Can green roofs provide benefits in climates with strong winters? According to Karen Liu of the National Research Council of Canada's Institute for Research in Construction, the answer is "Yes!"
In an article posted in EnviroZine, green roofs provided a 10% improvement in the heat loss of tested buildings in the winter. The results of the first round of testing were so significant the NRC-IRC began performing a multi-year study on green roofs to verify the results.
Once the benefits were confirmed, the article indicates that NRC-IRC designed green roofs specifically for winter climates. From the article:
"... green roofs are often planted with drought resistant plants ...which grow in a six-centimetre thick layer of a light weight growing medium (such as volcanic rock), the winter green roofs were composed of juniper shrubs... The purpose was to reduce the effect of wind speed (which draws heat from the building) and to increase the building's resistance to heat loss. Junipers were particularly well-suited for the experiment because they are evergreens and spread easily."
The research on both traditional home structures and homes built specifically for cold climates found that the juniper-based, winter green roofs increased heating efficiency by 10 and 8.5% respectively. Additionally, the winter green roofs helped to reduce the fluctuation in roof temperature, thus improving the life of the roof itself.
It would be very interesting to see if cities such as Chicago could benefit from the addition of junipers to their large number of green roofs.