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Developments in areas with high external noise levels are unlikely to be able to rely on opening windows to mitigate overheating risk and may be required to install comfort cooling. Approved Document O (Overheating), published on 15th December 2021, sets internal noise limits that must be met in bedrooms during the night. If these limits are not met with windows open, then the overheating mitigation strategy cannot utilise open windows during the night.

This is a change from the current approach often used at the planning stage where the frequency of night-time window opening in bedrooms is agreed with the acoustician, following the Acoustics Ventilation and Overheating Residential Design Guide. Strategies often allow some window opening during the hottest months of the year, even if this will expose residents to unreasonable noise levels.

Depending on the noise levels, passive measures may be sufficient to mitigate summertime overheating in some instances. Such measures are likely to include fixed external shading (shutters, overhangs, awnings, brise soleil), small windows with large reveals and low G-values, and acoustically treated louvres or vents. Building Control will want to see evidence that all of these measures have been used first before resorting to the use of mechanical cooling, which should only be used where overheating cannot be mitigated using openings.

This will also impact developments with air quality issues and ground floor bedrooms where open windows could present a security risk. Easily accessible bedrooms should be provided with fixed or lockable louvred shutters, window grilles or railings to allow secure window opening.

The new AD-O takes effect on 15 June 2022 and will not apply to work subject to a building notice before that date, provided the work is started on site before 15 June 2023. This is the same timeframe for the implementation of the new AD-L and AD-F.