Common detail challenges
When designing an FPIS system, the transitions present the most challenging details. These details should not simply be delegated to the installing contractor, as often occurs. Failing to incorporate desired transition details may result in construction phase design changes and the associated change orders. Designers should pay close attention to roofing flashing transitions, at-grade transitions, and anything that may result in substrate and/or plane changes.
Depending on the type of roofing, direct attachment to FPIS may not be an option; hot-rubberized asphalt, which is frequently utilized at occupied terrace and plaza areas, cannot be applied directly to FPIS. Instead, the design may need to incorporate a separation layer (e.g. plywood or roofing cover board, see Figure 7). This separation layer will create an offset in the wall plane requiring accommodation.
Consider metal counter-flashings. Even where compatibility is not an issue, the use of FPIS will likely impact the roof flashing termination; for starters, every termination will be surface-mounted, with functionality entirely reliant on a bead of sealant or liquid flashing. Additionally, the backup wall may require metal strapping at the height of the termination bar to support appropriate fastening requirements.
Along the same lines, designers should be aware FPIS systems cannot typically be installed below-grade. For purposes of continuity, there should be a transition detail between the FPIS and the below-grade system, where applicable. XPS is frequently the insulation of choice below grade, with the damp proofing or waterproofing layer inboard of the XPS. In addition to the change in substrate, this transition may create a geometric offset that flashings will need to span. Broadly speaking, these are minor modifications. When incorporated into the design during the construction phase, however, their cost impact may be outsized.
Coordinated concepts
The importance of system coordination is universal, regardless of FPIS integration. That said, the design team for an FPIS assembly needs to consider there may be conflicting interests of the components. For example, structural engineers are taught to select the most efficient steel sections, including for structural connections. Where an exterior structural element ties to the backup wall system, it may be more valuable to enlarge the size of each connection to limit the overall number of connection points as each creates a breach in the air, weather, thermal, and vapor barrier system, and requires special detailing. It is at these penetrations where breaches are most likely to occur, so limiting the number of penetrations provides the best strategy for long-term success.
This becomes more complex where delegated design enters the picture. Cold form metal framing (CFMF) is often completed through the delegated design process. CFMF systems frequently incorporate movement provisions. Where the movement changes planes, the FPIS system must accommodate the same movement. For example, where a projected balcony interrupts a balloon framed CFMF wall, one segment of the CFMF may be loaded on the slab, while the adjacent portion extends down to grade (Figure 8). From the exterior there is little indication the FPIS system requires a specialized detail. While this could be addressed in shop drawings, it is uncommon to receive a shop drawing submittal from FPIS installers.
Field modifications and remedial repairs
Nearly every project necessitates select field modifications, due to any number of variables. These modifications may present added challenges where FPIS wall systems occur. Consider a box header in cold-formed steel construction. Design with an FPIS system would likely direct cladding attachment points to occur above of a box header. However, cold-formed steel framing is frequently completed through delegated design, with that party often unaware of any specific detail requirements outside of structural load requirements or deflection criteria. The lack of coordination may cause cladding attachment points aligned with box headers, inhibiting an interior seal at the penetration; this type of detail is often overlooked until the initial installation. This is one of many potential design busts that may occur during the construction phase.
While undesirable, in-construction issues are more manageable than in-service issues. Consider the scenario in which a masonry cladding system includes improper lateral ties. Undoubtedly, this would present an issue regardless of the wall barrier construction. A typical repair may include remedial anchors drilled through the masonry and into the backup wall. However, every fastener creates an unprotected penetration. It is difficult to imagine a retrofit that avoids damaging the FPIS short of removing and replacing the wall assembly in its entirety.
Structural Considerations
There are several structural considerations that the design should incorporate. These include the code required cladding attachment parameters as well as strategies to address lateral bracing alternatives as the sheathing layer is often relied upon to fulfill the role. While an important consideration, this is beyond the scope of this article, though there are helpful resources available.
Conclusion
All-in-one FPIS systems offer considerable value to the construction process, and market forces will likely demand further implementation of these systems and increased variation in product offerings. As with any product, certain building types enhance the benefits of FPIS systems, specifically those with limited wall openings.
When considering whether to use FPIS systems, a designer should understand the potential product limitations, especially the lack of redundancy. Select appropriate claddings to minimize difficult-to-seal penetrations, include enhancements to facilitate installation (e.g. furring strips), illustrate the system to the extent possible without defaulting to a manufacturer’s published details, and identify specific conditions requiring coordination between multiple trades. Keep a Plan B in mind if a system critical material is no longer available.
Author
David Sacks is a registered architect with more than 16 years of experience. His expertise encompasses building enclosure design, consulting, and forensic investigation work for new and existing buildings. He sits on the ASTM D08 Committee on Roofing and Waterproofing and is a former board member for the Western Great Lakes Chapter of The Association for Preservation Technology.
