Designing with foam plastic insulating sheathing

by arslan_ahmed | January 23, 2023 9:00 am

[1]By David E. Sacks

In the 20th century, wall assemblies discretized into multi-layered systems with each playing a distinct role. Each new layer arose with the emergence of building science technology, including rain screen design principles and the evolution of building materials. What began as an assembly of one primary material offering thermal mass, weather protection, and esthetics, fragmented into a series of components, including cladding, thermal barrier, air barrier, vapour retarder, weather resistive barrier (WRB), and separate structural wall.

Over time, designers developed various wall configurations to provide thermal mass and weather protection suitable to particular building and climate parameters. Building codes ultimately canonized each wall assembly layer, creating prescriptive requirements for the construction industry.

Driven by economics and construction material development, the 21st century brought about an inversion to this trend. Today, there are a plethora of multi-functional material options, including structurally insulated panels (SIP), insulated metal panels (IMP), and integral weather barrier sheathings, all of which reduce the number of discrete layers to limit field labor. They improve quality control (QC) while compressing construction schedules.

Foam plastic insulation manufacturers realized the physical performance of their products presented an opportunity to capitalize on their existing characteristics beyond just thermal properties. The origins of this trend emerged in the 1970s and 1980s when manufacturers and trade organizations promoted features such as fire exposure behavior of certain polyisocyanurate (polyiso) insulation boards to eliminate the need for a thermal barrier.1 This was marketed as a differentiator between polyiso products and extruded polystyrene (XPS) products.2

When modified and/or accessorized properly, the same insulation board providing thermal resistance may serve as an air, weather, and/or vapour barrier. Select foam plastic insulting sheathing (FPIS) products merge all barriers and the wall sheathing into a single layer; these FPIS systems are the focus of this article.

All-in-one products pose drawbacks, as well. Difficulties may arise due to multiple factors, including the inherent nature of foam plastic as a combustible material, the lack of structural capacity,3 the suitability of the FPIS substrate when detailing transitions, and the potential for dimensional instability. Additionally, by their very nature, all-in-one products lack redundancy; a failure in one component creates a full assembly failure.


Insulting sheathing, as defined by the National Building Code of Canada (NBC), “Insulation that is uncompressed and continuous across all structural members without thermal bridges other than fasteners and service openings.” For the purposes of this article, insulting sheathing is installed on the exterior surface of the building envelope, and should have a minimum thermal resistance of R-5 in the core material.4 FPIS simply use foam plastics as an insulting sheathing.

Figure 1 s (FPIS) system utilizing foil-faced polyisocyanurate insulation. Image courtesy PIMA

FPIS systems most commonly utilize expanded polystyrene (EPS), XPS, or polyiso insulation enhanced with appropriate facers to create multifunctional products for use in an exterior wall envelope (Figure 1). This article examines polyiso products due to their broad market adoption as an efficient, multi-functional barrier (thermal, air, weather, and vapour) and wall sheathing. Here, efficiency refers both to thermal performance relative to other common insulation types and the related material properties enabling polyiso to serve multiple wall enclosure functions, thereby minimizing necessary wall assembly thickness.

There is a direct correlation between building energy code requirements and the depth of a conventional wall assembly. A conventional 20th century cavity wall would typically include a cladding system (e.g. brick masonry), a 25 mm (1 in.) air space, and a wood or steel stud backup wall system, with all insulation provided in the backup stud wall cavity. As energy code requirements become more stringent, a conventional wall provides limited options to comply. Prescriptive code requires exterior continuous insulation5 (e.g. insulation outboard of the backup stud wall); this improves the entire wall thermal performance and also enhances cavity insulation performance, which is otherwise diminished due to thermal bridging effects at studs, especially steel studs.6

However, the insertion of continuous insulation creates an added dimensional thickness within the assembly previously unaccounted for in a conventional wall. This additional layer has a cascading effect on other wall components. It pushes the cladding further from the structure, which may trigger upgrades to the structural system, necessitate different wall accessories, and potentially reduce the building’s effective usable area. This is not to mention the potential impact to construction schedule associated with the additional exterior layer.

FPIS systems, which incorporate foil facers onto a polyiso board, maximize the R-value per inch relative to other foam insulations. With reported R-values of R-6.5 per inch, one would need to use 30 per cent more XPS (R-5.0 per inch) or 50 per cent more EPS (R-4.2 per inch for high density EPS) to construct an equivalent R-value wall assembly.7

Figure 2 foam plastic insulting sheathings (FPIS) System shown with optional spray polyurethane foam in the stud cavity. Image courtesy DuPont PBS

For a building with little flexibility in terms of wall depth, the space savings afforded by an FPIS system can be the difference between prescriptive code compliance and non-compliance. A valuable online resource is, which allows the user to input basic building characteristics and receive corresponding exterior wall construction requirements specific to assemblies that may use foamed plastics in general, not just FPIS systems.

Wall assemblies featuring FPIS systems may be enhanced with cavity spray polyurethane foam (SPF). In the same efficient vein, SPF complements FPIS well. It is a highly efficient thermal insulation that also creates an effective air and vapour barrier and encapsulates the FPIS anchors, thereby minimizing thermal bridging and/or air leakage potential at board fastener penetrations.

Figure 3 Fenestration head design with use of proprietary products within one system.


Designing with FPIS requires some deviation from conventional wall detailing. Typically, one can differentiate between the field of a wall­—which would be covered by a continuous fluid or sheet good—and the edges of a wall, where the more laborious detail work occurs. FPIS installation is essentially all detail work. The field of the board is manufactured with a foil facer; the foil itself provides an air and water resistive barrier. To maintain barrier continuity, every board edge must be detailed. Any penetration through the facer creates a deficiency that must be remedied, right down to the fastener washer. Anything other than a usually solid washer requires a patch.

The detail work incorporates a series of accessory products to create a continuous air and weather resistive barrier along the exterior surface of FPIS. These include fluid-applied flashings, self-adhered tapes, sealants, and/or spray foams, among other products. Figure 3  illustrates one manufacturer’s basic FPIS wall assembly, with a trowel-applied fluid over all board joints.

In general, the application methods for most FPIS systems are similar, with minor component-specific variations, such as the required fluid coating edge distances (e.g. 25.4 mm [1 in.] minimum onto pipe penetrations). Tape flashing products universally require a wider application than fluid flashings. Accessory products, including masonry veneer ties, are not incidental components, but rather coordinated components as part of an integrated assembly.

There are a few differences from one system to another for designers to be aware of. While many systems include both a fluid and tape accessory to provide flexibility, some manufacturers do not incorporate a tape flashing in their accessory line or allow for perimeter sealant application (i.e. at window openings) to occur over their fluid membrane at a cut edge. As a result, the system requires additional metal trim at the entire perimeter for every rough opening.

Additionally, certain systems will require special detailing at the fenestration head (e.g. mineral wool at a box beam) and/or fire-resistive treated wood blocking. This is especially critical where SPF is introduced into the system and there is a requirement to protect the SPF from exposure at the fenestration.


In terms of code requirements, the potential building applications are nearly limitless. That said, some applications are better than others, often those which capitalize on the efficiency of FPIS. Building designs featuring a low window wall ratio (WWR), such as data centers, cold storage buildings, and other industrial facilities, derive more value from FPIS systems than higher WWR designs. This is not meant to imply FPIS systems cannot be utilized for commercial, daylit buildings, however the return on investment diminishes.

Consider a single 1.22 x 2.44 (4 x 8) FPIS panel in an opaque wall. Where this panel abuts all opaque wall, it will contain 2.9 m2 (32 sf) surface area with 7.3 m (24 ft) of perimeter. All detail work (i.e. application of liquid or tape goods) to maintain barrier continuity occurs along the perimeter (in this idealized scenario with no penetrations). The efficiency rate of that panel (surface area/perimeter distance) will be
0.4 (1.33). The higher the rate is, the less detail work that is necessary. One can quickly extrapolate that for a simple rectangular box, more continuous opaque wall equals more shared perimeter joints, driving the efficiency rate up.

For contrast, consider a narrow panel utilizing a half-size standard board (2 x 8) between two fenestration openings. Examining the perimeter detailing of the panel only (and ignoring any impacts of specialized fenestration detailing), the efficiency rate tumbles to 0.24. While these are simplified scenarios, they provide a clear differentiation in terms of installation efficiency.

Cladding specific applications

Each manufacturer publishes their own test report outlining different overall wall assemblies viable with their FPIS. These reports outline numerous allowable cladding types, including, among others: masonry, metal panel, cement board, and stucco along with air/weather barrier accessories.

Masonry systems often require a single anchor style lateral tie plus solid cap washer to limit the penetrations through the insulation board. Thermally, these single penetration lateral ties outperform dual anchor lateral ties; however, they may limit installation tolerance when compared with a standard two-piece adjustable lateral tie. In addition, the single attachment point style of lateral tie inherently lacks the redundancy of a two-point system.

Figure 4 Head detail requirements where alternate surface plasmon resonance (SPR) products are introduced–green highlights illustrate the deviations. mage courtesy Jensen Hughes Letter Report, Project No.: 1JJB05306.011

Stucco systems over metal lath involve a series of regularly spaced fasteners offset from the FPIS substrate. The offset fastener configuration prevents an adequate seal where the anchor penetrates the FPIS. To accommodate this issue, manufacturers may require either pre-striping anchorage points with FPIS accessory tape or fluid at stud locations, or the introduction of furring strips and pre-striping. This is not to mention quantity of attachment points for stucco (175 mm [7 in.] spacing at framing members)8 compared with standard 400 mm (16 in.) lateral tie spacing for masonry.

Metal panel systems, which are frequently attached to a subgirt system, present relatively fewer installation challenges. The girts can be sealed to the FPIS prior to cladding attachment, and no blind fastening through FPIS occurs. Metal cladding systems may be utilized in an open joint fashion; in this case, long-term ultraviolet (UV) stability should be factored into the design of the air and weather barrier system. Additionally, metal conductivity and related heat buildup may present a separate set of issues related to dimensional stability. Research has consistently shown that foamed plastics experience both elastic and inelastic (i.e. permanent) board deformation at elevated service temperatures.9 However, these studies typically focus on roofing type assemblies where conductive heat transfer should be anticipated.

Each cladding type will have its own series of nuances. This is not meant to sway a designer away from FPIS systems or from a certain cladding and FPIS pairing, but rather to bring attention to potential challenges presented by various product combinations. In general, any cladding system requiring direct attachment, concealed or exposed, will likely require supplementary steps to ensure a sealed penetration, as compared with a cladding attached to sub-framing.

Construction challenges

For every benefit, there is always a countering cost. This may become an actual financial cost, but typically this presents as a construction variable. There are several challenges designers should consider as they proceed with an all-in-one FPIS system.

Proprietary-like assembly

To achieve code compliance, FPIS systems require full assembly testing (ULC S134) to confirm several performance metrics–the result provides rating for an assembly of components working together. Testing is an expensive endeavor, and thus most manufacturers have only performed limited tests relative to the available products on the market. This means assembly components are not interchangeable. In construction, it is routine for product shortages, time constraints, or other limitations to necessitate a building component substitution.

In short, if a single assembly component is not available, the system may not be viable or may require modifications to meet testing or code requirements. For non-combustible assembly components, substitution options may abound. For combustible components, on the other hand, options may be severely limited. For example, if a manufacturer has a tested assembly with a specific closed cell spray foam which is not available, substituting an alternate spray foam may trigger significant detail changes (Figure 3 and Figure 4).

Figure 5 Foam plastic insulating sheathing (FPIS) system with a series of surface punctures due to temporary placement of construction debris against the material. Photo courtesy

Alternative solutions may be proposed by a qualified architect/engineer through a process commonly referred to as equivalency. As noted in the code commentary, “to do something different from the acceptable solutions described in Division B, a designer must show that their proposed solution will perform at least as well as the acceptable solution(s) it is replacing.” However, much of the language provided includes qualitative rather than quantitative metrics. Accordingly, alternative solutions may be unattainable in practice.

Material resilience

Polyiso is low density; this is an inherent property necessary to create the desired thermal resistance. However, low density also creates a material vulnerable to site damage during construction (Figure 5). Since FPIS in the non-redundant scenario provides all the building enclosure barriers, damage to FPIS means damage to the entire building enclosure assembly.

This is easily remedied where an issue is exposed; however, concealed deterioration is just as likely to occur. Where a cladding contactor lacks familiarity with FPIS systems, breaches may go unnoticed and become buried within the completed wall assembly. Requiring coordinated mockups with all parties participating in a joint discussion may limit this issue.

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.

Figure 6 Base of wall flashing with plywood added as substrate for thermoplastic polyolefin (TPO) roofing.

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 6). 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 product. 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 co-ordination 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 vapour barrier system, and requires special detailing. It is at these penetrations where breaches are most likely to occur, therefore limiting the number of penetrations provides the best strategy for long-term success.

Figure 7 An expansion joint provision where a balloon-framed parapet wall meets a slab supported vertical wall.

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 7). 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.

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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