Roofing materials for thermal performance and environmental integration of buildings - Bernoulli’s principle

Roofing materials for thermal performance and environmental integration of buildings

J. Jones, in Materials for Energy Efficiency and Thermal Comfort in Buildings, 2010

19.6 Natural ventilation

As introduced in Chapter 3 natural ventilation is a design strategy that can both reduce energy consumption and provide occupants with a positive psycho-physical connection between inside and out. When designing buildings for natural ventilation, the design of the roof can play a significant role in the ventilation strategy. Bernoulli’s principle indicates that as wind speed increases, pressure will decrease. Because of this, as wind flows over a building, it typically accelerates producing a low pressure uplifting force. This is why roofs often fail during high wind events such as hurricanes. This condition may be enhanced by moderately sloping the roof while smoothly compressing and accelerating the airflow on the windward surface of the roof and near the ridge. Utilization of the Bernoulli principle through the design of the roof can lead to solutions that intentionally create low pressure conditions to draw stale air out of the building. By analyzing wind speed and direction, and orienting the roof slope into the prevailing winds during periods when natural ventilation is appropriate, properly located roof openings can draw air out of the building. Figure 19.11 shows a proposed design of a middle school for Blacksburg, VA where wind tunnel studies indicated that the sloping roof could serve the natural ventilation strategy. In this building wind is compressed and accelerated over the moderately sloped roof creating a low pressure zone at the top of the corridor/atrium (Henderson 1999). This low pressure draws outdoor air through the adjacent classrooms and out of the building. Similarly Thomas Herzog’s Hall 26 was designed with a wave-form roof that compresses and accelerates the wind flowing over the building to create a low pressure region that draws air out of openings near the ridge of the roof. In both designs the roofs speak to their role in the natural ventilation system.

19.11. Image of a proposed naturally ventilated middle school for Blacksburg, VA .

(source: Henderson 1999)

A special case of Bernoulli’s principle is shown in Fig. 19.11 where air is smoothly compressed through an opening with increased air speed and lower pressure. Termed the Venturi effect, this condition can be taken advantage of for the design of roofs for natural ventilation. The image in Fig. 19.11 is for the proposed design of a middle school that includes a roof section with an upper inverted aerofoil element supported above a convex lower element. As the prevailing winds smoothly compress through this roof element, low pressure is created. By coupling this low pressure to the building’s interior, outward airflow will be induced. Similar roof designs are employed in the Ionica building in Cambridge UK by RH Partnership as shown in Fig. 19.12(a) (Jones 1998: 80) and the RWE Ag building by Overdick and Partners in Essen, Germany (Fig. 19.12(b)) to achieve natural ventilation (Jones 1998: 217). In the Ionica building, inverted pyramids are raised above the top of the central atrium to create the low pressure condition and draw air up and out of the building. In the RWE building, an inverted aerofoil disk is lifted above the top floor to compress and accelerate the wind to induce upward air flow out of the central ventilation shaft of the building. In all of these buildings the roof not only plays an important role in the natural ventilation strategy but becomes a visual expression of environmental response.

19.12. (a) Ionica Building (Jones 1998: 80) and (b) RWE Ag Building (Jones 1998: 217) utilizing roof elements to create a low pressure region to draw stale air out of each building.