Acoustic design of ventilation systems for residential buildings

Acoustic design of ventilation systems for residential buildings

By Ruth MacEachern

Product Manager

There are many factors that must be considered when designing and commissioning a mechanical ventilation system in a new build or during the renovation of an existing building. These include measures to ensure indoor air quality and adherence to prescribed emissions levels. This article will instead focus on acoustic ventilation design, first examining what Approved Document F has to say about noise from mechanical ventilation systems, before considering the best design when soundproofing residential buildings and adequately reducing noise from ventilation systems.

Approved Document F on noise from ventilation systems

With numerous specifications and tables stipulating requirements regarding air flow rates, whole dwelling ventilation rates, and intermittent extract rates, Approved Document F is decidedly vague with regard to noise emissions. This may be on account of the fact that the noise affects only the occupants of the property. However, the majority of unwanted noise from ventilation systems is a result of poor installation and inefficiency, and as such a noisy installation likely serves as an indication that an installation is not performing as it should with regard to conserving energy and reducing carbon emissions. More detailed information on the importance of energy-efficient ventilation systems can be found here.

So what does Approved Document F suggest?

The guideline provided within the building regulations is that “mechanical ventilation systems […] should be designed and installed to minimise noise”. This includes:

• Correctly sizing and jointing ducts
• Ensuring that equipment is appropriately and securely fixed, such as using resilient mountings where noise carried by the structure of the building could be a problem
• Selecting appropriate equipment

The document also highlights the importance of the correct sizing and installation of fans so that they operate at full capacity (our dedicated article on noise from fans is available here). It advocates the consideration of external noise when considering the suitability of open windows as a means of purge ventilation, and suggests maximum ventilation noise levels of 30 dB for noise-sensitive rooms, and 45 dB for less sensitive rooms such as kitchens and bathrooms.

Why is soundproofing and insulation important?

The fact that there is no actual requirement for the testing or noise levels from ventilation systems should not be taken as an indication that reducing noise from your MVHR system is not important. In fact, a noisy system could be a sign that the installation is failing to provide many of the benefits for which it was installed. For example:

• Improved indoor air quality – noisy ducts or vents may indicate that the system has not been properly cleaned and maintained. This may be on account of a build-up of dust in the system, thus adding dust and other allergens into the living environment rather than eradicating them.
• Environmental credentials – noise is itself a form of energy that is being dissipated into the surrounding environment. Whether as a result of an inappropriately dimensioned system or poor installation, noise can be a symptom of issues that will require the system to work harder, thus reducing its efficiency.
• Cost savings – with the aforementioned drop in efficiency, any reduction in heating costs will be undone by an increase in the electricity bill of your client.

Tips for soundproofing and acoustic ventilation design

1. Ensure that the system is appropriate for the residential project – an improperly dimensioned ventilation unit may produce excess noise

2. Ensure that the central ventilation unit is properly positioned and mounted so as to avoid excess vibration. This includes considering the space required for the unit, and where it will be positioned in relation to noise-sensitive rooms such as bedrooms.

3. Fan units should be appropriately sized and installed

4. Choose soundproofing or sound absorbing materials for the  vent or duct work

5. Insulate the ducts with liners & vents with acoustic foam

6. Cover unused vents to dampen sound

7. Consider flexible ductwork at joints and sections that may be prone to excess vibration. This should be kept to a minimum and ideally would only be used for final connections.

8. Ensure that ductwork is appropriately dimensioned and is as airtight as possible - the more airtight the ductwork, the easier it is to control pressure loss and acoustic noise transmission. You should also carefully plan the routing of ducts and the positioning of ventilation openings.

9. When commissioning the system, use appropriate technical methods and equipment to determine sound levels  – do not rely on your ear.

10. Determine permissible sound levels in the various rooms in advance, and take appropriate measures to counteract excessive noise. These might include silencers to control duct noise transmission.

11. Have the system cleaned and maintained, as a build-up of dust can affect air flow, resulting in an increased noise level.

Conclusion: quiet, efficient, affordable, and future proof…

Noise pollution from a ventilation system that is too large or small, from clogged vents, or due to vibrations resulting from poor installation undoubtedly offers sufficient motivation to take the issue of soundproofing and noise reduction seriously. It should now also be apparent that the matter is inextricably linked with a vast array of other concerns relevant to architects, house builders and homeowners alike. With energy costs rising, and the government’s environmental targets requiring an increasing effort to ensure compliance, it is essential that you consider noise emissions when using ventilations so you benefit from  a system that is quiet, efficient, affordable, and future proof.