December 2, 2016
By Guillaume Vadeboncoeur
With an increased focus on careful stewardship of resources and rapidly rising utility costs, it is now mandatory for new and remediated buildings to be more energy-efficient. This tends to be easier said than done. There are numerous challenges that must be addressed during the design and construction phases of such a project. (An earlier version of this article appeared in the August 2016 issue of RCI’s Interface magazine.)
One of the main challenges the building envelope industry faces is designing exterior wall assemblies addressing thermal bridging, with lower-conductivity components ideally located outboard of the sheathing. Standards and building code requirements are trending in this direction.
One example—the American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings—is mandated by some building codes (e.g. British Columbia Building Code [BCBC]). To meet its requirements using the prescriptive method, one can either demonstrate low overall heat loss for an assembly or incorporate a continuous layer of insulation in the exterior wall design. Recently, manufacturers have introduced several varieties of thermal clips to facilitate the construction of exterior walls with insulation located outboard of the wall sheathing.
However, these various types of thermal clips are not equal in performance. Each has its own advantages and disadvantages, ranging from thermal properties to ease of installation. This author is an engineer with WSP Canada, which has been involved in projects involving these thermal clips. This article offers a trio of case studies with three different clips to discuss some of the advantages and disadvantages of each product, including thermal effectiveness, adjustability, cost, and design considerations.
Even with these new thermal clips, exterior insulated wall designs can be quite challenging during building envelope remediation projects, as existing buildings are often constructed using the traditional method of insulating between the wall studs. During these types of projects, it is crucial to properly design the new wall assemblies so the air barriers and vapour retarders are at the correct locations to prevent potential condensation problems.
Besides exterior insulated finish systems (EIFS)—wall assemblies that are adhered to the wall sheathing and structure—most exterior walls have framing components that can bridge the insulation layer. The impact of these components on the thermal performance of exterior walls is significant. Numerous strategies and wall cladding assemblies can reduce thermal bridging through an exterior insulation layer.
As mentioned, there are significant design and construction challenges with exterior insulated wall assemblies. These challenges should not be taken lightly, as they could become problematic and, in some cases, possibly catastrophic. Design solutions for these assemblies need to perform thermally, but also in accordance with good moisture management practices.
Moisture management should be considered in all exterior wall designs, but it is of primary importance in the coastal climate of British Columbia, where this author practises engineering. Current versions of ASHRAE 90.1 do not explicitly state this (as earlier versions did), but the standard’s primary focus is energy-efficient design. As insulation levels increase and walls become more airtight, it becomes increasingly difficult for the assembly to dry out—this makes it even more important to ensure the assembly does not get wet in the first place. Thus, ensuring reduced heat loss is important, as is proper management of wind-driven rain, air leakage, capillary action (pressure), and water-vapour diffusion. A wall or roof design that fails to account for these can be subject to leaks or condensation within the assemblies.
These exterior wall assemblies must also perform structurally and economically—in other words, they need to be relatively cost-effective and not overly difficult to construct.
Improving a healthcare facility
A recent project involved a challenging building envelope remediation for a healthcare facility. This building had a history of envelope failures due to air and water leakage. Originally, it was constructed with insulation between the steel framing. To increase the thermal performance of the exterior walls and to enhance the airtightness of the building, the design team decided to have the air barrier/vapour retarder (also the weather barrier layer) and the insulation layer outside the sheathing.
During the demolition phase, the deterioration to the existing steel framing and interior components were found to be worse than expected. Due to the condition of the existing sheathing, studs, and interior wall components, it was necessary to remove all wall components down to the framing.
Another challenge was to properly scaffold and weather-protect the building during demolition, as this is a hospital setting. Since the building was required to remain operational during construction, strict infection-control procedures were needed to safeguard patients and staff. The exterior walls and scaffolding area were fully enclosed, and the interior of the building was pressurized to prevent any contamination of the occupied space. This pressure was monitored at all times, and air quality testing was performed before, during, and after construction.
The new wall design incorporated engineered wall cladding assemblies to allow for drying and drainage, while reducing thermal bridging with fibreglass clips to reduce heat transfer.
The new wall assembly comprised:
The cladding types utilized for this remediation project were fibre-cement panels and horizontal corrugated metal panels.
Challenges during design and construction included providing proper structural support at all the joints in the new fibre-cement panels, and at transitions between wall cladding types. This required careful planning prior to installation of the fibreglass thermal clips and Z-girts. One of the disadvantages of this particular thermal clip is the lack of possible adjustment—both laterally (i.e. side to side) and front to back—to allow for construction tolerances. Therefore, the clip must be shimmed if the exterior wall sheathing is not plumb.
Thermal break project
This author’s firm is also currently involved in a healthcare construction project that uses galvanized steel clips with a ‘thermal break’ incorporated at the back of the clip. This project is the construction of a new healthcare building. The exterior wall construction is similar to the previously mentioned project, with a self-adhered air barrier/vapour retarder (also the weather barrier layer) outside the sheathing and semi-rigid exterior insulation.
One of the clip design’s advantages is a slot allowing for the installation of horizontal and vertical girts outboard of the clip. This allows the girt to be easily adjusted if the substrate is out of plane. It allows for the installation of both vertical and horizontal girts, if needed.
A few disadvantages were also noted. Due to its material and design, this clip does not provide the thermal effectiveness offered by the others. The design team may want to model the clip using a thermal transfer software or employ thermal transfer equations to determine its thermal effectiveness.
Several factors can influence the thermal effectiveness of the clip and exterior wall assembly. A small change in the exterior wall assembly’s effective RSI or R-value could have a significant impact on the building energy model. While other clips come in multiple sizes, the T-clip used on this project is only available in a depth of 100 mm (4 in.).
Thermal clips and aerogel
A third type of thermal clip was used for a project on Vancouver Island. Made of stainless steel, these products are paired with an aerogel material that can be placed on either or both ends of each clip. Each aerogel layer has an RSI of 0.7 (R-4), and multiple layers can be used. Aerogel is a synthetic, ultralight material derived from a gel; it is an excellent insulator as it consists generally of gas. The aerogel insulation comes into rolls and can be cut to the desired width.
An advantage with this clip (similar to the previously discussed thermal break clip) is the fact it is adjustable. This gives installers the ability to account for wall imperfections that are common in renovated buildings with concrete, brick, and steel studs that are non-parallel or inconsistent to each other. One of the disadvantages of this clip is its cost—the materials are fairly expensive, especially the aerogel insulation.
There are several thermal clips from which a design professional can choose, but these are not necessarily created equal. It is up to the design and project team to research, test, and model the various types in order to determine which thermal clip is the best for an upcoming project.
Whether they are used in new construction or in building envelope remediation projects, these types of wall assemblies will become more prevalent as the industry moves toward more energy-efficient buildings. Such projects are challenging, but with careful planning, design, and proper review during the construction processes, they can be accomplished with great success.
Guillaume Vadeboncoeur, P.Eng., LEED AP, is a professional engineer with more than 11 years of experience in building science. WSP Canada Ltd.’s building science group leader for the Fraser Valley and Southern Interior in British Columbia, he has also managed several building projects that include building envelope remediations, building envelope assessments, roofing assessments and replacements, and wall monitoring. Vadeboncoeur attended Laval University and graduated with a diploma in mechanical engineering. He is the president for the Western Canada RCI Chapter and the vice-chair on the Association of Consulting Engineering Companies British Columbia (ACECBC) Building Engineering Committee. Vadeboncoeur can be reached at email@example.com.
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