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Double-skin façades and natural ventilation performance | building roster

Double-skin façades and natural ventilation

Heat extraction double skin façades | Night-time ventilation | Mixed-mode and natural ventilation

Night-time ventilation 

During the summer and in the some climates where there is sufficient variation in diurnal and outdoor temperatures and a good prevailing wind, night-time ventilation can be used to cool down the thermal mass of the building interior, reducing air-conditioning loads. Heat gains generated during the day are absorbed by furnishings, walls, floors, and other building surfaces then released over a period of time in proportion to the thermal capacity of the material. Removal of these accumulated heat loads can be achieved with a variety of cross-ventilation schemes that rely on wind-induced flow, stack effect, and/or mechanical ventilation. 

In recent years, the concept of radiant cooling has been coupled with traditional cross ventilation schemes. For some climates and building types, this strategy can be used to completely eliminate the need for mechanical air-conditioning. Heavy-weight thermal mass is strategically located in exposed concrete ceilings. This mass is "activated" or cooled at night using outdoor air directed to flow over its unobstructed surface. During the day, occupants exposed to this chilled thermal mass perceive a cooler environment due to a radiative exchange with the low surface temperature of this thermal mass. 

"Adaptive" thermal comfort is a key concept that must be accepted by the building owner, facility manager, occupants, and code officials. Interior temperatures are expected to exceed the limits defined by the ASHRAE Standard 55, which was originally intended for conventional HVAC applications. Field studies suggest that behavioral adaptations (changes in clothing level and air velocity, via local fans or operable windows) and psychological adaptations widen the range of acceptable interior temperatures - acclimatization or physiological adaptations are unlikely to result in significant changes (Brager and deDear 2000). 

Therefore, occupants of these new buildings who are accustomed to air-conditioned space should be made aware of the design intent of naturally-ventilated buildings so that their expectations for thermal control will be more relaxed. Employers might also make greater accommodations such as a more relaxed dress code during peak summer periods and allow employees to shift work hours or even telecommute if thermal conditions are unacceptable. 

Double-skin facades have been designed for the purposes of allowing night-time ventilation, with the reasons of security and rain protection cited as main advantages. However, single-skin facades are capable of having a larger proportion of unobstructed operable windows. The required percentage of facades openness is proportional to the internal heat load: for milder European climates or northern California coastal climates and for buildings where daytime solar loads are controlled, such a scheme may be feasible with a moderate degree of façade openness. 

The building exterior and interior are often shaped to minimize obstructions to air flow. The exterior façade tends to be planar with few horizontal projecting obstructions, particularly if there is no strong prevailing wind direction. The depth of the building is minimized. The interior is designed to have minimal floor-to-ceiling obstructions. Furniture systems located near the window are designed to have an open structure. Privacy screens between offices are kept to minimal heights. Ceiling heights are greater than 9 ft (10-14 ft) and no plenums are used. Lighting fixtures are pendant hung. The ceiling surface may be shaped to encourage laminar flow and to channel air from the window wall to the opposing window wall. 

As with any natural ventilation scheme, other factors must be considered: night-time humidity, moisture, and condensation control; magnitude of forces exerted on the windows, shading devices and internal furnishings by gusts or negative pressure; pollutant control; fire and security protection. Screens may be required to keep out birds and insects, reducing ventilation potential. 

Implementation of such a scheme involves the use of motor-operated flaps and windows that are controlled via a centralized building automation system. The sequence of operations must be designed and programmed for each unique site to accommodate the strategies for night-time cooling ventilation, heating conditions, fire emergencies, avoidance of condensation, closure against heavy rains, and occasional night-time occupancy. Exterior and interior sensors are used in each thermal zone to provide feedback for real-time operations. Commissioning and tuning the building must occur to ensure proper operations. 

References 

Brager, G.S., R. deDear. 2000. "A Standard for Natural Ventilation." ASHRAE Journal 42 (10): 21-29. 

Personal communication with P. Haves, LBNL and E. McConahey, Ove Arup and Partners California, November 2001.

Question/Information: eslee@lbl.gov