Its common to hear that green roofs will provide better insulation for your home, and that in turn your energy usage will be lower. However, the term “insulation“ here is quite misleading.

In the context of building science insulation is a material with low thermal conductivity, an example would be batt insulation. Now lets go back to green roofs which are primarily made of a watered growing medium, basically wet soil which compared to insulation materials does NOT have a low thermal conductivity. Just imagine laying in wet soil, its going to be cold because all of your heat is being easily conducted into the soil. So if you’re heating your house in the winter and your roof system only consists of a wet growing medium, all that heat is going to rise and conduct outside, meaning your heating costs go up.

The confusion of green roofs being insulating likely comes from them being used for “thermal mass”. Thermal mass is a term used to describe materials that can store lots of heat (energy), and are useful for passive cooling when you want to absorb heat from the environment and release it later. Read more about thermal mass

In short, green roofs are primarily wet soil that will conducts heat out of your warm house into the cold outside. Instead they should be used as thermal mass which means they absorb heat without getting too hot, making them good for passive cooling.