A Novel Laboratory Procedure to Determine Thermal Conductivity of Green Roof Substrates

Green roofs are recognized as a solution able to reduce building energy consumption and to increase envelope thermal performance. Nowadays, new materials are continuously proposed and investigated for green roof layers to optimize environmental and economic sustainability. However, in absence of specific national and international standards for laboratory calculations of the thermal performance of substrate for green roofs, thermophysical characterization of substrates generally excluded that thermal conductivity should be modified as water content changes from dry to saturated. In this paper, a novel laboratory testing method was proposed to assess thermal conductivity of substrates for green roofs. The tests were performed by varying the water content into substrates and saturated, dry, and moist conditions were analyzed. Three commercial substrates different in composition were investigated. Thermal conductivity measurements were conducted using the HFM 436/3/1 Lambda and the TLS-100 devices to cross-referencing the measurements obtained. The results for all the substrates demonstrated that thermal conductivity fluctuates by varying water content. At moist conditions, the thermal conductivity value is an average between the one at saturated and dry conditions. The lower thermal conductivity is obtained in dry condition while the higher one is obtained in saturated condition due to water influence. Since the thermal conductivity was consistent with those provided by the company manufacturing commercial substrates, the proposed testing method could be considered reliable and it could be applied to different substrates, especially concerning the HFM Lambda device. This laboratory testing can potentially provide a useful guideline for assessing thermal conductivity of green roof substrates