The microclimate of an intensive green roof was monitored in subtropical winter.
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Mixed ANOVA found interactions among factors related to microclimatic regulation.
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Vegetation canopy suppressed retention of solar inputs and passive warming.
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Intensive green roof arrested net thermal radiation export observed on bare roof.
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Intensive green roof could significantly cool surface and air in subtropical winter.
Abstract
As urban green space, green roof has been increasingly installed in cities mainly for amenities, temperature regulation and energy conservation. Nevertheless, in subtropical urban setting and during winter, the thermal and radiation behaviours of green roofs, especially the intensive type, have remained poorly understood. This paper examined the effect of a woodland-type intensive green roof (IGR) on the roof-level microclimate by comparing temperature and radiation parameters against a reference bare roof (BR). In-situ microclimatic monitoring was conducted on an IGR and BR pair in subtropical Hong Kong throughout a winter. Using objective selection criteria, 30 sampled days with representative winter weather, namely sunny, cloudy and rainy conditions, were identified for statistical analyses of interactions among key factors. The results showed that the woodland canopy intercepted about 90% of incoming solar radiation regardless of weather, thus substantially suppressing the energy source of passive warming, and instead creating undesirable cooling of ground surface and canopy-enclosed air. Surface and air temperatures on IGR were lower than BR, with the highest mean difference reaching 2.77 °C and 2.27 °C respectively in sunny daytime. The vegetation canopy retained some but insufficient outgoing thermal radiation to counteract the observed cooling. Limited solar inputs could only slightly warm the soil which experienced heat loss in cloudy weather to raise indoor heating load. The empirical findings provided the basis to recommend changes in landscape and soil designs to allow IGR to yield thermal benefits in winter and to improve building energy efficiency.
