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York Computer Science Building acts as vessel for natural ventilation

By John E. Czarnecki, Assoc. AIA, with Sara Hart

Although designed prior to the implementation of the LEED (Leadership in Energy and Environmental Design) green building rating system, the York University Computer Science Building would likely qualify for a LEED Gold rating, according to the architects.

The building’s shape and external shading provide an efficient envelope. Exterior walls, composed primarily of curtain-wall systems, precast concrete, and copper, have a thermal value of R23, and the roof has a value of R35. Ordinary concrete emits greenhouse gases, but the concrete used in this structure is composed of a mixture with at least 50 percent fly ash. The fly-ash concrete will emit far less greenhouse gas than conventional concrete. The building also features a planted roof that allows for evaporative cooling in summer and added insulation in winter. Excess rainwater is collected and stored in a rooftop tank for controlled dispersal into a drain to the ground.

According to the architects, the Toronto climate will allow for natural ventilation without heating or air-conditioning 55 percent of the year. When natural ventilation is adequate for maintaining comfortable temperatures, temperate air is drawn through intake grilles into an underground plenum where air is passively cooled, then drawn into rooms through floor- or wall-based diffusers. In this condition (see middle section diagram, opposite), fan coils are locked out, and windows and stack dampers open to allow cross ventilation. The air rises in rooms and is warmed by building occupants and computers. The exposed concrete structure absorbs, holds, and slowly releases heat before it rises through openings in the building’s two atria, creating a pressure differential. Stack effect draws air into operable windows and wind pressure raises the air-change rate, allowing for passive cooling. As outdoor air temperature rises, exhaust fans are turned on to assist ventilation. When heating or air-conditioning is needed, fan-coil units draw in mixed air, heat or cool it, and deliver it to building spaces. A direct digital control system, which manages the heating, cooling, and ventilation, determines the switch-over point from natural ventilation to normal systems and vice versa.