Green roofs can be very efficient in cooling a building in summer due to the heat loss caused by the evapotranspiration process. In simple terms: water is converted to gas, heat is lost, and the roof is cooled. The resulting energy savings are multifold: (1) due to direct cooling of the roof, (2) but also due to the lower temperature air that is taken in by potential air conditioning modules.
Despite this effective cooling, people are often confused when they see green roofs being rated as having moderate-to-poor insulation capacity in permits. In these permits, it is common to see the green roof (and other components of the building) rated according to “R” or “U” values/factors.
R-Value is a measure of thermal resistance. It is a measure of the ability of a material to resist heat flow. On the contrary, a U-Value is a measure of thermal transmittance or heat loss through a structural element. Both are used as ratings; the higher the R-value, the better the material is to resist heat flow, e.g., leading to a warmer house in winter. The opposite is the case for U-values.
The simplicity and straightforwardness of R and U-values have made them standard procedures for building insulation. They are great for static systems.
Mineral wool (a substrate component of high-retention green roofs) has a very high R-value when dry. When wet, its R-value approaches zero. Similar to other components that cycle through wet and dry on the roof, but most dramatic with mineral wool. There really isn’t a static R-value that can be assigned to these materials, though their R-value can be assigned at fully dry, fully wet, or some point in between. But that’s not how R-values tend to be used.
An R- or U-value is not an optimal way to describe the ability of a green roof to manage heat energy. This is also because R and U-values are static and do not incorporate or describe the heat loss from the living part of the roof: the plants. Hence, the green roof’s capacity to manage heat energy is not so much dependent on the static building components but on the upper part of the profile where water is evapotranspired – especially during wet and hot conditions.
The energy lost via evapotranspiration is much more difficult to calculate and describe than the static R and U-values and involves dynamic modeling. Many factors affect the roof’s ability to cool, such as climate, vegetation, water availability, slope, substrate type, and much more. A green roof with no access to water cannot evapotranspire and hence is not able to cool the building. On the contrary, a high retention green roof with access to water is an effective cooling engine.
Again, biology has crashed the engineering party and messed up the straightforward equations.
In many cases, R- or U-values are required to be given for the green roof. They are likely calculated for a worst-case scenario, i.e., for a soaked profile, and won’t be spectacular looking. However, if you are interested in how much money you can save on energy using your green roof, you can contact an energy modeler or use one of the available green roof energy modelers that are available, e.g., the PET tool from Knauf. Also, take care to design your green roof for your local climate to ensure effective evapotranspiration during the hot months. You can use our evapotranspiration modeler for this (free of use).
In the end, it is about the real functionality of the roof, and no matter what the R or U-values say, a well-designed and optimized green roof will cool your building in summer and will save you energy and $.
Please, feel free to contact us if you have any questions!