Controlling thermal bridging with single-component wall assemblies

by Katie Daniel | May 27, 2015 11:14 am

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By Andrew Williams
Although many advances over the last decade have improved energy efficiency, not every new building is designed and built with these technologies. In some other instances, older buildings that have undergone retrofits have better thermal control capabilities than newer structures.

Thermal bridging is a primary issue affecting energy performance and occupant comfort in modern buildings, occurring where conductive materials ‘bridge’ between the interior and exterior environments, allowing less resistance to heat transfer. Common examples of thermal bridging are flashings, windows, studs in cavity walls, and fasteners.

There are several methods to enclose a building and improve energy efficiency, but most are multi-part systems and do an inadequate job of helping control thermal bridging. To resolve this, a specified cladding material must provide the highest R-value, have a labour- and cost-reducing installation process, and contribute to green construction certification.

According to the Building Envelope Thermal Analysis, published last year by engineering firm Morrison Hershfield:

The plane of heat transfer for the building envelope is a theoretical projected area between the interior and exterior conditions through which heat flows. In order for there to be a heat loss or heat gain through the building envelope, energy must pass through this plane of heat transfer. A building assembly may have some elaborate features that extend out past the building envelope; however, all that is important for thermal performance is where the heat flow passes the plane of heat transfer into or out of the building.

Insulated metal panels
Specifying insulated metal panels (IMPs) can assist in controlling thermal bridging, while helping create a more efficient exterior wall system and meeting construction code requirements. Inadequate thermal performance can create a structural failure that may require costly retrofits. By solving thermal bridging issues in the design phase of new building construction, the need for expensive deep-energy-retrofit projects in the future can be eliminated.

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A building’s design must have insulation membrane continuity to control moisture at all interface joints. Designers should focus on thermal load reduction rather than trying to solve thermal performance issues by just specifying high-efficiency mechanical equipment. The challenge is to select an efficient exterior insulation product that can help control thermal bridging as much as possible, while also reducing energy consumption.

Ideally, a building’s envelope should be designed to optimize energy conservation performance by allowing it to achieve net-zero energy targets. Single-component wall systems, built with insulated metal panels, form the basis of a building envelope that maximizes thermal performance while simplifying design and construction. Although thermal bridging may never be fully eliminated, energy modelling has shown an IMP wall system can help reduce a building’s energy consumption over its lifespan.

Insulated metal panels are manufactured by bonding a rigid foam core between two prefinished metal facings. These steel or aluminum panel facings create a vapour barrier and provide long-term thermal stability. The foam core of an IMP is made with polyisocyanurate (polyiso). This thermosetting type of plastic, closed-cell foam contains a low-conductivity, hydro-chlorofluorocarbon-free gas within its cells. The high thermal resistance of that gas is what gives polyiso insulation materials an excellent R-value range.

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Even with its thermal performance capability, many designers choose to specify insulated metal panels because the installation process is simplified. Exterior cladding systems that require multiple components are more prone to construction delays—partly due to a need for scheduling multiple trades. A typical exterior insulated finish system (EIFS) requires at least 10 separate components and must be fabricated onsite. IMPs are a single-component product, which arrive ready to install, allowing for faster construction.

IMPs can be installed in almost any kind of weather without risk to system integrity, as opposed to multiple installation steps required for other insulated wall and roof assemblies. Insulated metal panels are installed with a concealed fastener system further reducing the number of field-assembled components. With this simplified process, IMPs can be erected quickly even for large commercial and industrial facilities.

When insulated metal panels are joined together with an interlocking system, the result is a solid exterior wall that reduces thermal bridging. Using these integrated side joints makes it easier to incorporate additional components such as sunshades or window systems. This seamless integration creates a thermally efficient building envelope with an esthetically pleasing appearance, weather resistance, and an increased lifespan.

The Ontario dilemma
To meet the Ontario Building Code’s (OBC’s) Supplementary Standard 10 (SB-10), “Energy Efficiency Supplement,” which governs thermal efficiency, many designers specify a costly high-efficiency mechanical system.

Initially adopted in 2007, OBC was revised in 2012. This excerpt is from the revision and states when thermal bridging regulations can be ignored:

Division 3, Chapter Two, Revision to American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1-2010, Energy Standard for Buildings Except Low-rise Residential Buildings, 5.5.3.7/8:

For the purposes of Section 5, the effects of thermal bridging are waived for:

• Continuous steel shelf angles (or similar structural element) used to support the building façade provided there is a thermal break.
• Structural connections of load-bearing elements where a thermal break cannot be achieved.
• Exposed structural projections of buildings where the total cross-sectional area of the exposed element does not exceed 2%.
• Ties in masonry construction, flashing, and top exposed portion of foundation walls, provided the exposure does not exceed 200 mm [7.8 in.].

However, this code might need further revisions again because all thermal bridges should be resolved, not ignored. Few new buildings in Ontario meet the requirements of the provincial code’s SB-10 energy efficiency requirements. This is because rather than find a way to reduce energy use, some projects have been designed with high-performance HVAC equipment to compensate for poor thermal performance.

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Thermal bridging on the roof
An integrated wall and roof panel system has many benefits. Standing seam insulated metal roof panels are rigorously tested to ensure they remain both airtight and weather-tight over a building’s life.

A single-component insulated metal panel roofing system provides enhanced performance, design flexibility, and a fast-track construction installation method. However, above all these benefits, it also offers a high R-value thermal performance and superior airtightness with no thermal bridging. It can also result in energy savings higher than given by conventional built-up roofing (BUR) systems, as a result of the thermal performance provided by each panel’s polyiso core.

Standing-seam insulated metal panels have resistance to outdoor corrosion. Unlike conventional roof systems, these panels will neither crack nor peel when exposed to the sun and rough weather. Insulated metal roof panels also eliminate the cost of re-shingling or replacing a BUR system. Made with recaptured low-weight metals, they weigh only 1.36 kg/m² (3 lb/sf), reducing transport and installation energy costs.

Weather conditions are a significant concern in the construction process with most exterior envelope systems. However, IMPs can be installed any time of year because they create non-reservoir walls that do not absorb water. Exterior walls made of brick, precast concrete, or tilt-up absorb rainwater. If moisture enters a wall cavity and is not drained or vented, mould and mildew growth usually occurs.

The green factor
Installing the insulated metal panels will create superior airtightness and moisture control, as well as offer high R-value. It is the crucial initial design step if a building is to achieve Leadership in Energy and Environmental Design (LEED) certifications.

A roof built with insulated metal panels will help to reduce operational heating and cooling demands and, as a result, contributes to lowering the carbon footprint of a building. This can contribute to the LEED rating program’s Sustainable Sites (SS) credits, earning up to two points for reducing urban heat islands. IMPs can also satisfy requirements for the EnergyStar and cool roofs programs for reflectivity benefits. These metal panels, along with
their polyiso core, are completely recyclable, which offer multiple end-of-life reuse options—including creation of new IMPs.

Due to the fact IMPs are installed with a one-step method, onsite work hours can be reduced by as much as 50 per cent when compared to multi-component roof systems. Some insulated metal panels use a hidden-clip design that minimizes the need for through-fasteners that would otherwise penetrate the external facing, resulting in considerable degradation of panel performance.

Specialized buildings
The resolution of thermal bridging issues should be the leading concern in the design of all structures made for the built environment. However, there are certain building types where closing all thermal bridges is the primary concern. This is especially the case with cold-storage buildings.

For example, the BC Tree Fruits Cooperative (formerly Okanagan Tree Fruits Cooperative) in British Columbia was hampered by aging and out-dated facilities, combined with higher food safety standards that led to rising production and operating costs.

The existing refrigerated storage systems were antiquated and inefficient from an energy utilization standpoint. They required major upgrades with a more sustainable technology in order to significantly improve the quality of the product.

It commissioned TriCorp Developments (Vancouver Island) to assist with facility performance measures aimed at maximizing its growers’ fruit returns. TriCorp achieved this through creating 23 controlled atmosphere storage rooms at the group’s Winfield packinghouse. TriCorp, which specializes in the design and build of highly specialized controlled atmosphere rooms, specified IMPs to fulfil the insulation requirements and esthetic needs for a complete building renovation and upgrade. This reduced the building’s carbon footprint, lowered maintenance costs, and gave the building a modern appearance.

Integrated systems
Bringing together single-component insulated metal wall and roof panels creates an exterior integrated system meeting thermal demands. Such a wall and roof system also offers design flexibility with vertical and horizontal applications easily incorporated with traditional construction methods and building systems.

These assemblies can offer superior energy efficiency with a stylish look sought by many in the architectural community. IMPs are fabricated in many styles and sizes depending on the selected design application. On top of the energy-saving benefits and ease of installation, insulated metal roof panels offer an alternative to traditional metal roofs. Certain panels have an architecturally appealing 51-mm (2-in.) vertical side-lap with minor ribs in between, which presents an attractive linear accent. Architects and design professionals can select from a wide range of finishes and colour palettes.

Conclusion
Not every type of building can utilize the benefits that insulated metal panels offer. Structural design variations, plus a wide range of factors, can pose challenges to a project that cannot be resolved with IMPs. The answer is determined by a judgment call made by the architect.

Every building project comes with challenges and designers are always confronted with standard issues of structural performance, visual appearance, budget, and project deadlines. Design features now require a building—and its individual elements—to act as an environmental steward.

Whether a structure is built for office use, warehousing, or manufacturing, high thermal performance is key to conserving natural resources while simultaneously helping with the bottom line. Buildings constructed in a sustainable method will have a higher and more retainable market value, while also contributing to help conserve resources and improve the bottom line. One primary factor for achieving this is to control thermal bridging; insulated metal panels are single-component products that can be specified to fulfil this role.

[5]Andrew Williams, Dipl. Eng Tech (Mech), serves as Kingspan Insulated Panels’ national business development manager for Canada. He has been with the company for five years, and in the metal cladding industry for almost a decade. Williams can be reached by e-mail at andrew.williams@kingspan.com.

Source URL: https://www.constructioncanada.net/controlling-thermal-bridging-with-single-component-wall-assemblies/

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