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Combined with high thermal insulation in the building envelope, the passive solar energy system was designed to pay for itself in 25 years through energy bill savings. The wind towers and solar walls were devised to be openly expressed as part of the overall architectural form.

A novel system of natural ventilation has been incorporated into all three house types, exploiting the natural stack effect of warm air. The shower cubicles, toilets, kitchens and communal spaces have all been designed to be ventilated without mechanical fans.

A separate solar gain trap centres on the communal staircase in each villa and helps to heat the communal spaces. A window wall has been built alongside the south flank of each staircase to admit sunlight that heats the staircase of exposed precast concrete and the solid brickwork partitions lining the communal corridors. Heat is retained by the solid elements of the staircase and partitions and recirculated at night when the air temperature drops.

Natural ventilation The wind tower, which is a prominent feature of one of the dwelling types, has been designed along traditional Arabian lines. It has been built alongside the open stairwell of the building, where it rises straight up through all three floors and then rises another 5.2 m above the roof ridge. Measuring a substantial 2 × 1.5 m, it is split vertically into two stacks. At the top of the tower, one stack ends in a side opening that scoops in cool winds and distributes the fresh air through grilles on the floors below. Passing winds draw exhaust air from the house and discharge it through the other stack.

Where Arup Associates’ version differs from a traditional Arabian wind tower is that a heat exchanger has been installed to extract heat from the warm exhaust air and transfer it to the fresh incoming air. In addition, a fan has been incorporated to operate the ventilation system on the few days when the external air is completely still.

Fire safety The main problem in the design was how to prevent the natural ventilation system spreading smoke in a fire. Student residences are classed as hotels for fire safety purposes, which means that the study bedrooms must be separated from communal corridors by fire-compartment doors and partitions. Fire doors are also needed between corridors and staircases, but at Durham, these have been left open and fastened with electro-magnetic releases that close the doors automatically when activated by a smoke alarm.

Providing ventilation to the en-suite shower cubicles of every study bedroom through open vents from the corridors was more difficult. The option of installing fire dampers in each vent was dismissed as too expensive and would involve fire alarms that could be troublesome if set off by students. Instead, a dispensation was eventually agreed with the local fire officer that each shower cubicle should form part of the same fire compartment as the corridor, rather than of the bedroom it served. As a result, the door between bedroom and shower cubicle has been specified as a solid fire door with spring closer.

Noise, kitchens and cost Another problem was posed by sound being transmitted through the open vent from the shower cubicle to the corridor.

This has been overcome by installing separate air-intake grilles opening to shower and corridor and connecting them with a narrow duct incorporating a sound attenuator.

A post-occupancy appraisal, carried out in February, reported that the wind tower ventilates well. Peter McEwen, the university’s estates officer, claims that there have been no problems of condensation around the shower and no complaints of draughts on cold windy days. One minor weakness was identified in the intensively used kitchens. The appraisal suggested that the problem could be remedied by fitting a small air-recirculation canopy with grease filters above each cooker.

The biggest drawback of the wind tower system was its unexpectedly high installation cost. The post-occupancy evaluation calculated it would take 100 years to recoup the cost through energy saved.