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.
Design challenges
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.
