First-ever analysis of green roofs’ ability to keep buildings warm in winter
by John Laumer, Philadelphia 
on 11.29.05
A Univerity of Toronto Professor will be presenting his findings on the abilty of green or "living" roofs to offer energy benefits during the cold season. Appropriately, his poster session will debute at the Toronto Design Exchange, featured in the "Design for Cold" exhibition. Professor Brad Bass' work is a vital element of spreading Green Building concepts. Because Green Building designs were pioneered in warm temperate zones, work like this is essential not only to validate performance merits in different settings but also to avoid getting arrows in our backs. The specializing-in-climate-change-denial Think Tanks are doubtless ready to draw their bows at whichever high profile green building project can be first claimed as a "failure", blaming TreeHuggers for idealogical over-reach.
Key excerpts from the Univerity of Toronto press release follow:
“Everyone studies how green roofs operate in warm conditions,” says Brad Bass of the U of T Centre for Environment. “No one else has looked at winter design.” Bass analyzed a test roof built in Ottawa by Karen Liu of the National Research Council’s Institute for Research in Construction, to offer the first conclusive data that winter green roofs can help reduce heat loss and energy consumption during cold months..."
"The winter green roof uses evergreens – juniper shrubs – and a thicker soil base than typical leafy green roofs, which generally provide passive benefits to the environment by reducing the need for air conditioning on hot days. The winter roof was installed on both a standard test house and an energy-efficient winterized house. Bass used environmental systems performance software to chart the indoor temperature fluctuations in both buildings".
“The results for the winterized house were good, and the results for the regular house were dramatic,” says Bass. “The assessment opens up designers to considering winter roofs as part of a year-round energy efficiency strategy.”...
Tip of the stocking hat to Professor Brad and his team, eh!
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I visited a house in Utah that needs no heat or airconditioning. It is one extremely large arched roof, half cylinder, steel, with glass on both ends. Roughly 2500 sq. ft. The roof is covered with several feet of earth. It takes 6 months for the outside temperature to seep through the earth, so in winter, the interior is heated by the summer's sun, and cooled in the summer by winter's chill. The owners love it.
I've been wondering how green roofs handle snow. On a regular roof, if you get 3 feet of snow (rather common in northern climes) it will slide off. I'd think snow would have more resistance to sliding off if there is grass there adding friction.
But this guy wants to add shrubs like juniper? That seems like that would be worse. atleast with the grass if all else fails you can push it off with a shovel.
=== author's response follows =====
Good question. Obviously the load bearing capacity of the structure must be taken into account, ensuring that it would safely hold all manmade materials plus wet frozen soil, snow, and plants. For designed to purpose structures there is a positive tradeoff when a green roof is included . An ordinary flat roof commonly has an inch or two of gravel over bitumin and/or butyl rubber membrane. That gravel presumably is dispensed with in the areas covered by plants, saving some weight.
On a flat topped commercial or public building located in climate where "thaw days" are rare, snow accumulates througout the winter months, and loses mass only by sublimation and by wind erosion. With plants on top, sublimation processes should be identical to those on ordinary roofs. However, we can safely assume that junipers would create a wind break effect, and tend therefore to keep more snow on the roof.
In the final analysis, for a flat roof, this seems to come down to whether the net weight of manmade materials is greater with green design. For a sloped roof, the analysis is entirely different and the issues you identify would be problematic. Make sense?
Flann,
I would be interested to learm more about this home. Is there a website link about this design or a link to the architecture firm that built it?
Having co-lead this study, I would like to add some additional comments and respond to some other comments.
This on-going research emerged out of previous study involving Canada's Federal Government and the roofing industry. That study indicated the potential for green roofs to reduce stormwater runoff and energy consumption during the summer. At a wrap-up session in October, 2002, the industrial partners expressed in interest in a green roof design for winter. Although the market for green roofs in North America is growing, the additional up-front cost is still seen as a barrier.
We are hoping that this research on a green roof designed for the winter will increase the public's willingness to embrace this technology and help convince policy makers to support this technology.
This roof cannot be used on every building as it is heavier than the standard, extensive green roof. However, there are buildings that can support the extra weight, and the number of intensive green roofs in Europe suggests that a heavier green roof, designed for winter, is not too far-fetched for northern climates. We hope that the results of this research will encourage the construction of more buildings to accomodate a heavier green roof.
By the way, I have not heard of anyone pushing the snow off their green roofs, and I have visited green roofs in Ottawa and Winnipeg. Most green roofs include a barrier, which would preclude pushing the snow off of the roof.
The cold-design home that was used in this study, is still a design. We did not build the home, so I cannot provide additional information or website. The field site is located on the campus of the National Research Council, in Ottawa. It is set up with a partitioned roof, so that we can compare data between a green roof and a conventional roof.
We are planning on attending the green roof conference in Boston, in May 2006. The poster will be displayed, and you will have the opportunity to speak with me and members of my research team. I look forward to meeting you in Boston.