Thermal behavior of green roofs under Nordic winter conditions

Abstract

To understand how green roofs affect building energy performance under cold climatic conditions, a proper thermal analysis of the roof and its components is required. To address this, we measured the thermal conductivity of each layer of experimental green roofs, as well as the equivalent thermal resistance of the complete green roof system during winter conditions in southern Finland. Green roofs were compared to bare roofs (without substrate, vegetation and other green roof layers) to assess the basic functioning and relative performance of the green roof system. Layer analysis at various intensities of frost penetration showed that the thermal conductivity of each layer decreased when penetrated by frost. In particular, thermal conductivity of the substrate and vegetation layers decreased from 0.41 Wm-1 K-1 and 0.34 Wm-1 K-1 prior to freezing, to 0.12 Wm-1 K-1 and 0.10 Wm-1 K-1 after freezing, respectively. This phenomenon is explained by a reduction in bridge-water connectivity during freezing and a volumetric water content that was below the critical threshold value. Overall, a frost depth that extended through the complete green roof yielded the greatest equivalent thermal resistance at a mean value of 2.01 m 2 WK-1. During times of snow cover, snow acted as an insulator and reduced the relative energy saving benefits achieved by green roofs. These results provide information for designing the substrate and vegetation layers of green roofs for optimal insulation.

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