Thermal Behaviour of green roof in winter condition

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http://urn.fi/URN:NBN:fi-fe20180115182
Title: Thermal Behaviour of green roof in winter condition
Author: Collins, Steven G.
Other contributor: Helsingin yliopisto, Bio- ja ympäristötieteellinen tiedekunta, Ympäristötieteiden laitos
University of Helsinki, Faculty of Biological and Environmental Sciences, Department of Environmental Sciences
Helsingfors universitet, Bio- och miljövetenskapliga fakulteten, Miljövetenskapliga institutionen
Publisher: Helsingfors universitet
Date: 2016
Language: eng
URI: http://urn.fi/URN:NBN:fi-fe20180115182
http://hdl.handle.net/10138/197731
Thesis level: master's thesis
Discipline: Urban Ecology
Multidisciplinary studies on Urban Environmental Issues
Abstract: Cities of the 21st century consume massive amounts of energy, and indoor climate control within the built environment is responsible for a large fraction of the total demand. With pressures to make buildings more environmentally friendly, new energy efficient technologies and designs are continually sought after. A green roof, or a living roof, is a structural design approach that can provide a variety of ecosystem services along with the reduction of building energy demands. It has been shown that green roofs are effective tools for reducing cooling energy demands in warm and sunny climates; however, in cold climates, where heat energy demands dominate, there is a lack of research and general uncertainty about how beneficial a green roof may be. This thesis, conducted during the winter of 2013-2014, focused on the thermal performance of green roofs in cold weather (winter) conditions. The aim of the study was to quantify the reduction in energy loss that a green roof achieves and to examine the thermal behaviour of each of the green roof layers. Extensive green roofs with hot boxes underneath were constructed in Lahti (southern Finland). Heat sensors were placed vertically through the bare and green roofs to measure linear heat transfer from the interior to the exterior. Heat transfer by conduction was assessed, and a steady state analysis was used to quantify heat flux values. Furthermore, a green roof thermal conductivity model was developed to estimate the thermal conductivity of each of the layers under various environmental conditions (changing moisture contents, frost depths, and during freezing and thawing periods). Monthly comparisons of the energy lost through the two roofing structures were quantified. My results showed that green roofs reduced the amount of energy loss through the surface compared to bare roofs throughout the winter season. The overall reduction in energy loss, due to the presence of green roofs, was on average, 32.6%. Layer analysis showed that thermal conductivity of each of the layers decreased when penetrated by frost. A frost depth that extended through the whole green roof yielded the highest thermal resistance for the green roof at 3.96 m2KW-1. Comparatively, the bare roof had a thermal resistance of 0.27 m2KW-1. During times of snow coverage, the snow acted as a good thermal insulator, reducing the relative benefits achieved from green roofs. During refreezing and thawing, the green roof experienced the lowest values of thermal resistance at 1.83 m2KW-1. These results can be used for quantifying possible heat loss reductions in similar climates using a similar green roof, and the layer analysis provides information of how to best design green roof components for thermal resistance.
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