Passive warming of indoor space induced by tropical green roof in winter

Thermal-energy performance of green roofs in winter is seldom investigated. With poor building thermal insulation and lacking artificial heating, indoor space in winter in subtropics can become uncomfortably cold with health implications for elderly and weak people. This experimental study established two extensive green roofs on a residential building in humid-subtropical Hong Kong. Broadleaved Perennial Peanut (Arachis pintoi) and succulent Mexican Sedum (Sedum mexicanum) plots were compared with bare concrete-tile Control Plot. Temperature sensors were installed along a holistic vertical temperature profile. Three apartments below the plots were left vacant to permit undisturbed monitoring. At Control Plot, notable heat loss especially in nighttime induces upward heat flux to cool indoor air. Vegetated roofs provide receptor and repository of solar energy as sensible heat to generate green-roof heat-sink effect (GHE). The porous substrate stores interstitial water to augment thermal capacity, conductivity and GHE. Warmer green-roof vis-à-vis cooler ceiling generates a thermal gradient to induce downward heat flux to warm indoor space. Peanut Plot with thicker substrate creates a stronger GHE than Sedum. Extensive green roofs in subtropical areas offer passive warming to indoor space in winter, with implications on indoor-heating energy consumption for a progressively aging population and climate-change adaptation.