HAVAN eLearn

Background

When talking to builders about locating all the thermal insulation outboard of the wall structure, there is a consensus that this wall configuration delivers a holistic outcome. Among all of its benefits, “maintaining the wall structure close to indoor temperature year around” appears to resonate the most with builders due to the elimination of the condensation risk on the structure; not only this translates to significantly less liability for builders, but it also provides end users with a solution that is durable, flexible, and forgiving to lack of perfection. But then, one may wonder, “how come this assembly has so far only had a small share in the construction market?”. When asked about the reasons, builders across BC shared legitimate concerns in response to high cost and installation challenges associated with the exterior insulation and the rainscreen cladding system.

Generally speaking, two approaches are mainly utilized in British Columbia for the cladding attachment of the exterior insulated wood-framed walls:

• In the first approach, the cladding is fastened onto the wood strappings, which in turn are directly connected to the wood studs using long screws. One of the key advantages of this system is that it minimizes the cutting of the insulation boards; additionally, since the long screws are installed after the insulation boards are in place, the disruption to the insulation boards is minimized. These result in a more uniform insulation layer, which ultimately translates to a thermal performance closer to what is anticipated via thermal modeling. With that said, one of the reported challenges with this approach has been the associated labour cost in ensuring the long screws hit the studs. This especially becomes a challenge when using insulation thickness beyond 4” as slight movement of the hand can easily result in missing the studs. In addition, some structural engineers have raised concerns on solely relying on the long screws for the transfer of the cladding load, in particular when dealing with heavy cladding types. Furthermore, since the insulation is part of the structural load transfer of the cladding (strapping will be pressed against the insulation), depending on the compressive strength, some insulation types can become compressed in localized areas, increasing the risk of waviness of the strapping. Last and not the least, long structural screws generally come with a premium pricing.
• In the second approach, the wood strappings are replaced with the combination of intermittent thermally broken clips and vertical metal girts; the cladding will then be connected to the vertical metal girts. In most of these systems, the intermittent clips are first fastened onto the wall studs (using short screws); then once the insulation boards are in place, the vertical metal girts will be installed onto the intermittent clips (using other sets of
  short screws). This system generally has a higher material price than that of the first approach. But at the same time, it eliminates the need for using long screws, thus the process of finding the studs and fastening onto them is less labour involved. Furthermore, the insulation is no longer part of the cladding load transfer, thus, it does not need to have a high compressive strength, meaning cost saving. With that said, when dealing with board type insulation, almost every piece of insulation needs to be custom cut to fit around the intermittent clips, something that was not the case in the first approach. This can be especially labour intensive when dealing with intermittent clips that have bulky profile. And to add to that, some systems require that the vertical metal girts be installed at the same time as the intermittent clips (prior to insulation installation) which compromises the ease of access for insulation installation. The combination of these factors has, in many cases, led to widespread gaps between the insulation boards and the intermittent clips which are mostly ignored. The cumulative impact of such gaps can result in a thermal performance outcome quite different than what was anticipated in the thermal modelling.

In addition to the items discussed above, both approaches require special considerations for exterior wall corners, around windows/doors, etc., to ensure sufficient back-up is available for fastening the trims and cladding.

Session Overview
Based on two recently completed fully exterior insulated wood-framed projects, this session is going to introduce a new “hybrid approach” for the attachment of the exterior insulation and rainscreen cladding system that incorporate:

• The advantages of both approaches discussed earlier.
• A highly reliable and robust structural system for handling various cladding loads
• Minimum thermal bridging
• Great saving on labour cost associated with the installation of exterior insulation and rainscreen attachment system.
• Flexibility in handling cladding and trim installation in exterior wall corners, around
  windows and doors, etc.

The presented “hybrid approach” is developed via various interactive brainstorming sessions with builders and structural engineers of the two projects with the goal to address the high cost and installation challenges that, over the years, have kept the industry away from embracing the fully exterior insulated wall construction. The step-by-step of the design development and site installation process were documented with photos to assist in communicating the intent intuitively.

Learning Objectives
By the end of this session, the audience will gain a holistic and practical insight towards:

• The comparison between the following two cladding attachment systems for exterior insulated walls: wood strapping with long screws VS combination of intermittent clips and vertical metal girts.
•  A new “hybrid approach” for cladding attachment that combines the best of the above two-mentioned approaches while offering noticeable cost saving.
• Structural and building envelope considerations associated with the proposed “hybrid approach”
• The necessary in-advance coordination for a streamline site installation of the proposed “hybrid approach”.

Thank you to our Sponsors:
Presenting Sponsor: FortisBC
Patron Sponsor: BC Housing