Thermal behavior of green roofs under Nordic winter conditions

Steven Collins, Kirsi Kuoppamäki, David J. Kotze, Xiaoshu Lü

Research output: Contribution to journalArticleScientificpeer-review

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 equivalent thermal resistance of the complete green roof system during winter conditions in southern Finland. Three experimental green roof platforms (1 m x 2 m) with heated boxes and three identical bare roof platforms (without substrate, vegetation and other green roof layers) were equipped with thermocouples that continuously measured a vertical temperature profile through the roofs. A steady-state heat transfer analysis was performed to assess the functioning and relative thermal performance of the green roof systems. 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. 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. (C) 2017 Elsevier Ltd. All rights reserved.
Original languageEnglish
JournalBuilding and Environment
Volume122
Pages (from-to)206-214
Number of pages9
ISSN0360-1323
DOIs
Publication statusPublished - 2017
MoE publication typeA1 Journal article-refereed

Fields of Science

  • 1172 Environmental sciences
  • Green roof
  • Thermal conductivity
  • Heat flux
  • Energy saving
  • Winter conditions

Cite this

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title = "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 equivalent thermal resistance of the complete green roof system during winter conditions in southern Finland. Three experimental green roof platforms (1 m x 2 m) with heated boxes and three identical bare roof platforms (without substrate, vegetation and other green roof layers) were equipped with thermocouples that continuously measured a vertical temperature profile through the roofs. A steady-state heat transfer analysis was performed to assess the functioning and relative thermal performance of the green roof systems. 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. 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. (C) 2017 Elsevier Ltd. All rights reserved.",
keywords = "1172 Environmental sciences, Green roof , Thermal conductivity , Heat flux , Energy saving , Winter conditions",
author = "Steven Collins and Kirsi Kuoppam{\"a}ki and Kotze, {David J.} and Xiaoshu L{\"u}",
year = "2017",
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language = "English",
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Thermal behavior of green roofs under Nordic winter conditions. / Collins, Steven; Kuoppamäki, Kirsi ; Kotze, David J.; Lü, Xiaoshu.

In: Building and Environment, Vol. 122, 2017, p. 206-214.

Research output: Contribution to journalArticleScientificpeer-review

TY - JOUR

T1 - Thermal behavior of green roofs under Nordic winter conditions

AU - Collins, Steven

AU - Kuoppamäki, Kirsi

AU - Kotze, David J.

AU - Lü, Xiaoshu

PY - 2017

Y1 - 2017

N2 - 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 equivalent thermal resistance of the complete green roof system during winter conditions in southern Finland. Three experimental green roof platforms (1 m x 2 m) with heated boxes and three identical bare roof platforms (without substrate, vegetation and other green roof layers) were equipped with thermocouples that continuously measured a vertical temperature profile through the roofs. A steady-state heat transfer analysis was performed to assess the functioning and relative thermal performance of the green roof systems. 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. 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. (C) 2017 Elsevier Ltd. All rights reserved.

AB - 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 equivalent thermal resistance of the complete green roof system during winter conditions in southern Finland. Three experimental green roof platforms (1 m x 2 m) with heated boxes and three identical bare roof platforms (without substrate, vegetation and other green roof layers) were equipped with thermocouples that continuously measured a vertical temperature profile through the roofs. A steady-state heat transfer analysis was performed to assess the functioning and relative thermal performance of the green roof systems. 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. 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. (C) 2017 Elsevier Ltd. All rights reserved.

KW - 1172 Environmental sciences

KW - Green roof

KW - Thermal conductivity

KW - Heat flux

KW - Energy saving

KW - Winter conditions

U2 - 10.1016/j.buildenv.2017.06.020

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EP - 214

JO - Building and Environment

JF - Building and Environment

SN - 0360-1323

ER -