Phys.org: Give and Take of Temperature Mitigating Technologies
"Cool roofs are a good example. In an effort to reflect incoming solar radiation, and therefore cools buildings and lessen energy demand during summer, painting one's roof white has been proposed as an effective strategy. Cool roofs have been found to be particularly effective for certain areas during summertime.
However, during winter these same urban adaptation strategies when deployed in northerly locations, further cool the environment and consequently require additional heating to maintain comfort levels. This is an important seasonal contrast between cool roofs (i.e. highly reflective) and green roofs (i.e. highly transpiring). While green roofs do not cool the environment as much during summer, they also do not compromise summertime energy savings with additional energy demand during winter."