Sensitivity of Low Sloped Roofs Designs to Initial Water and Air Leakage

Abstract

Liquid water in low sloped roofs almost always causes problems. Roofs are designed only to control the migration of vapor, if at all. Small amounts of water leakage/penetration, may cause mold growth or catastrophic corrosion in current roofs systems. In a recent paper by the authors the effect of exterior surface emissive and absorptive properties was found to have a significant effect on the moisture performance of a roof that had a leak. Depending on the surface characteristics, roof systems can be designed to effectively manage water penetration, but at an energy cost. In the roofs system examined previously, air leakage was not included. In the present study, the authors reinvestigated the effect of water penetration and the influence of air leakage on the hygrothermal performance of a few selected roofs. The drying potential of a groove ventilated roof is examined. The performance concept is based on the fact that warming up of air in the groove increases it's ability to transport moisture to the outside. Solar radiation raises the temperature of air in the grooves and on average, during a sunny summer day 0.5 L of water can be ventilated out of the roof per 1m width of the roof. In this paper, one climatic condition was investigated; a hot and humid Climate representative of Houston, TX. The specific questions that the paper addresses are: What are the vapor and liquid control dynamic involved in the moisture migration of a roof in Houston TX? and how does airflow influence the performance of a roof that is initially wet ? A state-of-the-art numerical model was used to address these issues. Results showed that the drying potential depends on the ventilation rates. The roof system with ventilation grooves dried out faster from the initially wet stage than the roof without the ventilation grooves. The total increase in heat loss of the roof was found to be between 0 - 5 % depending on the thickness of the insulation. The ventilation can cool down the temperature of the roof in the middle of a hot and sunny day thus reducing the heat load to the inside.