December 7, 2009

WILLIAM CONWAY/PROGRESS PHOTOGRAPHY

A curtainwall is part of the design of the Ritz Carlton in Toronto.

Building envelope

Curtainwalls best suited for high rise condominiums

DON PROCTER

correspondent

Most condo highrises in Toronto are clad in window-walls, but a few developers are going with more expensive unitized curtainwall systems normally associated with office buildings. Two downtown projects currently on the rise, the Ritz-Carlton Toronto and Trump International Hotel & Tower (both with large residential components), are good examples.

There are plenty of reasons for selecting a curtainwall system. Its ability to resist snow, seismic and thermal forces, for example.

It is also designed to sustain excessive wind loads common at high heights.

At 683-feet and 924-feet the Ritz-Carlton and the Trump tower respectively are cloud scrapers — even by Toronto’s standards.

The key difference between curtainwalls and window-walls is that the former hangs off a building, attached with metal embedded plates to concrete floors, whereas window-walls are glazing assemblies installed between floor slabs or a floor and roof structure, said Bruce Norman, construction manager, PCL Constructors Inc.

One of the advantages of curtainwall systems is a rainscreen, a gap between the assembly and the building that allows moisture to drain down and out, explained Steve Gusterson, manager of pre-construction design services with Alumicor, a Canadian-based company that manufactures architectural aluminum building products such as curtainwalls.

By comparison, many window-wall systems still rely on prime seals (caulking and tapes) to keep out the weather and minimize deflection — an approach employed 25 years ago.

Gusterson said the problem is sealants crack and deteriorate over time, causing moisture problems and the potential failure of the glazing units.

Some higher end window-walls today, however, feature receptors at the top and bottom sides of the window frame, which allow the frame to expand and contract within the receptor, helping to minimize cracking of sealants, he pointed out.

Also, some window-wall manufacturers have added drainage holes in the prime seals.

To prevent the prime seal from being compromised a heal bead can be installed around the insulating glass unit to protect the air vapor barrier and allow for moisture drainage, he said, adding it is a pricey labour-intensive measure because a compatible sealant is required around every glass unit.

“I think we are going to see some (window-wall) failures because there are some really poor quality window-wall systems that are still being used, but I wouldn’t want to paint the entire market segment with that brush. It would sound like sour grapes from a guy in the curtainwall business,” he said.

Darius Rybak is vice-president of highrise operations for Aspen Ridge Homes.

His company is a developer that has seen many window-wall systems go into its residential highrises and others around Toronto.

He is confident window-walls measure up and won’t leak, unless they are poorly installed.

But one reason to consider a curtainwall is it is more airtight — an important energy-savings benefit, he said.

Aspen Ridge chose a curtainwall for 77 Charles St., a 16-storey upscale condo under construction in Yorkville, Toronto.

The system is three times more expensive than a window-wall.

“One of the reasons its cost is much greater is it is truly a commercial product,” he said.

Rybak added that Aspen Ridge decided on a curtainwall rather than a window-wall partly because of esthetics.

Curtainwalls allow for more mullion-free vision glass — typically five feet by five-feet glass panels, whereas window-walls are typically three feet by five-feet.

Window-walls were developed from window products.

“Essentially, they started out as punched windows in masonry walls that were supported by anchors in the walls,” said Gusterson.

“Eventually, we started losing the masonry and the windows became larger until they were being installed into the slabs of the building.”

Among the different curtainwall assemblies on the market is stick curtainwall, a system of aluminum and glass components assembled on site typically for lowrise commercial buildings.

Unitized curtainwall, common on large office buildings, is prefabricated in shop in panel sizes and snapped into place on site.

Unitized curtainwall requires fewer tradespeople on site — a big plus because skilled assemblers are very difficult to find for stick curtainwall, said Gusterson.

While none of Alumicor’s curtainwall assemblies have made it into the highrise condo market in Toronto, the company has done work for residential towers in New York City and Seattle.

Gusterson said the owners were convinced to go with curtainwalls by building envelope consultants on the projects.

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