Measurement of Thermal Properties of Growing Media for Green Roofs: Assessment of a Laboratory Procedure and Experimental Results

Abstract

While the Italian standard UNI 11235:2015 establishes minimum performance criteria, the thermal resistance of the growing medium of green roofs is not included in national regulations. Instead, thermal transmittance limits for roofs are obtained by referring to other stratigraphic layers. In the absence of specific national and international standards for laboratory calculations of the thermal performance of growing media for green roofs, a multi-year laboratory testing campaign was carried out on 8 samples which aimed to define the thermal resistance reference values of growing media as a function of density and water content. Thermal conductivity varies between 0.046-0.179 W/mK for dry samples as a function of density and between 0.046-0.47 W/mK as a function of moisture content. Defining a reference method, laboratory tests and restitution of the output in performance curves, was based on 108 tests carried out according to and deviating from the standard based on both guarded hot plate and heat flow meter methods. The significance of the results has prompted researchers and industrial partners to engage in further investment and ongoing tests in this area, targeting the definition of a standard laboratory method to be presented worldwide.

Performance of Blue-Green Roofs in Cold Climates: A Scoping Review

“…It is evident that determining the U-value for a green roof is a difficult endeavour, and the actual value will fluctuate significantly depending on several factors, most prominently water content as shown by Barozzi et al [61]. As shown in the studies described in this section, green roofs have a limited insulating effect, which becomes slightly higher when the roof freezes, but the overall insulation potential is both lower and less reliable than that of conventional roof insulation.…”

“…Ouldbouhkitine and Belarbi [60] measured the thermo-physical properties of green roof substrate materials and plants, for usage in building simulation models. In Barozzi et al [61], guarded hot plate and heat flow meter tests were carried out on 108 samples of growth media in the laboratory. Thermal conductivity (λ) was found to vary between 0.046 and 0.179 W/mK as a function of density, and between 0.046 and 0.470 W/mK as a function of moisture content (up to 50 weight-%).…”

The Laboratory Definition of the Thermal Resistance of Growing Media for Green Roofs: New Experimental Setups

“…The Construction Technologies Institute of the National Research Council of Italy (ITC-CNR) has many years of proven experience in the laboratory measurement of the thermal characteristics (thermal resistance, thermal transmittance and conductivity) of building materials and components. This is the reason why, in the attempt to give a first answer to these outstanding issues, during the years from 2009 to 2013, some ITC-CNR researchers set up an experimental campaign focused on the thermal characterization of growing media for green roofs based on laboratory tests for the calculation of thermal conductivity [13].…”

“…The Italian standard UNI 11235:2015 [12] analyzes growing media from some different points of view, but it does not provide for any specific rule about thermal calculation. Conversely, the main characteristic layers of a green roof play a significant and sensitive role in the evaluation of the thermal performances of a green roof (growing medium [13], vegetative solutions [17]). Barozzi et al [13] calculated the curves of thermal performances as a function of density (compaction levels) and internal moisture contents of eight different samples of growing media, of which four were intensive and four were extensive.…”

“…Conversely, the main characteristic layers of a green roof play a significant and sensitive role in the evaluation of the thermal performances of a green roof (growing medium [13], vegetative solutions [17]). Barozzi et al [13] calculated the curves of thermal performances as a function of density (compaction levels) and internal moisture contents of eight different samples of growing media, of which four were intensive and four were extensive. The experimental results, which proved very interesting for both companies and researchers, underlined some existing gaps:…”