Theory of a self-drying flat roof


A black, self-drying roof model from Chicago.

Since 78 WUFI models were simulated, it would be difficult to describe all the models with their drying profiles. For the purpose of this article, four models are explained. Figures 2 and 3 are the self-drying models from Miami and Chicago with both black and white roof membrane shades, including ventilation moving moisture out of the metal roof deck’s flutes. It is important to note the baseline was no ventilation moving the moisture out. Ventilation had to be added, as there was increased moisture. As previously mentioned, the beginning of the WUFI model began October 1, 2016, and the simulated leak occurred April 1, 2017 (as indicated on the graphs). There was minimal moisture ingress into the roof enclosure during the winter months, even with a smart membrane due to the ventilation introduced between the metal deck roof flutes. This ventilation allowed for the vapor attempting to diffuse into the roof assembly to be greatly reduced—the effect of this reduction will be evident when ventilation is not occurring. The black roof membrane assembly in Miami and Chicago (Figure 2 and figure 4) dried in nearly half the time of the white membrane roof assembly in Miami and Chicago (Figure 3 and figure 5). A higher membrane temperature and high vapor pressure were critical to the effective diffusion of the moisture/vapor to the interior. Based on the graphs, it appears the roof drying was effective when a single wetting event took place. If several wetting events had occurred, the results would be different. In a colder climate such as Chicago (Figures 4 and 5), by not venting the moisture attempting to diffuse into the roof enclosure, there will be added diffused moisture due to the installed smart membrane. Even in Chicago, if a black membrane is used in conjunction with a self-drying roof enclosure there is still enough energy to diffuse the moisture to the interior, where it can be accommodated. On the other hand, the white roof does not have adequate energy to move all the moisture out of the system before the interior moist air reverses and diffuses back into the system (around mid-September). In this scenario, the author does not believe a self-drying roof with a white membrane will be an acceptable approach.

A white, self-drying roof model from Chicago.

The tight time constraints of planning, designing, and constructing have placed a strain on available resources, especially finances and labor. A saying in the construction industry is, “Speed, Quality, and Cost—you can pick two, but you cannot have three.” Developing durable and efficient building enclosures, even when problems arise, should be the way of the future. Developing a self-drying roof enclosure would be a giant step forward in minimizing the monetary impact of construction on an owner and the environmental burden on society.

The proposed self-drying roof design was developed with economics, environment, durability, and ease of construction in mind. The roof membrane is to be continuous and have very low vapor permeability and a specified color to suit the respective climatic zone. The thermal insulation in the assembly is to be a highly vapor permeable insulation material with a varying smart membrane below the insulation. Ventilation of the metal deck may be required in some climatic zones to remove the vapor attempting to diffuse into the roof assembly.

Results from the 78 hygrothermal models prove the potential for a self-drying roof enclosure is suitable in almost every location (Figure 6). Several outliers show self-drying roof enclosures with a white membrane were not ‘functional’ in colder climates. The performance of this self-drying roof enclosure design could minimize the rate of litigation and provide the additional benefits of increased resiliency and reduced financial burden for building owners and the environment. Though there is much more work required with modeling and testing, the design of a self-drying roof enclosure appears to have some promise.

With the life cycle of standard roof systems (typical industry types) averaging 17 years, the potential for increased durability and resiliency is needed. A self-drying roof system will allow for increased performance and longevity, as the insulations’ effectively recover when moisture loading occurs. The cost of the overall system will increase by 30 to 35 percent, but if the roof system can reach the industry standard 25-year warranty (with proper maintenance) the operational and environmental costs could be reduced.

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