EVER wondered why old colonial-style buildings are so cool and airy and today's modern flats are hot and stuffy? The answer is the design, says one architectural firm that has a display focusing on how environmentally-friendly concepts can be incorporated into buildings from the drawing board stage. ''Colonial buildings have thick walls,'' said K. S. Wong, project architect for Anthony Ng Architects. ''They make flats feel cooler.'' Ways of making flats cooler - without having to resort to air-conditioners - form a big part of the firm's display at the Environmental Pavilion. The display also covers ways to reduce noise, minimise water use, encourage waste recycling and to control wind effects. It looks at alternative building materials and environmental education. ''We have analysed the energy use or electricity bills of a typical household,'' Mr Wong said. ''Air-conditioners account for a large portion of the cost.'' Strategically created wind passages in a building could do much to make flats cooler, as could placing shades on facades and windows, he said. These simple methods could go a long way towards reducing energy use - and electricity bills. ''Improving ventilation inside the flats not only improves the comfort level of the tenants but also reduces the tendency to use the air-conditioner,'' he said. Mr Wong said the thickness of external walls, which acted as insulators in retaining or repelling heat, and the colour of a building affected a flat's temperature. Managing wind helped reduce energy use and also made the external environment of a building much more comfortable for residents, he said. Some locations could be quite windy: to control the wind at ground level, for instance, the designers should include features, such as podiums, around communal areas. Block of flats could produce a great deal of noise from equipment, such as lifts, and designers should strive to minimise this, he said. In a model of its environmentally-friendly building of the future, Anthony Ng Architects has included collection bins for the recycling of plastics, paper and aluminium. It has roof solar panels for heating water that takes advantage of Hong Kong's sunny climate, which is an ideal source of solar energy. Groups that use solar energy include the Architectural Services Department, the Urban Services Department and some New Territories residents. But Mr Wong added the technology had only limited usefulness in large blocks of flats because of the small size of the roof area compared to the building as a whole. The firm's proposal for reducing water use is, however, suitable for all buildings. ''A low-flush toilet uses less water per flush,'' Mr Wong said. The firm's environmentally-friendly building design was developed as a project for the Housing Society, although it has still to decide whether to use any of the ideas.