Designing with foam plastic insulating sheathing

Site modifications and remedial repairs

Nearly every project necessitates select site 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 co-ordination 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 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. According to the NBC’s Appendices:

insulting sheathing material may perform adequately as sheathing in a wall system that is braced by other means but may not perform adequately as sheathing in a wall system where the sheathing must provide the structural bracing. The structural performance of the insulting sheathing should be taken into consideration, with appropriate design solutions to demonstrate compliance with overall load resistance performance of the wall assembly.9


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 co-ordination between multiple trades. Keep a Plan B in mind if a system critical material is no longer available.


1 Consult the Insulation Handbook, Chapter 11, pg. 226.

2 For more information, see A Builder’s Guide to Residential Foundation Insulation.

3 There are manufacturers in the marketplace fabricating structural FPIS systems which incorporate a sheathing board into the FPIS sandwich; however, this article focuses on FPIS without this additional integrated layer.

4 Refer to National Energy Code of Canada for Buildings A-—Thermal Characteristics of Components of the Building Envelope.

5 Per ASHRAE 90.1.

6 Consult the Insulation Handbook, Chapter 11, pg. 220.

7 Refer to ASTM C1063 – Standard Specification for Lathing and Furring to Receive Interior and Exterior Portland Cement-Based Plaster

8 Read Rockwool and RDH, “Dimensional Stability of Rigid Board Insulation Products”. June 12, 2020.

9 Consult National Building Code of Canada A- Code Compliance via Alternative Solutions.


David Sacks is a registered architect with more than 17 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.

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