Green Roof Study Results for Cold Climates

November 29th, 2005 by Peter Schmelzer

--Green Roof System Image--

Green Roofs are becoming a favored way to design buildings to be more earth-friendly. At the most basic level, they consist of a waterproof roof membrane covered with soil and plantings. The benefits are many: increased membrane life, insulation, storm-surge reduction, and usable space on the roof. Mostly, however, this idea has taken off in warmer climates.

The Professor Brad Bass of the University of Toronto has been researching the performance of these roofs in cold climates, and the results are soon to be unveiled.

“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.”…

More from TreeHugger. Thanks, Michael, for the link.

Our own Carleton College is studying green roofs:

David Holman ’06, Jason Lord ’06, Jake Gold ’07, Andrew Kaplan ’08, and Mandi Fix ’08 continued their independent study on green roofs with Director of Facilities Richard Strong as their advisor. Last spring involved the construction of a 660-square-foot green roof on top of the Olin storage room. The roof is currently visible from the Olin/Mudd loading dock and walkway. Last fall and winter the group researched green roofs from a number of different sources and constructed several test modules, studied soil mixtures, prairie spices, and drainage options.

They used a drainage system, donated by American Wick drain, that prevents soil from reaching the roof rubber, allowing ample drainage when the soil is saturated and holds water to evaporate back into the plants. They carefully selected the soil of Vermiculite, Perlite, compost, and clay to be lightweight, hold nutrients/water, and not to break down. They created a list of almost 200 different native prairie hardy species they thought would survive on the desert-like roof. Construction began on Friday, May 13, and was finished and planted by Thursday, May 19.

The green roof has performed better then expected so far. They placed between 2” and 6” of soil on the roof and planted prairie plants on the roof. Even in only 6” of soil, the prairie plants have grown to a height of over three feet–without any water since early July and no fertilizers whatsoever. The past few weeks we have seen several natural wild flowers on the roof. Feel free to stop by the Olin/Mudd walkway and see it for yourself.

We have been following the development of green roofs for many years and have watched their growing use. Soil depth is an issue both for plant viability and structural requirements to support the roof system. Spencer Jones, a local Landscape Architect, has suggested that the long-term viability of the plantings is a concern to be watched in our climate. Many roofs look good the first year, then seem to decline thereafter. Carleton’s look into prairie plantings will be interesting to follow in this regard.

VIVUS embraces the concepts of green roofs, and invite other interested individuals into dialogue and design about green roofs.

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