“Very proactive and extremely helpful, especially with technical queries. I would not hesitate to recommend and will certainly be using them again in the future.”



Under CDM2015 regs we have found CASS ALLEN to exercise a level of professionalism and due diligence which is second to none. We would therefore definitely recommend CASS ALLEN as our preferred acoustic consultant, for all of our projects.’


SF - Taylor Wimpey South Thames

“I was very pleased with their technical knowledge and professional approach when dealing with a very awkward purchaser as well as the local authority.”


JH, Telford Homes

“Cass Allen went above and beyond to help with the Acoustic planning performance and conditions regarding the Greenwich Creekside project…”


LM, Capita plc

“Good comprehensive reports and advice at all design stages. Very flexible and professional, provided attendance at acoustic cladding tests at short notice.”


DF, Berkeley Homes

“Cass Allen have assisted us on a number of our projects. I’ve been impressed with their thorough and professional approach.”


MT, Lark Energy

Openable Windows in Residential Developments – Acoustic Comfort vs Thermal Comfort

Design Briefing Note

Councils are increasingly requiring residential developments in noisy areas to be ventilated so that future occupants do not have to rely on open windows to prevent overheating.

This is to protect residents from the situation during warmer months where they either have to keep their windows closed and tolerate uncomfortably high internal temperatures or open their windows and tolerate unacceptably high internal noise levels. This design briefing note sets out a process to achieve adequate ventilation in residential developments so that windows can remain closed.

The process is outlined in Figure 1 below and is designed to achieve the noise criteria set out in BS8233:2014 – Guidance on sound insulation and noise reduction for buildings. It is compliant with current legislation, regulations and guidelines and is based on our extensive experience in the acoustic design of residential developments.

As shown in Figure 1, where windows need to be kept closed to protect residents from external noise; mechanical ventilation will invariably be required.

The mechanical ventilation systems and associated ductwork will normally need to be more powerful than standard systems in order to achieve the high ventilation rates required to offset overheating in habitable rooms during summer. This can have significant implications for equipment costs and space requirements inside ceiling voids to accommodate the larger ductwork required to carry the increased volume of air movement.

The mechanical ventilation systems will also need to be designed so that noise from the systems themselves do not disturb residents (i.e. noise from supply air and extract air grilles etc). Recent research indicates that residents can be disturbed by noise from mechanical ventilation systems over around 25 dB LAeq. We therefore generally recommend that, where possible, mechanical systems are designed to achieve noise levels around this level under normal background ventilation rates. However, it is generally appropriate to adopt higher noise limits for the mechanical systems when they running at the higher ventilation rates required to offset overheating.

It should be noted that there is no requirement in the Building Regulations to consider the impact of open windows on internal noise levels in habitable rooms (although there is a requirement to control overheating under Part L1A of the Building Regulations). For additional information regarding building regulations requirements, please refer to the full PDF of the briefing note via the link below.

However, developers may want to consider the implications of relying on openable windows to control thermal comfort under their duty of care to future residents.

Additional Information

Cass Allen Associates are a specialist Acoustics consultancy focused on the development, construction and environmental industries.

We are a full member of the Association of Noise Consultants and all consultants are either full Corporate or Associate members of the Institute of Acoustics (MIOA or AMIOA). Senior staff are Chartered Engineers (CEng).

We are experienced in the design and testing of large developments for major developers (e.g. Barratt Homes, Berkeley Homes, Bouygues, Morgan Sindall, Bovis, Crest Nicholson, Durkan, Galliard, Gladedale, Hill Partnerships, Kier Group, Mulalley, Mace Group, Taylor Wimpey, Telford Homes, United House).

For details of specific services, staff, past projects and clients, please see www.cassallen.co.uk

Figure 1 – Thermal Comfort vs Acoustic Comfort Flow Diagram



For additional information regarding building regulations requirements, please refer to the full PDF of the briefing note via the link below.



Get in touch

We hope you’ve enjoyed browsing our website but if you have any questions feel free to call us to talk through any acoustic issues you may be facing on your projects. The best person who can help with specific queries can be found here.