by Brian Burton
The following article discusses some of the important elements involved in “overclad” retrofitting of older Multi-Unit Residential Buildings (MURB’s) that were constructed in an era where energy costs were not a prime consideration. The challenge presented from a building science perspective, as you will see, is literally an invitation to innovation.
The overcladding retrofit of MURB’s in Toronto is about to receive a great deal of attention as a result of three major initiatives in Ontario. The first is the City of Toronto’s “Tower Renewal Project” which is based in large part on research conducted by the University of Toronto, John H. Daniels Faculty of Architecture, Landscape and Design by Professors Ted Kesik & Architect Ivan Saleff and ERA Architects. (Other contributors are listed on the web-site)
The second is the completion of the “Tower Renewal Guidelines - For the Comprehensive Retrofit of Multi-Unit Residential Buildings in Cold Climates” by University of Toronto, John H. Daniels Faculty of Architecture, Landscape and Design Professors Ted Kesik & Ivan Saleff.
The other high profile venture is the recently announced Zerofootprint Re-skinning Competition. For more details visit:
The GTA has an enormous stock of MURB’s and although these buildings are showing their age they are considered an extremely valuable resource.
These buildings were constructed in an era when energy efficiency was not a priority consideration and subsequently the thermal performance of these buildings is poor. Structurally they are in reasonably good shape however some exposed structural components such as balcony slabs and projecting shear walls require attention.
Professor Ted Kesik and Architect Ivan Saleff ran numerous simulations and concluded that these buildings represented cost-effective candidates for retrofit strategies.
The solid exterior masonry walls of these structures offer an excellent substrate for the support of "overcladding" systems and can cut the total energy requirements by one half.
However overcladding design, installation and commissioning will require a tremendous amount of building science expertise. This is a challenge that Canada is well equipped to tackle.
Overcladding Controls Rainwater
Overcladding offers a relatively simple solution to the problem of keeping rainwater out of buildings and there are several variations.
1. Basic overcladding which involves the installation of an air barrier and insulation protected by an exterior cladding applied to opaque wall elements (excluding balconies) and includes a replacement of the windows in the building.
2. Comprehensive overcladding involves the same approach as a basic overcladding however the cladding is installed over the entire opaque wall area and over open balconies and also includes a replacement of the windows in the building.
3. Integrated overcladding is somewhat more complex in that it involves installation of a secondary framing system that enables updating of building services and is more like a double façade that allows for natural ventilation and sound control. The ventilated cavity uses the pressure effects of the wind to dissipate the energy of driven rainwater and includes drainage paths that direct the water away from the cavity.
Advantages of Overcladding Systems
Overcladding technology is almost certainly an “exportable” technology. It will also lead to a tremendous amount of job creation opportunities and significant overall economic stimulus. Additional advantages include:
• Compared to demolishing the buildings and reconstruction overcladding is almost certainly more financially and ecologically reasonable.
• Improves thermal performance and air tightness.
• Enables transfer of the dew point outside the structural wall element.
• Optimizes the use of thermal mass.
• Overcladding contributes to improved sound insulation.
• Quality control is improved as the insulation coverage is visible.
• Avoids internal building work and can be installed with minimal impact on occupancy.
• Increases the life expectancy of the building.
• Does not reduce the size of the rooms.
• Limits disruption to the fabric of the building.
• Renews ageing exterior facades and improves appearance of structure.
• Lowers maintenance costs and allows upgrading of building services.
• Improves air quality and general comfort
• When controlled ventilation is supplied overcladding helps eliminate internal problems such as dampness, condensation and mould growth.
And last but not least overcladding retrofits eventually pay for themselves.
Limitations of Overcladding Systems
• Overcladding systems may not be as durable as solid construction and as a result in damage can lead to dampness and weathering problems.
• Overcladding process can take a considerable length, requires state–of–the art scaffolding systems to complete and may be quite noisy. These factors may annoy tenants however there are strategies to address these issues.
• Detailing is critical and requires knowledgeable design and care during installation.
• The installation of overcladding systems is premature where an existing substrate is structurally unsound or where repairs have not been completed.
• Overcladding systems are not generally suitable for historic buildings.
The existing condition of these buildings ranges everywhere from reasonably good to urgently in need of attention and must be established by an initial conditional assessment to determine feasible strategies and associated costs.
When we examine the numerous high-rises in Toronto we also encounter almost every construction material and system employed over the past 40 years - not to mention balconies, ledgers, canopies, parapets, disconnect enclosures, roofing components and a whole host of building appendages.
The key performance issues in broad terms are safety, occupant comfort, stability, structural performance, acoustics, energy efficiency, control
strategies, air quality/ventilation, daylighting, maintenance/cleaning and cost.
Overcladding designs will need to consider a comprehensive list of performance criteria, of which many may be unknown quantities at the
present time, such as connection details, fasteners, thermal & moisture movement, loads on the system, maintenance and serviceability.
The designs will also need to consider acoustics, air permeability, cavity ventilation, water penetration resistance, condensation, fire performance, electrical continuity and the potential for corrosion.
In the author’s view what we’re really looking at is a building science challenge. This is a challenge that Canada is well equipped to tackle. Another key factor, as one might expect, is in need for financial incentives and issues related to the building permit process, building code and by-law requirements.
Keeping the Water Out –
The Rainscreen Principal
Traditional wall construction, which focused on caulking all of the joints in the wall assembly, has never worked effectively over the long term because of the practical difficulties involved in achieving a perfect seal and the differential expansion & contraction caused by the elements and UV radiation.
A large percentage of the buildings in North America have been constructed using this method and looking back now with the benefit of hindsight this approach appears misguided - and we are paying for it. Over the past 10 years the approach to wall construction has moved to the concept of employing “rainscreen” technology.
As a result most of these retrofit projects will likely involve the application of various forms of rainscreen overcladding systems that came into use in Europe in the 1970s.
Two excellent sites for readers wishing to obtain more detail are listed. www.nrc-cnrc.gc.ca/eng/ibp/irc/ctus/ctus-n17.html www.aaa.ab.ca/pages/members/media/RainScreenWallQAwa.pdf
The rainscreen principle involves intentionally leaving the joints in the façade open in order to allow air to move freely between the exterior environment and the interior cavity. This results in pressure equalization between the two.
In a rainscreen the air is trapped in the cavity. Therefore the air pressure equalizes between the exterior and the interior. In theory when the pressure outside and inside equalize the pressure to push water into the cavity is zero. The vent holes and the base flashings at the bottom of the wall permit any water that has penetrated the cladding to drain. Air that is continuously flowing through open joints also provides ventilation for the interior cavity. This ventilation is important because aids in the prevention of heat build-up and also maintains the effectiveness of thermal insulation by promoting the drying of any penetrating humidity in colder climates.