What overheating assessors actually need from a noise report — and why vague briefs cost money

Noise reports that don’t say enough are costing residential developers time, money, and unnecessary M&E clutter. Here’s what needs to change.

What the problem looks like

As overheating assessors, we regularly receive noise reports as part of the Part O compliance pack. The good ones are specific: they list each plot, each facade, which windows are affected, and the hours during which they must remain closed. We can work with those immediately.

Others are not like that. They state, broadly, that noise levels on site exceed the thresholds in Approved Document O — implying windows cannot open — but without specifying which rooms, which elevations, or which time periods are affected. When pressed, the response often points to the ANC Acoustics, Ventilation and Overheating Residential Design Guide (the AVO Guide) as sufficient reference. In our experience, that is not enough and leaves things open to interpretation.

The AVO Guide is a useful framework for acoustic consultants designing to the intersection of noise, ventilation and overheating. It is not a substitute for a clear schedule telling the design team exactly what they are working with.

Why it matters

Part O (Approved Document O, in force June 2022) sets night-time bedroom thresholds that determine whether windows can be modelled open. If external noise at a bedroom exceeds 40dB L_Aeq,T (8-hour night average) or 55dB L_AFmax more than 10 times per night between 11pm and 7am, that bedroom window must be modelled closed at night in the TM59 assessment.

If a window is closed, mechanical ventilation must cover the shortfall. Purge fans, acoustically attenuated supply vents, or MVHR with sufficient flow rates need to be specified — and each of those must also meet separate noise limits. Part O sets a 40dB L_Aeq,T limit in the bedroom for purge supply fans. Part F sets a stricter 30dB L_Aeq,T limit for continuous mechanical ventilation in bedrooms and living rooms at minimum low rate. In practice, the Part F limit is the binding constraint.

When the noise report does not clearly state which rooms are affected, the design team has two choices: ask the acoustic consultant and wait, or assume the worst case and specify accordingly. Too often, the second option wins on programme grounds.

The result is over-specification. Mechanical ventilation equipment gets sized for every habitable room in every affected plot. The external facade gets peppered with grilles that may not all be necessary. The cost goes up, and the building looks worse than it needs to.

A real example

We recently encountered a noise report on a multi-plot residential scheme that stated, when questioned, that all habitable windows on the affected elevation should be modelled closed. Not just bedrooms between 11pm and 7am — all habitable rooms, all hours.

That is a significant constraint. Every living room and kitchen on that elevation would require mechanical ventilation capable of meeting both Part F and Part O noise limits. The M&E cost implication is substantial.

A second acoustic consultant reviewed the same site and came to a different conclusion. They produced a clear schedule — by plot, by room type, by facade, and by time period — showing that the restricted window opening applied only to bedrooms, between 11pm and 7am, on the north-east facing elevation. The living rooms and kitchens were not affected during daytime hours.

Same site. Same noise data. Very different briefs and outcomes.

What a good noise report looks like for Part O and Part F purposes

A noise report that properly supports a TM59 or simplified method overheating assessment should include a schedule that confirms, for each plot:

• Which rooms are affected (bedrooms only, or all habitable rooms)
• Which facades are affected (not just “the site”)
• What time period applies (typically 11pm–7am for the Part O bedroom criterion, but not always)
• Whether the constraint is L_Aeq,T, L_AFmax, or both
• Does it comply with ‘Noise’ under Sections 3.2-3.4 of Approved Document ‘O’ of the Building Regulations.

Without this, the overheating assessor is second-guessing. That second-guessing either delays the assessment while we chase clarification, or it leads to conservative assumptions that inflate the specification.

When noise assessments happen — and when they should

Most acoustic assessments are carried out at planning stage. That is appropriate: noise exposure data informs whether a site is suitable for residential use and what mitigation is needed.

The gap is what happens after planning consent. The planning-stage noise report has served its purpose from the acoustic consultant’s perspective, but it has not yet answered the questions that the Part O and Part F assessors need before they can finalise the ventilation strategy.

Our recommendation is straightforward: brief the acoustic consultant to cover two stages.

Stage 2 does not need to be a full reassessment. In most cases it is a relatively concise document that translates the noise data into terms the design team can act on directly. It avoids the back-and-forth that delays programmes, prevents M&E over-specification, and means the overheating and ventilation assessors can complete their work with a clear brief.

This is not a criticism of acoustic consultants

The variation in report quality we have described is not unique to any one firm. It reflects the fact that the interface between noise, overheating and ventilation is a relatively recent problem — Part O only came into force in June 2022 — and the industry is still working out how to handle it consistently.

Some acoustic consultants have already expanded their scope to address this directly. Others rely on the AVO Guide and assume the design team will work it out. A clearer commissioning brief from the client or architect from the outset would help both sides.

The ANC AVO Guide is a good document. It just needs a clearer scope of service alongside it so that everyone on the design team ends up with the information they actually need.

What to do now

1. Check whether your noise report includes a room-by-room, facade-by-facade schedule of window opening restrictions.
2. If it doesn’t, ask the acoustic consultant to provide one — specifically referencing Part O and Part F requirements.
3. Brief new acoustic appointments to include this as a deliverable, either within the original scope or as a post-planning supplementary note.
4. Make sure the brief references the 11pm–7am window relevant to the Part O bedroom criterion — not just a general “windows closed” instruction.

If you need help reviewing what a noise report says (or doesn’t say) from a Part O or TM59 perspective, get in touch with Low Carbon Box → We work through these issues regularly and can advise on what the overheating assessment needs before work begins.

Frequently asked questions

Does Part O require bedroom windows to be closed all day if there is a noise problem?
No. The Part O night-time bedroom criterion applies between 11pm and 7am. If noise levels exceed 40dB L_Aeq,T (8-hour night average) or 55dB L_AFmax more than 10 times in that period, the bedroom window must be modelled closed at night — but this does not automatically affect daytime operation. Habitable rooms such as kitchens and living rooms are subject to different criteria, typically set through planning conditions.

What is the ANC AVO Guide and is it enough to rely on for Part O?
The Acoustics, Ventilation and Overheating Residential Design Guide (AVO Guide) was published by the Association of Noise Consultants in 2020. It provides a useful framework for addressing the three disciplines together. It is not, however, a substitute for a specific plot-by-plot schedule of window opening restrictions. Overheating assessors need that schedule to complete a TM59 or simplified method assessment correctly. It is also worth noting that the AVO Guide predates Part O of the Building Regulations (June 2022) and does not directly reference its requirements.

What ventilation noise limits apply when bedroom windows must stay closed at night?
Two limits apply. Under Part O, a purge supply fan must not exceed 40dB L_Aeq,T in the bedroom. Under Part F, continuous mechanical ventilation in bedrooms and living rooms must not exceed 30dB L_Aeq,T at minimum low rate. The Part F limit is the more demanding of the two and is effectively the binding specification requirement in most cases.

Why does a vague noise report increase construction costs?
Without a clear schedule of which rooms and facades are affected, the design team tends to assume worst case — specifying mechanical ventilation for all habitable rooms across all affected plots. If the restriction applies only to bedrooms at night, that over-specification means unnecessary equipment, higher costs, and more external grilles than the building actually needs.

When should a noise assessment be commissioned for a residential scheme?
The initial noise assessment should be commissioned at planning stage. For sites with elevated noise, a supplementary post-planning note is worth commissioning to translate the noise data into a clear schedule of Part O and Part F requirements. This is a relatively small additional cost that prevents significant design team delays and M&E over-specification later in the programme.

Low Carbon Box are accredited On-Construction Domestic Energy Assessors (OCDEA) providing TM59 overheating assessments and Part O compliance services for homeowners, architects and contractors across the UK. Assessor reference EES/011387.

For more information or to discuss your project: info@lowcarbonbox.co.uk | 01858 322011