Impact of dynamical thermal behaviour of external constructions on indoor temperature in warm locations: The case of Sao Carlos, Brazil

Abstract

In warm locations the effect that (strong) solar radiation is first absorbed on the outer surface, then transferred through the construction materials and finally arrives at the inner surface heating it up may be remarkable. The corresponding physical models describe this dynamical thermal behaviour, which is described by the quantities decrement factor (also known as temperature-amplitude-damping) and delay. There are two measures to reduce negative effects; thermal insulation and thermal inertness (mass). The paper investigates this effect in the warm and humid climate of Sao Carlos, Brazil. A typical living room is defined and both traditional and contemporary external wall constructions are proposed and assessed in regard to their thermal behaviour.

The corresponding local standards NBR 15575 and 15220 give thresholds for stationary and dynami- cal physical behaviour. The proposed constructions were assessed, and it was found they fulfil all the obligatory NBR 15575 standards (U-value and thermal storage mass) but not the wider recommendations of NBR 15220. All constructions were also assessed with the European standard ISO 13786 that describes the dynamical physical behaviour precisely, whereas the Brazilian standards use approximate formulas.

The selected room was investigated with the transient simulation software Primero-Comfort and the resulting indoor operative temperatures were assessed. Traditional constructions (brick) show very good values, whereas contemporary constructions (concrete slab, hollow concrete element) do not. The differences in indoor operative temperature lie in the range of two degrees, especially for the daily peaks.

Furthermore, wood-based constructions were proposed and simulated. The room already behaved better with the simplest construction (two sheets of plywood) than contemporary constructions. A filling with 5 cm thermal insulation leads to very good values.

Finally, recommendations for the currently used standards are given. Only the quantities of ISO13786 correlate well with the results; it describes the physical behaviour well and should be used to derive thresholds. NBR 15220 describes the decrement delay in satisfying correlation with the results but fails in the corresponding quantity for the decrement factor. Additionally, the obligatory NBR 15575 excludes light wood-based constructions even though such constructions behave well in the regarded climate. Both standards should be adapted.