“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

How to Choose a Ventilation Strategy for Residential Apartments

The ambient noise environment is likely to dictate the ventilation strategy for residential apartments in larger, urban developments. There are normally only two strategies to consider:

  • Full Mechanical Ventilation with Heat Recovery (MVHR), as per System 4 in Part F of the Building Regulations. This system uses a centralised fan to ventilate each room individually via ductwork in the ceiling, or
  • Trickle ventilators in the external facades and continuous mechanical extract from wet rooms, as per System 3 from Part F.

Systems 1 & 2 from Part F are normally impractical for larger urban developments. System 1 is best suited for quiet rural housing developments. System 2 is impractical for all developments!

Diagrams from Part F showing how System 3 and 4 work are shown below.

Each system is quite different from an acoustic perspective and it is important to understand these differences when considering which ventilation system to specify.

The pros and cons of each system are outlined below.



  • Very energy efficient due to heat recovery and therefore great for compliance with sustainability targets (i.e. SAPS, BREEAM).
  • No need for trickle ventilators in facades and therefore higher levels of sound insulation are achievable. This is particularly important for noisy sites where it may be difficult to achieve acceptable internal noise levels.
  • Air can be taken from facades away from busy roads to achieve better indoor air quality.
  • Boost and summer bypass modes can be used to help offset thermal gains in summer. This means residents in noisier areas can keep their windows closed for longer periods without the habitable rooms overheating.
  • Background ‘white’ noise from the MVHR system can be very effective at acoustically masking noise from other sources, such as neighbours or nearby external sources that would otherwise disturb residents.


  • Expensive compared to System 3 due to MVHR unit cost but also cost for associated ducting, grilles etc.
  • Careful specification and design is required to ensure that noise from the MVHR system does not disturb residents. Click here for more guidance on the acoustic design of MVHR systems.
  • The MVHR units and associated ductwork take up space. Ceiling voids need to be sufficiently deep to carry the ductwork and space is required to house the MVHR units.
  • Require more maintenance than other systems.


Trickle vents


  • Very cheap and normally easy to integrate into the window frames.
  • Easy for residents to control with simple open and close mechanism.
  • Less risk of onsite issues and/or customer complaints due to simplicity of the system.


  • Standard ‘hit & miss’ trickle vents will significantly limit the sound insulation performance of the façade. Therefore, they will need to be acoustically upgraded at noisier sites to achieve acceptable internal noise levels.
  • Acoustically upgraded trickle ventilators may need to be quite big and are often considered to be ugly.
  • There are limits to the levels of sound insulation that are achievable with acoustic trickle ventilators
  • Less thermally efficient than MVHR.
  • No boost function and therefore more reliance on openable windows to offset thermal gains in summer months, although additional independent MEV systems can be added to individual rooms if increased ventilation rates are required to offset thermal gains.
  • No background noise masking provided.

It is appropriate to adopt MVHR for sites in noisier areas that are also likely to have poorer air quality. However, for quieter, cleaner sites, the cost savings available with trickle ventilators make this a preferable strategy.

Therefore, the noise levels at the site can be used to determine the best ventilation strategy.

A diagram outlining the likely best ventilation strategy based on external noise levels is given below.



In summary, MVHR is appropriate for the noisiest sites whereas trickle ventilators are appropriate for the quietest sites. Acoustic trickle vents can be used where noise level are not excessive however MVHR may be preferable for non-acoustic reasons, principally aesthetics and thermal performance.

We hope you find the above useful. Cass Allen are experienced in the design and specification of ventilation for residential developments. Please contact us for further information.



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.