An experimental study of the effect of roof cover on indoor thermal comfort

Abstract

Among the building envelope elements, the thermal importance of roofs in tropical buildings is immense. Due to the high sun angles in the tropics, roofs are exposed to the sun more often than the walls. The solar altitudes in the lower tropics in particular are so high that all slopes of all roofs are exposed to the sun virtually from sunrise to sunset (Figure 1 a). Thus, climate-conscious building designers in the tropics refer the role roof plays in sun protection as the “Roof Problem” (Cf. Koenigsberger & Lynn, 1965: Iff).

Figure 1a: Solar geometry in low latitudes Figure 1 b: Solar geometry in high latitudes Additionally, roofs are important in rainwater runoff in the wet tropics. Materials and methods of construction that keep the rain water away are important considerations in this region. Roof is also the costliest among the super-structure elements in a low-cost tropical building and its aesthetic appeal too, cannot be ignored in the tropical context.

However, contemporary tropical buildings pay little attention to the thermal aspects of the “roof problem.” As far as thermal comfort is concerned, contemporary tropical buildings seem to pay greater attention to providing cross-ventilation than the roof. Even when shading is provided, tropical buildings follow the dictates of temperate practices where walls receive greater care than roofs.

In Sri Lanka too, contemporary designers and builders pay little attention to the thermal aspects of the “roof problem.” Roof cover, pitch and external color are determined more on the basis of aesthetics and/or cost than their thermal performance. There is little or no emphasis on the thermal comfort influences of roof cover. Lack of pirical research applicable to the tropics might be one of the reason for this weakness in contemporary Sri Lankan buildings.

It is in this light that the present study empirically evaluates the thermal comfort performance of three most popular roof cover options with and without ceiling. The roof cover options studied are, “Calicut” clay tiles, corrugated asbestos-cement roof sheets and half-round clay tiles on corrugated asbestos roofing sheet (“tiles-onasbestos”). These are studied together with flat asbestos, flat timber and a “no ceiling” options.