Why window walls are favoured for condos
Report aims to shatter misperceptions about performance of "glass towers"
CondoBusiness
By Michelle Ervin
15 August 2017
A new report aims to shatter what it suggests are lingering
misperceptions about the performance of Toronto’s so-called “glass
towers.”
The window walls commonly used to enclose condos have improved,
addressing some of the issues associated with past generations of the
glazed cladding, say the authors of Window Wall and Curtain Wall: An
objective review. The comparative analysis, conducted in collaboration
by the Residential Construction Council of Ontario (RESCON) and the
University of Toronto’s (U of T) Building Tall Research Centre, found
that window walls can compete with, and even best, the performance of
curtain wall.
If comparable performance can be achieved with either system, then the
question isn’t which system is superior, but rather which system is
preferable for the application. Its authors conclude that in the case
of high-rise residential construction projects, window wall systems are
the way to go, arguing that they are easier to customize, install and
maintain; typically cost less as a result; and help to isolate smells
and sounds in units.
“As long as systems are properly designed and installed, it (the
report) shows that a window wall can perform equally to a curtain wall,
and the benefits of a window wall are tailored to residential
construction,” said Paul De Berardis, director of building science and
innovation at RESCON.
Cladding based on application
The report specifically compares window wall systems with the unitized
curtain wall systems typically used in high-rise construction. This
kind of curtain wall system is structurally engineered and anchored by
crane to building exteriors in prefabricated panels that can span
multiple units and floors, while window wall systems are inserted
between slabs from building interiors in panels that span single
storeys.
At around twice the price of window wall systems, aesthetically
pleasing curtain wall systems give architects more design freedom with
their sweeping, uninterrupted spans of glazed cladding, said Dr. Arash
Shahi, associate director of Building Tall and a post-doctoral fellow
at U of T’s department of civil engineering.
“That’s why a lot of commercial buildings are using curtain wall —
because you don’t have balconies, you don’t want people opening their
windows and you want them to have a sleek look,” he said.
“Plus, in a commercial building, you don’t have occupants 24 hours a
day, and nobody sleeps in the building at night, so the use-case is
inherently very different. Also, there’s generally no cooking and no
smoking in the units, so you’re not worried about the odours passing
through units.”
Window walls help to compartmentalize units, preventing the transfer of
smells and sounds that can cause quarrels between neighbours in
residential buildings. Cost aside, that — and the ease with which the
system can integrate balconies and windows — explains why it has become
the preferred option for residential applications, according to the
report.
Stigmas persist despite improvements
The Building Tall Research Centre was established to undertake research
relevant to the residential construction industry that might otherwise
get overlooked.
The group zeroed in on a comparison of curtain walls and window walls
due to what De Berardis characterized as the stigmas surrounding window
wall systems. Earlier versions may have had their challenges, he
acknowledged, but the cladding type has matured with experience and
time.
“Condo buildings nowadays definitely are different from condo buildings
which may have used window wall 20 years ago, so some of the issues
that may have existed back then are not commonplace anymore,” said De
Berardis. “It’s just like any product out there; a modern car will
likely have better fuel efficiency than a car which was produced 20 or
30 years ago.”
Along with industry innovation, building code changes and energy
efficiency requirements have driven improvements to window wall systems
by updating minimum standards, he said.
De Berardis pointed to balcony slabs as an example. Their concrete
edges, which used to be exposed to the elements, are now insulated with
covers, increasing their resistance to heat loss.
Dr. Shahi added that frames in window wall systems are now being formed
from less conductive materials, with vinyl and fiberglass starting to
take the place of aluminum, in another move that has advanced the
thermal performance of window walls.
And water-draining rain screens have helped to address problems with
moisture penetration, which, according to the report, were observed in
both curtain and window wall systems when exterior face sealing was the
main defence against leaks.
Performance hinges on installation
The report underscores that, if these performance improvements are to be realized, these systems must be installed properly.
Accordingly, it prescribes the use of field mock-ups of the systems so
workers can get familiar with their assembly, among other things, and
cautions against applying sealant in inappropriate weather in the final
push to get the job done, as subpar workmanship can compromise its
integrity. Some of the sealant used to give window walls air tightness
and water resistance is now applied in factory, adding a measure of
quality control.
The work isn’t done after installation, as maintenance is integral to
ensuring that perimeter sealants reach their expected service life of
10 to 15 years, the report notes. It recommends inspecting and
repairing exterior sealant every three to five years.
And when it comes time to replace panels, the difference in the way the
two systems are installed makes window walls easier to maintain than
curtain walls, the report asserts. Work on a window wall panel is
limited to a single unit, while swapping out the sweeping spans of
curtain wall can disrupt multiple floors and tenants.
“If a window wall panel needs to be replaced, it’s a lot easier because
you can access it from inside the unit,” Dr. Shahi added. “When it
comes to curtain wall, you actually have to access it from outside, so
you need to have a crane in place.”
Outside expert questions conclusion
Dale Kerr, senior principal at Pretium GRG Building Engineers, who was
not involved in the report, said it provided a good overview of the two
systems, but she wasn’t convinced that window walls could outperform
curtain walls. She suggested the way curtain walls are sealed is
superior to the way window walls are sealed.
The report acknowledges the shortcomings of the sealant commonly used
in window walls compared to the gaskets commonly used in curtain walls,
but adds that gaskets are starting to be used in window walls.
Unfortunately, whichever type of sealant is used, it will generally
fail before the aluminum and glass components of the system, said Kerr,
and so the ability to replace the sealant easily is important. Window
walls may have an advantage here, since some of this work can be
completed from the interior.
“If you’ve got heel beads around the perimeter of the insulated glass
unit, you could replace that from the inside of the unit, and it is
true: that would be easier to do than a curtain wall, which you’d have
to repair totally from the exterior,” said Kerr.
However, she pointed out that some of this work would have to be
completed from the exterior for window walls too, citing the slab edge
spandrel panels as an example.
Kerr recognized that window walls are likely the preferable choice for
residential construction, but mainly from a cost perspective. Indeed,
the report concludes that window walls are not only less expensive than
curtain walls, but also provide an economical way to compartmentalize
units. However, Kerr noted that, because of the way fresh air is
supplied to apartment building units via corridors, there is a more
important factor in compartmentalization.
“The big thing with compartmentalization is not as much what you’re
doing with the window wall, or curtain wall, or whatever it is, it’s
more what you do inside the unit to seal one unit off from another,”
she maintained.
Cladding research to continue
The recent release of the report by RESCON and U of T’s Building Tall
Research Centre marks the end of phase one of a two-phase study. Phase
two, the results of which are due to be published before the end of the
year, will shed light on the discrepancy in the way window-to-wall
ratios are measured for high-rise versus low-rise homes.
For high-rise buildings the ratio is measured by dividing the total
window area by the overall building area, whereas for single-family and
town homes the ratio is measured by dividing the area of the windows by
the area around the unit. Dr. Shahi said early findings show that when
the window-to-wall ratio for high-rise residential buildings is
measured on a unit basis, condo and home owners enjoy roughly
equivalent proportions of glazing.
“The building might look all-glass from outside, but from an occupant
perspective, living in a unit in a condo, they may only have one wall
to the outside,” he said.
With unit sizes on the decline, the research will also look at
alternatives to lowering window-to-wall ratios to improve the energy
efficiency of residential buildings with glazed cladding. Some of the
other options available to raise the thermal performance of these
buildings include the use of low-emissivity coating and triple glazing.
top contents
appendix previous next