise is dealt with by Environmental Health officers, who have education, training and expertise in noise. However, the technical details of acoustics in building design has often been a more difficult subject for Environmental Health officers, who don’t have training or experience in this area. Under Building Regulations, enforcement falls to the Building Control officer (BCO), who is either a member of the local authority building control department, or an Approved Inspector. While BCOs have specific expertise in evaluating compliance of plans and designs with the Building Regulations, they usually have very little or no training and experience of considering environmental noise. Experience with other parts of the Building Regulations (especially Part F, for example) indicates that BCOs are often happy to accept a statement of compliance from someone who is better qualified than themselves to make the assessment. In Approved Doc O, this statement is on page (iii) Anyone using the approved documents should have sufficient knowledge and skills to understand the guidance and correctly apply it to the building work. This is important because simply following the guidance does not guarantee that your building work will comply with the legal requirements of the Building Regulations.

4.3 Risks of an unregulated industry The requirements for noise as currently stated are very brief and could be interpreted and misinterpreted in many different ways. BCOs are unlikely to have the expertise to interrogate any submission made to them regarding compliance with the noise constraints. The implications of having to design and build alternatives to opening windows to mitigate overheating are significant and costly; therefore there is a high level of significance in the outcome of the noise assessment. Thus developers may lean on consultants to provide a favourable outcome, and consultants may find little risk in making optimistic assumptions in their assessments. However, it is in the interests of the whole industry and our wider society, not only for the benefit of the future occupants of new dwellings ) that assessments are carried out consistently and reliably. As a society we need to build energy efficient, sustainable, comfortable and resilient dwellings that will not need to be retrofitted in a few years time, for example, with mechanical cooling; similarly, occupants should not have to choose between intolerable thermal or acoustic conditions; a reasonable balance between these aspects must be available, which is the ethos running through these Regulations.

4.4 Clarifications required for consistent practice To summarise, this review has considered the requirements of the new Approved Document O, identified significant complexities and anomalies requiring clarification, and has suggested the following measures where ambiguity exists or further information for the industry is required to appropriately undertake an assessment:

1. A noise survey and comprehensive prediction is required at the outset of any Overheating

strategy. An agreed prediction method in relation to the dB L AF,max metric is required. 2. Compliance should be demonstrated for both the L Aeq,8hr and the L AFmax criteria in AD-O. 3. The sources of sound for the assessment should be external and anthropogenic noise sources

only, not natural sources of sound. 4. A time interval of one minute in determining the 10 th highest L AFmax noise level is

recommended. 5. Surveys should be undertaken using Class 1 precision equipment as defined in IEC 61672-1

and IEC 61672-3. 6. Surveys should cover at least one full eight hour period between 23:00 and 07:00 hours. 7. When using survey data, a competent person should identify the likely location of the sound

sources contributing to the L Aeq,8hr and L AFmax readings. 8. The minimum bedroom free area requirements for the Simplified method equates to a level

difference of 5 dB for high risk areas and 10 dB for moderate risk areas, between freefield external and reverberant internal levels. 9. Acoustic louvres cannot be assessed without aerodynamic tests to demonstrate the equivalent

(or effective) area of the product for these to be used in calculations. 10. New guidance is required on how dynamic thermal modelling should assess a bedroom that

uses a mechanical system at night but an open window during the daytime. 11. Noise level limits are required for mechanical services (ventilation or cooling) that mitigate

overheating. 5. ACKNOWLEDGEMENTS

We are grateful to all our colleagues who helped form the ideas and response in this paper. 6. REFERENCES

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