I've been looking for information on green roofs in Ottawa and came across this interesting article on the benefits of green roofs in winter:
Their research showed that winter green roofs could reduce the energy used for heating by more than 10 per cent during the cold season in the test house that is used as the National Research Council of Canada's field roofing facility in Ottawa.
It turns out that a combination of added insulation from the porous soil medium, and protection from the wind offered by small juniper shrubs was able to partially shield the roof from the extremes of Ottawa's cold winter weather.