Natural Ventilation Feasibility

As per the introduction to this blog, MACH Acoustics aims to provide a useful blog providing ideas, details and knowledge, where people can discuss ideas and hopefully move forwards in a more suitable and low carbon way.

MACH Acoustics has recently published four conference papers on the acoustic design of low energy buildings. Subjects covered are the acoustic performance of open windows, the design of high performance natural air vents, the onsite performance of corridor walls and methods of achieving cross ventilation. We are currently working on a 5^th paper looking at acoustic treatments in combination with thermal mass cooling. 

The starting point for this blog will therefore be a description of these papers.  The titles of each of the papers are listed below. There is also a link to a PDF copy of these papers below.

Acoustic Scaled Models and Naturally Vented Facades
NAT Vent Cross Talk Attenuator
NAT Vent Attenuator and Facades
Acoustic Performance of Open Windows

Green acoustic consultants probably have the greatest effect on a buildings carbon emission when reviewing the design of vented facades. This element of design is also likely to have the most significant impact upon the level of energy used by a building.

The graph below provided by Brent Associates Architects presents key information with respect to the energy consumption of building types and their location. This information in turn demonstrates how best to reduce the energy consumption of buildings.

Figure 1 - Energy consumption for a range of building types and their location

This graph shows that the greenest buildings are those that incorporate three elements; natural ventilation, a town or city centre location and the reuse of a building in the form of a refurbishment.  This conclusion is made on the basis that natural ventilation is significantly more energy efficient than mechanical ventilation (1 to 2 Figure 1).  A city centre location for a building reduces a significant chunk of the buildings carbon emissions as a result of reduced transportation (2 to 3). The third significant reduction in energy is achieved through refurbishing a building.  Here, the embodied energy required to form a building is not needed and hence this is a significant design goal for low energy building design (3 to 4).   

With respect to acoustics, these requirements make things difficult.  It is always complex and more complicated to refurbish a building. The difficulties arise as a result of reduced levels of flexibility in terms of design, structural constraints, facade restraints, planning, historical limitations, as well as other factors. As consultants we therefore need to be more creative and have a wide range of products and/or designs to overcome these limitations.

Locating the building within a city or town centre will always bring issues with respect to noise break in. The figure below shows a noise map taken from www.noiserus.com. Table 1 shows the percentage of land where it is possible to comply with BREEAM and BB93 internal noise levels when a building is ventilated by means of opening windows. The sound reduction of an open window is assumed to be 10 dBA.

Figure 2 - Town centre noise map provided by www.noiserus.com.

Table 1 - Percentage of sites available within town centres, where simple openable windows provide adequate levels of acoustic resistance such to comply with BREEAM and BB93 requirements.

Space	Target Noise Level	Percentage of site suitable for  natural ventilation
BREEAM – single occupancy offices	<40 dBA	41%
BREEAM – multiple occupancy offices	40-50 dB LAeq,T	47%
BB93 – classroom	35 dBA	22%
BB93 – lab/workshop	40 dBA	41%

It is clear from the figure above that the development of green buildings within towns and city centre locations, will more often than not require acoustic solutions which provide high levels of acoustic attenuation to environmental noise break in.

My next entry will therefore look at methods of increasing the acoustics resistance of vented facades.