BCME - Autumn 2008

NATURAL VENTILATION

OF RESIDENTIAL

APARTMENTS

INAPPROPRIATE CONSIDERATION ON VENTILATION PERFORMANCE IN RESIDENTIAL APARTMENTS CAN LEAD TO UNNECESSARY DIFFICULTIES IN PLANNING APPROVAL AND CAN POTENTIALLY REDUCE ENERGY EFFICIENCY. SITE SPECIFIC WIND TUNNEL AND NUMERICAL STUDIES CAN OVERCOME THESE OUTCOMES.

Throughout history building designers have recognised the innate human desire to control air movement in the built environment and have incorporated passive design strategies into their architecture to encourage natural ventilation. At its most basic, natural ventilation is needed to remove air from a building interior and replace it with ‘fresh’ air. In warmer climates natural ventilation is used to remove accumulated heat gains during overheated periods, by encouraging enhanced rates of volumetric air change.

Also important in warmer climates is the role of ventilation in directly improving the perception of thermal comfort by occupants of a space. This is achieved when moving air passing over the skin aids the evaporation of perspiration. As long as there is some air movement, most people will tolerate higher temperatures before they complain of discomfort. In this context, ventilation is intended to achieve useful air velocities directed over the occupant.

The widespread adoption of air-conditioning in the second half of the twentieth century allowed building designers to effectively seal buildings and instead rely on mechanical ventilation systems to provide volumetric air-change and thermal comfort to occupants. In many building sectors architects no longer considered natural ventilation and fully sealed the building envelope. In the residential sector poor design for natural ventilation could be salvaged through air-conditioning, albeit often a simple package unit fitted by the resident post occupancy.

In more recent years the green building movement has recognised the high levels of energy typically required to drive air-conditioning in buildings and have refocussed attention toward opening up building facades again to incorporate natural ventilation. The benefits of natural ventilation

for residential apartment amenity have been recognised in NSW planning tools, initially State Environmental Planning Policy 65 - Design of Residential Flat Buildings and then through the introduction in NSW of the more complex mandated sustainable design compliance tool BASIX. In more recent years the CSIRO developed a real time simulation ‘network model’ for calculating air flow rates and incorporated a software module into the AccuRate heating and cooling load simulation supporting BASIX. Both SEPP65 and BASIX encourage design for cross-ventilation as a means of assuring natural ventilation performance.

The problem quickly becoming manifest is that both designers and planning officers are applying a rote interpretation of what constitutes a cross ventilated apartment simply looking for the openings in two different facades. Little if any attention is being paid to whether good natural ventilation will actually be realized. Worse, the often acceptable ventilation performance of single sided units is being overlooked. To properly design for apartment natural ventilation requires an appreciation of some fundamental wind engineering principles. Wind Engineering consultancy Cermak Peterka Petersen Pty Ltd (CPP) regularly conduct combined wind tunnel and numerical Computational Fluid Dynamics (CFD) studies to determine natural ventilation and space heat flushing potential of residential apartments, both dual frontage and single sided apartment types (Figure 1). CPP report that natural ventilation and infiltration through building facades will be driven by either wind pressure differences across a building envelope (pressure driven flow) or by buoyancy effects caused by air density differences (stack pressure). The greater the vertical distance between openings the

Figure 1: Contours of velocity magnitude with flow vectors through the living spaces of two adjacent single-sided residential apartments as predicted using CFD.