A number of companies have started constructing large greenhouses on the roofs of buildings to produce food, such as Lufa Farms in Canada or Urban Farmers in the Netherlands. Here is the clever bit; not only do these greenhouses reduce the energy required to heat the building, but the residual heat from the building can be directed into the greenhouse to increase productivity. This is already happening in the Ecco Jȁger ‘integrated greenhouse’ in Switzerland. So your lettuce and tomatoes may soon be grown on your local supermarket roof.
Interested? Read this article by Esther Sanye-Mengual and her colleagues for more info.