June 2, 2015
by Tom Remmele, CSI
Continuous insulation (ci) has been a component of exterior wall assemblies for almost a half-century in North America. By minimizing energy loss caused by thermal bridging and the risk of condensation caused by water vapor diffusion, exterior ci can improve building durability and benefit the environment. However, using rigid foam plastic comes with certain design considerations that must be reviewed early in the design process.
In the February 2015 issue of The Construction Specifier, this author wrote a piece entitled, “Continuing Education on Continuous Insulation: Design Considerations for Rigid Foam Plastic Insulation in Exterior Walls.” That feature looked at the basic types of materials available, and focused on fire safety and moisture-related durability. Now, this article serves as a sequel of sorts, examining the added complexity of design details, along with structural considerations, environmental impacts, and cost control.
Design detailing considerations
When exterior ci is added to a wall assembly, design details can become more complicated because of the sometimes competing or conflicting design requirements of fire safety, insulation continuity, aesthetics, installation practicality, cost control, and water management.
While exterior ci functions to minimize or prevent water vapor diffusion condensation in wall assemblies, condensation caused by air leakage (which is beyond this article’s scope) is a greater threat to durability, and even greater is leakage of rainwater into walls. Preventing rain’s intrusion and accumulation of moisture in walls is critically important—perhaps even more so in exterior foam plastic ci wall assemblies—because the ci component can limit drying of the assembly.
Design details that deflect rain and prevent water entry into walls are the best preventative approach. For example, the need for diverter flashing is critical at roof/wall intersections since the insulation and cladding project well beyond the roof intersection (Figure 1). The omission of this diverter flashing (and other flashing components) was one of many construction defects that caused water intrusion damage to wood-frame homes clad with exterior insulation and finish system (EIFS), stucco, wood, and other wall claddings over a decade ago in North Carolina.
Establishing a drain plane in the assembly and how and where flashing integrates with the drain plane, particularly with mixed claddings, requires thoughtful detailing. Figure 2 shows flashing integrated in the typical location over sheathing and behind the insulation for the EIFS assembly and similarly for the masonry wall.
While this has the advantage of keeping the air barrier/water-resistive barrier (WRB) in the same plane (and facilitating its detailing), the EIFS has its drain plane behind the insulation and the masonry has its cavity outbound of the insulation. The two drain planes/cavities are out of plane; water from one should not be directed into the other. Thus, the EIFS assembly is ‘flashed’ to the exterior above the masonry wall to redirect any incidental water that gets behind the EIFS (from a crack or other breach) to the exterior.
The situation could be worse if the masonry wall were above the EIFS cladding. EIFS has historically been proven as an effective barrier against water infiltration. On the other hand, masonry walls have been described as “reservoir” claddings where substantial amounts of water are expected to get into and through the masonry veneer, resulting in elevated moisture levels in the cavity (Figure 2). Water in the masonry wall cavity should not be directed into the drainage plane behind the EIFS cladding. Once again, ‘flashing’ to the exterior above the dissimilar cladding would be appropriate.
Perhaps the most critical detail of any wall assembly is the window/opaque wall interface detailing, since this is the location where multiple components produced by multiple manufacturers (and installed by multiple trades) typically come together. Additionally, it is often a repeated condition that is distributed over the area of building elevations, so the effects of any errors here can easily be compounded.
Issues of sequencing of trades, compatibility of adjoining materials, and window, sealant, and cladding termination detailing are all in play at this location. Further, it is the critical location evaluated in National Fire Protection Association (NFPA) 285, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Non-load-bearing Wall Assemblies Containing Combustible Components. Two assemblies are shown in Figure 3, both having met the NFPA 285 acceptance criteria. In Figure 3A, the insulation was placed inbound of the sheathing. This has the advantage of ‘normalizing’ construction sequencing since the sheathing with air barrier/WRB, then window installation, and then cavity component and cladding installation mimic what would typically occur without ci.
This also eliminates ‘blind’ fastening, which can occur when the insulation is installed over the sheathing without marking stud lines. The only trade-off here is the rough opening has to be oversized to accommodate the sheathing return into it. The return of the gypsum sheathing into the opening in tandem with the fire-protective stucco cladding on the face of the wall protects the foam insulation from fire exposure.
Another important part of this detail related to fire safety is the requirement for a 15-minute thermal barrier (typically 13-mm [1⁄2-in.] drywall on the interior side of the wall separating the foam plastic from the interior space). In Figure 3B, a masonry veneer cavity wall, the insulation is placed in the more typical location outbound of the sheathing. In this case, the cavity is blocked with fire-retardant-treated (FRT) solid wood to protect the cavity and combustible ci.
Once detailing is fully resolved on paper, mockup construction and testing is a good idea to verify waterproofing integrity of the window/opaque wall interface detailing. The materials, methods, and planned sequencing should be used in the mockup to verify air leakage resistance, as well as water penetration resistance (or control of water penetration with flashing), followed by structural loading to verify performance with appropriate safety factors in relation to design wind pressures. A sample test protocol is shown in Figure 4. The sequence of progressively more severe testing can provide insight into corrective measures needed for construction detailing and where components may need to be reinforced, reintegrated, or re-evaluated in the wall assembly design.
One other important detail area to consider when using foam plastic in wall assemblies is at grade. In Chapter 26, Figure 2603.8, the International Building Code (IBC) restricts foam plastic to use above grade in areas of the country prone to termite infestation. However, there is a conflict since significant energy loss can occur through the foundation wall or slab edge when left uninsulated. Some exceptions are permitted in the code to allow ‘work-arounds.’ For example, if the building structure is entirely made up of noncombustible materials such as concrete or masonry, or preservative-treated wood, or if an “approved method of protection” (i.e. termite-shield) is provided, the foundation wall or slab edge can be insulated. Alternatively, the interior side of basement walls can be insulated.
Continuous insulation is defined in American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, as:
Insulation that is continuous across all structural members without thermal bridges other than fasteners and service openings. It is installed on the interior or exterior or is integral to any opaque surface of the building envelope.
With this in mind, different claddings will have different structural considerations, depending on ci thickness, cladding type, and the degree of energy efficiency that must be achieved to comply with building code and project requirements. In masonry wall design, brick shelf angles and lintels interfere with ci continuity which compromises energy efficiency. A recent study by Morrison-Hershfield indicates a 57 percent loss in R-value (effective R-value of 8.7 versus a nominal R-value of 20.3) caused by thermal bridging in a masonry wall assembly with substantial R-value loss attributable to a slab edge/shelf angle detail.
A more thermally efficient detail that uses intermittent brackets to hold the shelf angle out beyond the floor line to enable ci continuity improves the effective R-value from 8.7 to 9.4—this is an eight percent improvement in efficiency, but still a 54 percent loss compared to nominal R-value, despite substantially more complex and costly construction detailing. In this case, a decision has to be made whether to sacrifice energy efficiency and accept the energy loss caused by thermal bridging, or to construct a more complex and costly set of design details with improved continuity of the ci but only slight gains in energy efficiency. Alternatively, other means of mitigating thermal bridging can be explored and modeled for improvements.
Continuity of ci is more effectively achieved beneath stucco and other wall claddings attached with fasteners through ci, however, the dead load imposed by the cladding and ci on cantilevered fasteners creates a special condition that, until recently, was not fully addressed in building codes, particularly for thick (i.e. greater than 25-mm [1-in.]) insulation).
Two research projects, one sponsored by New York State Energy Research and Development Authority (NYSERDA) and another by the Foam Sheathing Coalition (FSC)—now the Foam Sheathing Committee of the American Chemistry Council (ACC)—form the basis of prescriptive attachment schedules for claddings over ci, based on cladding weight for steel framing in the 2015 IBC (Figure 5).
While many wall assemblies are covered by the prescriptive tables in these codes, those that do not conform to the tables require special analysis or testing to verify adequacy of fastener shear strength in relation to dead loads, and to examine the effects of long-term creep on the cladding and its attachment. Such analysis and testing was done with stucco cladding for the assembly shown in Figure 6 with 25- and 51-mm (1- and 2-in.) insulation. This assembly differs from a ‘standard’ stucco assembly with exterior ci since it incorporates a drainage mat directly behind the stucco, which adds to the cantilevered length of fastener, and potentially sets up the stucco’s rotational movement (where it is no longer restricted by a supporting solid substrate of sheathing).
Testing of the assembly was accomplished by applying loads parallel to the face of the stucco until there was a loss of load, indicating failure within the assembly. Although quite high ultimate loads were achieved (i.e. 1165 N [262 lbf] per fastener with 51-mm [2-in.] insulation), the load capacity at 0.38-mm (1⁄64-in.) deflection was the important information in the test.
NYSERDA work identified 0.38-mm maximum short-term deflection as its basis for design values and prediction of connection performance. Thus, to the extent the results of the ci stucco assembly tests mirrored the load capacity predictions of the NYSERDA work, and to the extent applied loads (above and beyond the dead load of the cladding) could be held at the deflection limit, the test validated the wall design.
The results of the testing showed the fasteners and fastening schedule were adequate to resist the dead load of the cladding, particularly for the 25-mm insulation assembly, where measured load capacity was 95 percent of predicted values, and an applied load of 387 N (87 lbf) was held at the deflection limit. For the 51-mm insulation assembly, measured load capacity was 53 percent of predicted values, while an applied load of 120 N (27 lbf) was held at the deflection limit. Taking the applied load plus dead load of the cladding, actual loads at the deflection limit were then calculated (Figure 7).
Long-term creep effects, as evaluated under the test conditions, were also within allowable limits. In fact, for two different panels creep was a maximum of 0.15 mm (0.006 in.), and seemed to be influenced more by relative humidity (RH) conditions in the test environment than the dead load of the cladding over a 45-day period.
Similar creep tests have also been performed in the NYSERDA work and more recently in a study by Building Science Corporation for the U.S. Department of Energy (DOE) Build America program. These studies influenced the selection of the stringent short-term load deflection limit of 0.38 mm (0.015 in.) as a means to control the uncertainty of long-term creep effects which, in real-world conditions, may vary with factors—such as elevated temperatures, thermal cycling, and moisture cycling—that cannot always be simulated in lab tests.
Resistance to wind is another important structural consideration (as is seismic loading, but that is beyond this article’s scope). Once again, national building codes have not historically provided comprehensive prescriptive attachment schedules for ci-based wall assemblies. However, reference is made in the 2012 and previous editions of the IBC to American Society of Civil Engineers (ASCE) 7, Minimum Design Loads for Buildings and Other Structures, as a basis for wind load.
In turn, ASCE 7 establishes fastener embedment into wood or steel studs as the basis for adequacy of cladding attachment (assuming the cladding itself is adequate to resist loads). Ultimately, pull-out or withdrawal capacity of the fastener from framing is used to determine required fastener spacing in relation to design wind pressures. Whether ci is in the assembly is irrelevant, since the length of fastener can be specified to maintain the required engagement or embedment of the fastener into structural members.
Thus, building codes have historically provided some direction for the verification of ci-based wall assembly resistance to wind loading. The 2015 IBC references American National Standards Institute/Structural Building Components Association (ANSI/SBCA) FS 100, Standard Requirements for Wind Pressure Resistance of Foam Plastic Insulating Sheathing Used in Exterior Wall Covering Assemblies. This standard includes a prescriptive table of design wind pressure loads based on design wind speed and exposure, and also provides direction for testing. Once again, ASTM E330 is listed as one test method, and ASTM E1233, Standard Test Method for Structural Performance of Exterior Windows, Doors, Skylights, and Curtain Walls by Cyclic Air Pressure Differential, as an alternate.
Where engineering analysis or verification through testing is needed, it can generally be accomplished at a reasonable cost, but time, planning, and foresight are needed to fit into the pre-construction schedule. As mentioned earlier, construction of mockup assemblies for a project can also be useful for constructability and construction practice verification, as well as establishing a quality control (QC) benchmark for installation of exterior wall coverings and interface detailing.
The current common types of rigid foam plastic continuous insulation used in exterior wall assemblies contain zero ozone-depleting chemicals and can be recycled through reuse. While made of petroleum-based products, they have low environmental impact. The impacts of raw material extraction, transport, manufacturing, and installation are minor compared to the savings gained in building heating and cooling energy, provided one takes into account the building lifecycle.
In a study done by Franklin Associates on several wall assemblies, exterior ci almost halved building heating and cooling energy consumption over the 50-year lifecycle, and greenhouse gas (GHG) emissions were substantially reduced (Figure 8).
An ongoing area of controversy is the use of flame retardants, specifically hexabromocyclododecane (HBCD), which is currently used in most expanded and extruded polystyrene (EPS and XPS) insulations. HBCD is on the U.S. Environmental Protection Agency’s (EPA’s) list of “chemicals of concern.” Polyisocyanurate (polyiso) insulation board manufacturers have converted to Tris (2-Chloro-1-Methylethyl) Phosphate (TCPP), which, based on current information, is “not considered to be toxic or bio-accumulative, and its environmental persistence is considered to be lower than other common foam plastic flame retardants.”
In all likelihood, HBCD will be replaced with a less environmentally controversial flame retardant in EPS and XPS. In fact, at least one XPS manufacturer has already introduced an alternative, though it is not yet commercially produced in North America.
As building codes have evolved to the point where continuous insulation is now mandatory for many wall assemblies, rigid foam plastic ci wall assemblies have become more prevalent than in the past. They have special design considerations, including fire safety, moisture-related durability, added design detail complexity, and cost control. Each category can be addressed at the design stage with an awareness of what the building code requires in relation to the use of foam plastics and their effects on the physics of the wall construction, as well as design details and the environment.
In some cases, engineering analysis of fire safety or structural adequacy is needed where prescriptive requirements or other direction does not exist in building codes. In other cases, testing is needed to verify performance. International Code Council Evaluation Service (ICC-ES) Evaluation Reports can be a valuable tool to assist in verifying code compliance and limits of materials and/or wall assembly performance.
Tom Remmele, CSI, is the director technical services/R&D for Sto Corp., a manufacturer of air barriers, coatings, exterior insulation and finish systems (EIFS), and stucco products. He has held technical management positions in the construction industry for more than 25 years. Remmele is a past Technical Committee chair of the EIFS Industry Members Association (EIMA). He can be reached at firstname.lastname@example.org.
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