Highly reflective building envelope materials are widely identified as an effective design option to limit the peak surface temperatures of roofs in summer conditions, thus mitigating the urban microclimates and the energy demand for cooling. However, especially surfaces having high solar reflectance are subject to soiling (i.e. deposition of soot and other airborne particles), in addition to ageing, and biological growth. All these processes reduce the reflectance of bright surfaces (and increase the reflectance of surfaces having reflectance lower than roughly 0.20). As a result, an increase in the solar reflectance of a cool roof yields to higher surface temperatures and a reduction in the energy savings expected thanks to high albedo roofing. Furthermore, the durability is also impacted. To quantify this effects, we exposed in the urban environment in Milano and in Roma (Italy) 16 roofing materials. More in detail we selected 14 roofing membranes, including synthetic, factory applied coating on synthetic membranes, field applied coatings on modified bitumen, asphalt shingles, and terracotta tiles. For each product class (e.g. synthetic membranes) we selected a high reflectance product and a mid-low reflectance one. We measured the UV-Vis-NIR spectral reflectance of three samples per product before the exposure – begun in April 2012 – and after 3, 6, and 12 months. Herein we present the results of the first year of natural exposure of roofing membranes, most of which show a remarkable loss in the solar reflectance (sometimes exceeding 15% of the initial value) already after the first three months of natural exposure, depending on their initial value. With the measured curves of solar reflectance over time as input data, we performed finite differences numerical modelling (by means of the software tool WUFI 5.2) of heat and moisture transport through typical roof assemblies. We analyzed the variation in the surface temperature and heat flux due to the change in solar reflectance, and we obtained relevant differences. For instance, in case of a highly reflective flat roof (with initial solar reflectance equal to 0.85, reduced to 0.70 after six months) over 10 cm of extruded polystyrene on a reinforced concrete slab (U-value of the roof assembly equal to 0.277 W m-2 K-1) we computed a significant increase in peak surface temperatures (up to 7°C, from 34.2°C to 41.9°C, assessed in the context of Milano, Italy). Knowledge about the soiling trends for different building envelope materials allows to better estimate the cooling energy demand of buildings, plan cleaning and maintenance operations (whether viable and sustainable), and to assess the benefit of anti-soiling treatments.
Paolini, R., Zinzi, M., Poli, T., Carnielo, E., Fiori, M., Mainini, A.g. (2013). Evolution over time of UV-VIS-NIR reflectance of cool roofing materials in urban environments. In AIVC, 34th Conference, Energy conservation technologies for mitigation and adaptation in the built environment: The role of ventilation strategies and smart materials.
Evolution over time of UV-VIS-NIR reflectance of cool roofing materials in urban environments
CARNIELO, EMILIANO;
2013-01-01
Abstract
Highly reflective building envelope materials are widely identified as an effective design option to limit the peak surface temperatures of roofs in summer conditions, thus mitigating the urban microclimates and the energy demand for cooling. However, especially surfaces having high solar reflectance are subject to soiling (i.e. deposition of soot and other airborne particles), in addition to ageing, and biological growth. All these processes reduce the reflectance of bright surfaces (and increase the reflectance of surfaces having reflectance lower than roughly 0.20). As a result, an increase in the solar reflectance of a cool roof yields to higher surface temperatures and a reduction in the energy savings expected thanks to high albedo roofing. Furthermore, the durability is also impacted. To quantify this effects, we exposed in the urban environment in Milano and in Roma (Italy) 16 roofing materials. More in detail we selected 14 roofing membranes, including synthetic, factory applied coating on synthetic membranes, field applied coatings on modified bitumen, asphalt shingles, and terracotta tiles. For each product class (e.g. synthetic membranes) we selected a high reflectance product and a mid-low reflectance one. We measured the UV-Vis-NIR spectral reflectance of three samples per product before the exposure – begun in April 2012 – and after 3, 6, and 12 months. Herein we present the results of the first year of natural exposure of roofing membranes, most of which show a remarkable loss in the solar reflectance (sometimes exceeding 15% of the initial value) already after the first three months of natural exposure, depending on their initial value. With the measured curves of solar reflectance over time as input data, we performed finite differences numerical modelling (by means of the software tool WUFI 5.2) of heat and moisture transport through typical roof assemblies. We analyzed the variation in the surface temperature and heat flux due to the change in solar reflectance, and we obtained relevant differences. For instance, in case of a highly reflective flat roof (with initial solar reflectance equal to 0.85, reduced to 0.70 after six months) over 10 cm of extruded polystyrene on a reinforced concrete slab (U-value of the roof assembly equal to 0.277 W m-2 K-1) we computed a significant increase in peak surface temperatures (up to 7°C, from 34.2°C to 41.9°C, assessed in the context of Milano, Italy). Knowledge about the soiling trends for different building envelope materials allows to better estimate the cooling energy demand of buildings, plan cleaning and maintenance operations (whether viable and sustainable), and to assess the benefit of anti-soiling treatments.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
