Airborne Algal growth on roofs of membrane-structured residences in cold area of Japan

Abstract

The discoloration of building facades due to airborne algae is observed in our surroundings. The growth conditions of these algae are not yet fully understood, and efficient measures for preventing the growth of the algae are not presently available. The objective of this study was to investigate the effects of the ambient environment and building structure on algal growth. A residential building in a cold region of Japan was surveyed. The roof was a multi-layered structure comprising a semi-transparent film, an air layer, and a layer of insulation from the outside, supported by rafters. The soiled state was visually observed by taking photographs. On the northeast (NE) and northwest (NW) roofs, several black stripes appeared 4 months after cleaning. The soiling increased in the spring and autumn. The soiling first appeared on the film backed by the rafter and then extended to the film backed by the air layer. The condensation time during the day in the rafter part was longer than that in the air-layer part. Condensation occurred during the night, but its frequency exhibited no dependence on the orientation of the roof. Algae tend to die when exposed to an environment with a temperature higher than 45°C. The NE roof had the shortest period with a surface temperature of >45°C. These measurements agreed well with the survey results, which indicated that the soiling mainly occurred on the NE and NW sides of the roofs. The time for algal growth was estimated under the assumption that algae can grow at surface temperatures ranging from 0 to 45°C, in agreement with the observed soiling. The observed soiling changes were well explained by the algal population calculated via a growth predictive model according to the algal temperature and relative humidity.