April 20, 2016
By Amir Hassan, M.Sc., P.Eng.
The scientific community has virtually agreed the planet’s atmosphere is changing due to greenhouse gas (GHG) emissions generated by human activities. The Paris Conference’s agreement, within the structure of the United Nations Framework Convention on Climate Change (UNFCCC) in December 2015, marks a change in direction toward a greener planet.
It confirms the target of keeping the rise in average global temperature below 2 C (3.6 F), as scientists believe a greater increase would be very dangerous. The agreement even establishes, for the first time, that a maximum rise of 1.5 C (2.7 F) should be the goal in a bid to protect island states, which are the most threatened by the rise in sea levels.
In typical buildings, space heating and cooling are usually the biggest consumers of primary energy, and consequently, are the largest contributors to carbon dioxide (CO2) emissions. Therefore, the logical approach for design/construction professionals is to reduce building energy use by creating better environmental barriers separating the interior and exterior climate conditions.
Codes and standards
In North America, an ambitious move toward sustainable building began almost two decades ago with the U.S. Green Building Council (USGBC) launch of Leadership in Energy and Environmental Design (LEED) program. The rating system, which promotes the use of building envelope systems with better performance by directly and indirectly offering some reasonable credit points, has diversified for different types of projects and spread to Canada and around the world.
While the LEED rating system (and others like it, including Green Globes) may have trail-blazed the green evolution in buildings, in many cases it remains voluntary and unenforceable. Prudently, then, recent building and energy code updates have refocused requirements on energy efficiency in buildings and a lower carbon footprint. The optimal goal is true net-zero-energy (NZE) projects—buildings producing at least as much energy as they consume on an annual basis.
Design/construction professionals and building envelope consultants in particular must do their part in ensuring building skins are adequately insulated when it comes to opaque walls, roofs, floors, fenestration, doors, and skylights.
In general, the newest energy codes are ahead of the present local market’s standards. Therefore, manufacturers need to keep up by improving their systems (especially exterior wall assemblies) to satisfy the new codes. Historically, that improvement process happened with glazing systems—today, one struggles to find exterior single-pane glass panels on a new Canadian building. Similarly, triple-pane insulating glass (IG) units, dynamic curtain walls, and double-skin façades will eventually replace mainstream double-glazed IG systems in the future.
The prescriptive path of the latest National Energy Code for Buildings (NECB) requires high-performance exterior walls with thermal resistance values ranging from RSI-3.2 to RSI-5.6 (R-18.2 to R-31.8), depending on climate. In other words, for a building in Edmonton to comply with the prescriptive path of the 2011 NECB, the above-ground opaque exterior wall system should have a maximum overall thermal transmittance of 0.21 W/m2.K, which is almost an RSI-4.8 (R-27.3) wall—this takes into account the effects of thermal bridging. In essence, this is a very high RSI-value for conventional exterior wall assemblies available in the local market.
Full adoption (or adaptation) of the 2011 NECB by all authorities having jurisdictions (AHJs) across Canada is expected by the end of this year. The code represents a significant reduction in building energy consumption levels over the 1997 Model National Energy Code for Buildings (MNECB) by around 25 per cent, and outperforms American Society of Heating, Refrigeration, and Air-conditioning Engineers (ASHRAE) 90.1-2010, Energy Standard for Buildings Except Low-rise Residential Buildings, by 18 per cent.
Accordingly, Canada’s building envelope industry needs to understand and accommodate the new energy requirements to construct sensible façades economically and environmentally. Thus, the first logical step is to increase the thermal insulation values of exterior walls in order to conserve heating and cooling energy in buildings.
In very basic terms, Mother Earth always wants to sustain balance. Therefore, heat transfers from higher temperature to lower temperature. There are three modes of heat transfer:
To reduce conduction, layers of material with high thermal resistivity—such as wood framing—should be used to construct the structural part of skin assembly as much as possible. Reducing radiation can be achieved by using material with low emissivity (i.e. poor absorber and good reflector of long-wave radiation), such as applying a very thin layer of metal film on low-emissivity (low-e) glass and laminating aluminum foil to sheathing board.
Air has the highest thermal resistivity among naturally existing materials, but circulation of air in big voids reduces its resistivity due to convection. Therefore, forming smaller compartments within the cavity would produce a good insulator. Depending on the arrangement of those compartments and used materials, the insulation types would rank in performance.
Many types of insulating materials are available and used in buildings. The most commonly used include:
Understanding the final thermal behaviour of an exterior wall, during the design stage, is crucial to achieve an acceptable building enclosure.
Aside from increasing thermal insulation and improving airtightness, reducing thermal bridging has to be carefully addressed. Solving thermal bridging in buildings is no easy task. To transfer structural loads between the components of exterior assemblies, many engineers and architects use metal brackets—Z-girts, masonry veneer brackets angles, and ties. It does not take a crystal ball to predict seeing more thermally broken connectors and innovative technologies to deal with this issue. Moreover, structural steel members protruding through the thermal layer pose an even harder challenge, since the transferable dead and live loads are usually higher in value—cantilevered framing at canopies or balconies provides a case in point. Upward trending thermography can be utilized to identify thermal bridging and verify repair strategies in new and existing buildings.
We have successfully made our modern buildings tighter and improved their thermal performance. However, we are now challenged to deal with some side effects, such as condensation, circulation, and indoor air quality (IAQ). Additionally, while contemporary construction is arguably smarter and more efficient, it can also be less durable. Products used in building envelopes that are less forgiving of water include traditional gypsum board and wood sheathing. In general, we expect the main structure of a building to serve for 75 years or more, while various components of the building skin are designed for considerably less than that, depending on the assembly’s overall construction and maintenance.
Caused by precipitation of water vapour from air onto colder surfaces, condensation is dampness on or inside building components (i.e. surface and interstitial condensation, respectively). Surface condensation is readily visible (e.g. misting of windows), and can be managed by providing sufficient ventilation and heat. (An alternate solution involves simply accepting the results and specifying the use of, for example, condensation gutters in some skylight assemblies.)
Surface condensation will eventually dry off. Consequently, for humidified spaces when it is cold outside, the suggestion is often to leave window curtains open to allow the heat from the interior to increase the inner surface temperature. This reduces the condensation risk, and lets the condensed water evaporate.
In humid, hot climates, surface condensation may form on the outside boundary of the wall system. This is not as critical to the building performance, provided moisture does not condense within the building’s assemblies. Although in high-rise towers, condensation on exterior glazing poses some challenges, as it is hard to be wiped off, and attracts dust.
Interstitial condensation—the aforementioned formation of moisture within the exterior cladding assemblies—is more problematic because it attacks the wall assembly from within. By the time it is detected, the assembly could be at a late stage of deterioration. In buildings, interstitial condensation could cause:
Water within the supporting structure would rot wood, corrode metal, and deteriorate concrete. This could compromise the building’s structural integrity, leading to very costly and inconvenient repair.
Mould is more of a growing problem within the building industry; there are many spore species known to cause adverse health effects in humans. Mould needs four essentials to survive: food (cellulose in most of building materials), oxygen, warmth, and water. Of these, three are challenging, if not impossible, to forgo from occupied spaces. Therefore, eliminating humidity in wall assemblies is the most viable way to ensure a mould-free building.
Besides the supporting structure (e.g. loadbearing masonry, stud wall, and metal deck roof assemblies) and the outer finish layer (e.g. brick veneer, vinyl siding, stucco, and metal cladding) serving as the weather shield, the building skin consists of three main layers: the thermal barrier, the water barrier, and the air barrier. The key to good design is understanding how those layers interact with one another and, most importantly, with the surrounding environment.
Depending on the position of the insulation layer, building skin types can generally be categorized as either:
When it comes to increasing the thermal resistance of new and existing walls by adding insulation, some think it is a case of the more, the better. This can be true for thermal performance; but for wall durability, the increased probability of condensation occurrence within the new wall has to be carefully analyzed.
In some cases, adding insulation arbitrarily can actually be quite damaging. For example, with an existing building clad in EIFS, adding batt insulation in the wall cavity would reduce the surface temperature of the main membrane, because insulation performance depends on the difference in temperatures across the thickness.
In this instance, without a vapour-permeable membrane, interstitial condensation will eventually occur in the new wall. Hygrothermal analysis should be implemented to evaluate heat and moisture movements through buildings during design stage in order to reduce the risk of condensation.
If architects and designers cannot find sufficient systems under the prescriptive approach, they can try to trade-off some of non-compliant elements of the exterior shell with more efficient HVAC, domestic hot water (DHW), and lighting. Architects usually leave reducing window/wall ratio as a last resort. Therefore, proposed buildings with a substantial percentage of glazed surfaces will need either incredibly high-performing glazing or significant HVAC upgrades to achieve compliance.
It is important to note a maximum fenestration-and-door-to-wall ratio (FDWR) of 20 to 40 per cent, depending on the climate region in which the project is located, is allowed in the 2011 NECB. Alternatively, the ‘performance path’ option can be taken, where there will be higher demand for employing thermal simulation software to be used in energy modelling software, such as National Resources Canada’s (NRCan’s) CAN-QUEST. (For more information, visit www.nrcan.gc.ca/energy/efficiency/buildings/eenb/16600).
While the performance path requires additional effort up-front in the form of energy modelling, it offers the greatest flexibility for demonstrating compliance. Further, it is often the only alternative when the design is non-compliant due to high FWDR. General consensus seems to be once energy modelling becomes a part of the design process, then exceeding the minimum code requirements is often cost-effective and desirable. Using energy modelling as a tool early on can also minimize costly redesigns and the need to introduce expensive technologies at the end of design.
To satisfy the energy codes as well as mitigate building envelope issues, architects and developers should engage sustainability and building science specialists to participate in or lead design choices for wall and glazing assemblies, provide or review building enclosure specifications, critique architectural and shop drawings, evaluate pre-construction mockups, and conduct onsite testing and commissioning. Testing and commissioning are important parts of the energy efficiency package to ensure systems operate in the way they were intended.
Amir Hassan, M.Sc., P.Eng., is a building science manager with WSP Canada Inc. He has more than 20 years of envelope experience in the Middle East, Europe, and North America, having worked with structural glass, curtain wall design and evaluation, frameless and skylight systems, structural assessment, weathertightness, thermal performance, cladding, roofing, and thermography. Hassan holds a bachelor’s degree in civil engineering, and a master’s degree in façade engineering. He can be reached via e-mail at firstname.lastname@example.org.
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