In designing climate responsive accommodation for the warm humid areas, where ambient air temperature and relative humidity are both above the desirable comfort level, the indoor conditions can be improved by keeping building open for natural ventilation, thus to outside conditions. As a result the outdoor and indoor air temperatures do not differ much. The primary function of building skin is to control the solar heat loads. It is necessary to shield any windows from direct sun penetration, and to reduce the heat transmitted through the sunlit walls and the roof. The east and west walls receive a good deal of radiation, but when the angle of incidence is small (early morning and late afternoon) the intensity of radiation is not at its maximum. The north and south walls receive comparatively little radiation and are much easier to shield with overhangs. The walls are easily shaded by overhanging eaves, verandas or verdant environment and therefore gradually acquire temperatures near to the air temperature. However, the roof is the most exposed to impacts of solar radiation, as it receives sunlight for practically the whole of the day, and in the tropics the angle of incidence is close to the normal in the hotter parts of the day. Heat gain through roof elevates ceiling surface temperature and causes radiant heat load on the occupants. The term ‘roof’ includes the roof structure, the outer covering, and layers of insulating materials or membranes and the ceiling.

The question arises, how much radiant heat load can be permitted without harmful effects on the life and work of the inhabitants? The answer to this question is guided by two conflicting motives: to satisfy comfort needs of the people of the warm humid tropics and to optimise the performance of the roof to provide good value of the investment. In other words, find a performance rating for roofs which is high enough to ensure progress and yet realistic enough to be generally attainable.

In tropical climates steady-state property, the overall air-to-air thermal transmittance (U-value) or thermal resistance (R-value), alone cannot provide a satisfactory performance criterion for rating roofs since the thermal performance of a roof is a function of the sol-air temperature wave form which is a function of climatic factors, surface colour and orientation. A performance criterion based on thermal comfort and actual thermal performance of a roof section would be appropriate.

Identification of problem is presented in the next section with a field experiment in India and followed by discussion on the state-of-the-art. The sections following it explain derivation and mathematical modelling of the new thermal performance index and its application for exemplar roofs in India and Australia.