In parallel with our deep involvement in writing the AVO Guide, we have been reading and disseminating the research that has been carried out around the world on solutions for natural ventilative cooling. We believe that natural or passive ventilative cooling could offer a more sustainable solution than a mechanical solution. The London Plan Policy 5.9 outlines a cooling hierarchy that places natural ventilation above mechanical solutions. The simplest – and usually the default – solution for natural ventilative cooling is to rely on opening windows. But if sufficiently opening windows to effectively mitigate overheating permits excessive external noise ingress, what options are there for the designer? You can watch the presentation below, and download the full paper.
In this paper we explore the possible application of attenuated vents to enable passive ventilative cooling in noisy environments, where opening windows for this purpose is not acoustically feasible. The paper starts with a little background on ventilative cooling and descriptions of the terms that is is necessary to understand. There is a refresher on ventilation opening terminology, including free area, equivalent area, effective area, and discharge coefficient. This is necessary to understand the ventilation performance of a vent, so that the sound insulation performance can be compared on a like-for-like basis with ventilation performance.
The ventilation performance in terms of effective area of an open window – with particular dimensions, open a specified amount – is used as a benchmark to compare against different attenuated vents. Firstly, different types and styles of open window are compared for their sound insulation performance, by combining data from the Napier Study NANR116 with emerging information on the effective ventilation area of partially open windows. The discharge calculator is available to assess environmental conditions in schools and is likely to be included in a future revision to CIBSE Guide AM10.
The vent performance is compared in the same manner, against a reference value for ventilation effectiveness, so that the sound insulation values can be directly compared. This demonstrates which type of vent is most effective acoustically. Cost can also be taken into account, to determine the most cost-effective vent for any given performance requirements.
The application of attenuated vents is considered for a typical London flat. This illustrates how a good thermal design is needed to mitigate overheating, rather than simply attempting to rely on ventilative cooling, as otherwise the vent sizes required can easily be excessive.
The use of hybrid options, combining natural ventilation and a mechanical option to provide “boost” air flow, are also discussed.