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Winnipeg’s Leaf conservatory to have unusual roof

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by Mary Baxter

The developers of a new conservatory in Winnipeg’s Assiniboine Park are banking on an unusual lightweight roofing material called ETFE to make a significant expanse of indoor growing space financially feasible.
Winnipeg’s Leaf conservatory to have unusual roof

"It would be cost prohibitive if we did it conventionally; we would have a much, much smaller facility, no question," says Gerald Dieleman, project director for Assiniboine Park Conservancy.

The conservancy, a non-profit organization, operates the 400 acres of park next to the city's southwest neighbourhoods.

The $75-million Diversity Gardens project, $61 million of which will be used to build the Leaf conservatory and landscaping, is the final phase of a larger $200 million park revitalization that launched in 2009.

The first phases updated the destination's duck pond and restaurant and revamped its zoo.

Doug Corbett, the conservancy's executive architect, says the idea to use ETFE (ethylene-tetrafluoroethylene copolymer) as the structure's main cover arose out of concerns that a conventional approach would create structural shading because of the large beams and structure needed to support a glass roof.

rees and plants are going to grow crooked and not get the really full sunlight as if they were outside," he explains.

The project's climate engineer, Stuttgart, Germany-based Transsolar, suggested using ETFE, a clear membrane material like polyethylene, Corbett says.

To date, ETFE has been used primarily in sports stadiums, says Corbett, referring to its use as cladding for the Beijing National Aquatics Centre during the 2008 Summer Olympics.

"It looks kind of like a down jacket that's quilted," he says, noting air is applied in three layers with air between "so it creates 'pillows.' These pillows are what actually give it strength to not sag."

The design team developed a lightweight cable net truss structure to support the ETFE, similar to a tent structure. Along with Transsolar, the design team members are as follows: KPMB, Toronto; Architecture A-49 and landscape architects HTFC Planning & Design, both of Winnipeg; and engineering firms Integral Group Inc., Calgary, SMS Engineering Ltd., Winnipeg and Blackwell Engineering, Toronto.

A steel-framed cylindrical "tent pole" which the team calls a diagrid supports the truss structure. Guy wires similar to tent poles along the perimeter will hold the roof to the ground.

"It's quite big," says Corbett. The diagrid will hold components such as two elevator shafts and double stairwells. The mechanical system is on the diagrid's roof.

Biomes containing a large tropical plant garden as well as smaller Mediterranean gardens, display and butterfly gardens will occupy the 30,000-square-foot structure. The biomes are greenhouse spaces physically divided by transparent ETFE and glass to facilitate different climate zones.

The goal, Corbett says, is to create a higher level of sustainability. To this end, the project's designers are eliminating as much of the mechanical and ventilation systems as possible, "appreciating that these are botanical gardens so you're really not trying to make it 22 C year-round all the time inside them."

The facility's transparency will allow the sun to help heat its interior but there are backups.

"We're going to have some normal perimeter radiation with a boiler system that makes heat," he says. "We're just trying to make sure that it doesn't get too cold that the plants will start to die. It's going to be a fairly passive mechanical system."

With plenty of parkland as a resource, the conservancy will develop a geothermal field to use as an energy source for the boilers.

A waterfall in the interior as well as misting jets inside the biomes will create clouds to control the indoor climates.

The other portion of the facility is a more conventionally built and mechanically serviced "box" building to house a restaurant, banquet facilities, multipurpose and classroom spaces and administration offices.

Exterior gardens created along different themes — an arboretum, traditional formal plantings and an indigenous peoples' garden — will bank the Leaf conservatory. The internal and external plantings together make up the Diversity Gardens.

The use of ETFE for roofing is a first in the prairie province and may be a first in Canada at this scale.

"I think there are ETFE elements at B.C. Place Stadium (in Vancouver) and in Mosaic Stadium in Saskatchewan," Dieleman says.

Public conservatory colleagues elsewhere in North America are watching closely, he says.

Obtaining LEED Gold is a goal, says Corbett, but there's a challenge to qualifying for the sustainable practice certification: the building's unique biomes don't fit into the criteria used for assessment.

"We are going to work with the CaGBC (Canada Green Building Council)," to establish an approach to its evaluation, he says.

Dieleman says the goal is to begin construction in the spring of this year with the aim to open the facility in late fall 2019.

Timing is tricky. The plants must be introduced during summer. A delay of three months in building the roof would end up pushing the project back a year. Conservancy staff is now awaiting the outcome of provincial and federal government funding applications for the project.

In the meantime, they are ensuring the project is shovel-ready.

Dieleman describes the building as a significant design project because unlike a building constructed in a densely packed urban centre, all of its sides have to be carefully considered to fit in with the park experience.

"You can view this building from all sides," he explains. "There's no back door, if you will. It's a real interesting way of approaching design in a park."

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