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Green Roofs – Acoustic Considerations

24 November 2010 No Comment

As green roofs become an increasingly popular solution, their image as an “alternative” method of construction has not completely disappeared. One might therefore consider that caveats lie in wait and that they do not offer all the common benefits of more established options.

In practice, however, nothing could be further from the truth.

And nowhere is this more apparent than in the area of acoustics where green roofs, supported by Corus decks, offer a completely comprehensive solution, with additional advantages of their own thrown in for good measure.

Although acoustics has moved more into the mainstream of desirable building features in recent years, it is still something of a “grey art”. We often talk to designers who are confused between the sometimes conflicting requirements of sound reduction and sound absorption.

Sound reduction refers to the control of either internal or external noise in a building. Typical applications include concert venues, where one is trying to avoid excessive noise levels from breaking out and buildings in areas near noisy facilities, such as airports, where one strives to achieve precisely the opposite.

Sound absorption is the more common acoustic feature required by designers. It is the control of attenuation, or internal reverberation of sound against building surfaces. A school sports hall is a very typical, everyday application requiring attention.

Green Roofs installed over Corus decks and trays can be fine-tuned to achieve sound reduction and/or sound absorption. They actually offer two significant advantages over other types of roof construction: –

  • Green roofs contain soil and sedum which gives them good mass. This is of critical importance in a situation where sound reduction is required, as they provide a natural sound barrier that can be further tuned by careful attention to detail with the underlying construction.
  • Reduction of the drumming noise created by rainfall on the external roof surface is an important acoustic consideration in many buildings. Green roofs offer a naturally soft outer surface that provides a “built-in” solution to an age old problem.

Choice of deck or tray plays an instrumental role in the success of a properly executed green acoustic roof project.

We offer a full range of perforated decks and trays, for applications requiring sound absorption. The perforations help to break up sound as it hits the underside of the roof construction, thereby reducing reflectivity.

Dense mineral fibre is the general insulant of choice for acoustic applications. It is therefore common for a tissue or foil layer, to prevent fibres from falling through the perforations, to be inserted in the pans of decks or trays, or directly above the deck.

The “open area” – the degree of perforation of the deck surface – is critical to the success of sound absorption. We can provide decks and trays with open areas between 5% and 30%. In an ideal world, everyone would specify the 30% maximum. However, as the open area increases, so the structural performance of the deck or tray in question reduces. This has implications for the amount of bracing and secondary steel that will be required. Therefore compromises often have to be made, either in favour of better sound absorption levels and the consequent increase in steelwork, or vice versa. Cost is clearly a key driver here.

Moreover, if a project requires both sound absorption and reduction, the bigger the open area, the more sound breaks into the roof construction, increasing noise break in and/or break out levels.

As mentioned earlier, the superior mass of the green roof construction helps to reduce the deployment of other sound reduction “counter measures”.

Bespoke mineral fibre acoustic infills that fit flush into the deck/tray pan profile are the most beneficial option, because they act as a direct barrier to sound entering the construction.

Flexible, high density polymer mass layers with exceptional sound reduction properties are also commonly used in conjunction with insulation, particularly when very high levels of control are required, especially where very low frequencies need attenuation.

We hope that this blog entry has given you some general food for thought when considering acoustic performance in green roofs. Please contact our technical support team if you require any specific advice on 0845 30 88 330.

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