Vegetated Roofs

There were some published studies done many years ago. Basically, there is some and it changes depending on the weather. During the summer, you only experience savings on cooling load for the level immediately below the vegetated roof. In the winter, you have a block of ice up there.

soil is close to sand...which is close to cement. it's pretty conductive and has similar lamda and thermal capacity values. moisture influences this somewhat. you can model it. previous modelling experience  shows little in regards to energy savings, but for rainwater run off rates and habitat as well as heat island impact, they are pretty nice.

I agree that in a reasonably well insulated roof the green roof itself will have little energy impact. The thermal properties do vary so at times it will help reduce thermal losses and other times it will make those losses worse. So I would not worry about the thermal properties. Where this can help energy performance is how it might affect the air temperatures at the roof level which could influence the temperature (and moisture level) of the outside air being brought into the building depending upon the location of the outside air intakes. This tends to have more of an impact in hot climates and can provide similar benefits to a white roof.

A few additional factors are that if the building has condensing units and these are on the roof, there will be a benefit for these operating at a lower ambient temperature. Another factor is that the thermal impact to the space below should be assessed for different configurations. In a loft configuration the roof will be directly proportional to load on the space. If there is a plenum then there would be a partial relationship where the load is partly going to return air and part to the space. For a ducted return system with a plenum, the impact may be much less than the return air plenum or the case without a plenum.    

This has been kind of a pet peeve of mine for many years. I was involved in an early vegetative roof project some years ago now, and the vendors of the system were touting energy savings, as many do. However, as indicated above, we could find no real evidence of these savings and neither could the energy modeling firm that was working for the local utility. As a sensitivity test, a model was run that eliminated the roof as a heating or cooling load to the building...as if there were no roof loads imposed, and the savings were actually a very small percentage of this particular project. There are so many other reasons for doing this kind of roof that trying to add energy savings is not appropriate and detracts from other very significant benefits.