Green roofs are known to help insulate a building in winter time, cool it in summer time and help manage rainwater.  Retail giant Walmart has been comparing the benefits of green roofs to the benefits of white roofs and worked out how much money each type of roof is saving them.

sedum green roof

This article from the spring 2014 issue of living architecture monitor describes their findings.

Big box retailers embracing green roofs

Walmart reveals the results of their green roof life-cycle cost-benefit analysis

The cost-benefit analysis

Walmart has completed a study of the performance of green roofs in collaboration with Arup, roofmeadow, Pennsylvania State University, the Green Team and University of Toronto researchers.  

This work centred on a research roof build in 2006 at Walmart store #5402 in Chicago.  The 133,000 square foot flat roof is just over half green (vegetated) and just under half white (cool) roof.  When constructed, this roof was the largest, most intensively monitored green roof in the world.  Each side was intrumented with equipment to measure variables impacting store energy use and stormwater runoff dynamics.  Monitoring ran for three years.  Data analysis included incorporation of field results into detailed models.

Energy

The green roof saved energy compared to the white roof.  If a full store had a green roof, the data suggestes it would save 1-6% storewide in a range of climates, with a 2.2% savings estimated in Chicago.  

Roof temperatures were up to 40 degrees cooler during hot weather on the green roof, but savings from the green roof were greater in cold weather than in warm weather. Summer savings would be higher if the basis for comparison was a black roof rather than a white one, but Walmart builds new stores in the United States with white roofs.

The green roof has an added benefit of peak load shifting due to its thermal mass.  

This study is one of the first to also demonstrate that the green roof moderates air temperature at rooftop HVAC unit intakes. This translates to air conditioning savings in summer ans well as heating savings in winter.

flooding

flooding in Norfolk UK in the 1950's - stormwater management aims to avoid incidents like this

Stormwater

Stormwater retention and peak rate attenuation are tow important criterial affecting city storm sewer capacity and water quality in lakes, rivers and bays.

This study showed that a green roof can retain more rainfall than perviously found by most studies of smaller roofs.  Up to 3.5" were detaitned during some rainfall events.  The soil layer on the roof is only 4" deep, with 3" of media over a 1-inch foam-type drainage mat.  No ponding or drain clogging on the green side was observed.

Of the 100 precipitation events that were studied, the green roof retained an average of 74%.  

Peak runoff from the green roof was delayed 1.25 - 3.25 hours from the rainfall peak, and the rate was 50% to 85% lower than the precipitation peak rate (average 65%)

Maintenance and roof life

The Walmart Chicago green roof costs roughly 45% less to maintain than a typical white Walmart roof.

This is due primarily to the protection the green roof affords the membrane and drains, preventing damage and clogging. Minimal selective weeding and avoidance of irrigation at this location helps to keep green roof maintenance costs low.

Green roofs can also be expected to extend the life of the waterproof membrane from a typical 15 years to 40 or more years.

The bottom line

A financial analysis was done for green vs white roofs in 9 locations (6 metro areas).

The approach to this financial analysis differed from a typical one, in that it compared not just the two roofing systems in isolation, but a development package that includes a green roof with one that uses more conventional means to comply with relevant regulations.

The results showed green roofs have a less than 20 year payback for all locations studied.

One third of the total show a  0-3 year payback.

This study demonstrates that in the right policy environment, green roofs can be an attractive voluntary choice for a retail developer like Walmart.

By Don Moseley

Read this article and more at www.livingarchitecturemonitor.com

 

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