Combination of Different InsulationTechnologies to Enhance Performance

of Exterior Wall Assemblies

Jean-François Côté, PhDSoprema, Inc.

1688 J-B Michaud St., Drummondville, QC J2C 8E9 Phone: 819-478-8166 • E-mail: jfcote@soprema.ca

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Abstract

The author will discuss modeled thermal performance of 13 types of exterior wall assemblies for nonresidential construction. The theoretical relationship between U-value and amount of insulation was determined to validate acceptable assemblies for climate zones 4 to 7. Nontraditional assemblies using continuous exterior insulation, coupled with a dual-composition cavity insulation mixture as the thermal control layer, were predicted to perform well. U-values measured experimentally (following ASTM C1363) were compared to those predicted by thermal modeling. Hygrothermal simulations were also performed to allow evaluation of the impact of the choice and positioning of the various control layers on the thermal and moisture management performance.

Speaker

Jean-François Côté, PhD — Soprema, Inc.

JEAN-FRANÇOIS CÔTÉ holds a PhD in materials science from INRS-Université du Québec, obtained in 1998. In his current role, he represents Soprema on technical committees of industry associations (ARMA, PIMA, SPRI) and is actively engaged in various North American standards development organizations. He is chair of the CSA A123 technical committee on Bituminous Roofing Materials, and is co-chair of the ASTM D08.04 subcommittee on Felts, Fabrics, and Bituminous Sheet Materials.

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Combination of Different InsulationTechnologies to Enhance Performance

of Exterior Wall Assemblies

BACKGROUND At the RCI, Inc. 31st International

Convention (Côté et al., 2016), we presented a study that demonstrated that the use of continuous insulation in exterior wall assemblies has significant impact on mois ­ture management and the drying/wetting potential of these assemblies. Claddings that could provide conditions for inward vapor drive were also identified as major contributors and moved the permeance level of the water-resistive barrier (WRB) or permeable air barrier to a minor or even insignificant role. In some extreme cases, evidence was provided that increasing WRB permeance above 20 U.S. perms can increase the risk of moisture-related problems in the assemblies.

This previous study covered assemblies with R-19 batt insulation in the stud cavity, with and with­out continuous insulation (CI). When CI was used, vapor-permeable and vapor-impermeable insulation mate ­rials were evaluated at levels of R-6.5 and R-18. Many of these assemblies would not be compliant with the thermal performance requirements for one or more of the Canadian cli­mate zones. Consequently, the pres ­ent study was undertaken to predict the required amount of insulation (both cavity and CI) in climate zones 4 to 8 for specific wall assemblies. These assemblies were then verified for moisture management based on hygrothermal simulations.

THERMAL SIMUL AT IONS Description of the Assemblies

In the current study, a total of 14 types of exterior wall assemblies were modeled for thermal perfor­mance following the same guidelines and principles that led to the publi­cation of the BETB Guide (BC Hydro, 2016). Of those, nine assemblies with 2- x 6-ft. steel studs, spaced 16 in. (406 mm) o.c., were selected as a

base case for the various modeled scenar­ios. Exterior-insulated assemblies (Series A) were investigated with nominal R-value of CI reaching R-35 or greater. The specific type of material used as CI has not been taken into consideration and was kept generic. For a given R-value, only the thick­ness of the insulation would be impacted by the selected type. For example, R-10 can be reached by any insulation type, but mineral wool thickness required to obtain R-10 will be greater than if polyisocyanurate insula­tion is used. Although large CI thicknesses may pose challenges for some assemblies (in other aspects than thermal perfor-

Figure 1 – Legend and schematic view of nine basic assemblies.

mance), the impact of insulation thickness on thermal performance was considered negligible in this study.

For many scenarios, it made sense to compare thermal performance of the exteri­or-insulated assemblies with split-insulated equivalents. Base case assemblies were also modeled after the addition of R-20 cavity insulation (Series B). Again, the type of cav­ity insulation was not pertinent to the study and was kept generic.

In order to evaluate the influence of thermal bridging on the overall per­formance of the assembly, three types of cladding attachment were evaluated:

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Figure 2 – Thermal performance of exterior-insulated assemblies (Series A).

1) vertical z-girts aligned with studs (poor), 2) vertical aluminum rails aligned with studs and connected using aluminum clips at 24 in. (610 mm) o.c. vertical spacing (good), and 3) standard face-mounted steel brick ties 16 in. (406 mm) o.c. horizontal and vertical spacing (better).

It is worth noting that the parameters above may be considered as worst-case scenarios. Using wood studs instead of steel studs, increasing stud spacing to 24 in. (610 mm) o.c., using horizontal z-girts as opposed to a vertical configuration, and substituting the aluminum rail and clip system with a stainless steel or other more thermally performing version, would all be improvements over the base case. The objective of this study wasn’t to sponsor a particular type of material or cladding attachment, but rather to establish the rela­tionship between the level of thermal bridg­ing of an assembly and its overall thermal performance. Figure 1 shows drawings of nine basic assemblies that were part of the current study.

Although drawings presented in Figure 1 may exhibit a particular cladding materi­al, the cladding itself has not been included in the thermal performance evaluation of these assemblies. Exterior-insulated assem­blies (A1, A2, and A3) differ only by the method used for the attachment of the clad­ding described above. The same can be said about split-insulated assemblies with R-20 cavity insulation (B1, B2, and B3).

Assemblies identified as C1, C2, and

C3 also are split-insulated assemblies, but their cavity is insulated with a combination of 2-in. (50-mm) spray polyurethane foam (SPF) and blown cellulose insulation. This combination brings the nominal R-value in the cavity to R-24. This combination of mate ­rials is very similar to the one referred to as “Hybrid Wall 3” (Grin and Lstiburek, 2014). This combination was chosen because Grin and Lstiburek depicted it as “the optimal in terms of cost, thermal, hygrothermal, and structural analysis.”

The overall thermal performance of each

assembly was determined by considering all structural components, CI, cavity insu­lation, and air films. The results were expressed as the effective R-value and were determined for various amounts of CI in each assembly.

Thermal Modeling Results The thermal modeling in the current

study was performed using the Nx software package from Siemens, a general-purpose computer-aided design and finite-element analysis (FEA) software package. The ther­mal solver and modeling procedures were extensively calibrated and validated (Roppel and Lawton, 2011). The analysis contained in the current study follows the same meth­odology and is an extension of the catalog (BC Hydro, 2016).

A steady-state conduction model was used. Air cavities were assumed to have an effective thermal conductivity that includes the effects of cavity convection. Interior/ exterior air films were taken from ASHRAE (2009), depending on surface orientation. From the calibration (Roppel and Lawton, 2011), contact resistances between materi­als were modeled. The temperature dif­ference between interior and exterior was modeled as a dimensionless temperature index between 0 and 1.

For every exterior wall assembly evalu­ated in the current study, thermal modeling was performed to obtain the assembly’s overall thermal transmittance (also referred

Figure 3 – Thermal performance of split-insulated assemblies with R-20 cavity insulation (Series B).

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to as “U-value”). This transmittance result was then converted to customary “effec ­tive R-value,” expressed in imperial units of hr∙ft2∙̊ F/BTU, representing the weighted average thermal resistance—including all thermal bridges but excluding any penetra­tion (such as door, window, pipe) and con­nection to slabs—of the assembly.

Figure 2 shows the relationship between the Effective R-value of the exterior-insu­lated assemblies (A1, A2, and A3) and the amount of CI used, expressed as the nomi­nal R-value for CI.

As expected, assembly A1 is the worst performer of all with an effective R-value maintained under R-15, even when R-40 nominal CI is used. One could argue that positioning the insulation between verti­cal z-girts is not “continuous insulation” in the pure definition of this term. The result of assembly A1 provides evidence that in order to be effective, CI has to really be continuous. Another observation for A1 is that as the amount of CI is increased, the curve flattens, and incremental benefit is gradually reduced. Moving to assembly A2 provides a significant improvement of effec ­tive R-value. The only modification between A1 and A2 is the change of cladding attach­ment method from vertical z-girts to alu­minum rail and clips. This change brings considerably less thermal bridging in the assembly and allows the effective R-value to rise much faster. Further improvement of thermal performance is achieved by reduc ­ing even more the level of thermal bridging from assembly A2 to A3, where brick ties are used for cladding attachment. In assem­bly A3, the relationship between effective R-value and nominal CI is much closer to linear, indicating that incremental use of CI still provides the same additional perfor­mance, unlike assembly A1.

Figure 3 shows the same relationship as above but for split-insulated assemblies with R-20 cavity insulation (B1, B2, and B3).

The addition of R-20 cavity insulation (while keeping stud dimensions and spac­ing constant) has systematically increased the effective R-value of all assemblies, as one would expect. Again, the assembly with high levels of thermal bridging (B1) remains poor, with effective R-value only reaching 20 when CI is used at a level of R-35 or greater. Assemblies B2 and B3 are very similar—the latter being slightly better performing and showing a greater slope.

The impact of the addition of R-20 (nominal) insulation in the cavity has been evaluated for all assemblies. Depending on the assembly type, the contribution to the effective R-value of this addition ranges between R-6 and R-8.

Figure 4 shows an example of the dif­ference in effective R-value provided by the addition of R-20 cavity insulation in otherwise identical exterior-insulated and split-insulated assemblies (B3 versus A3). In these cases, the difference in effective R-value provided by the addition of R-20 cavity insulation is R-7.3 on average.

Figure 4 – Impact of R-20 cavity insulation on the thermal performance of assemblies.

Figure 5 – Thermal performance of innovative exterior-insulated assemblies (Series C).

Further increasing the performance of cavity insulation to an R-24 level without any change in cavity dimensions, through the use of an innovative combination of insulation materials (Hybrid Wall 3 from Grin and Lstiburek) has the impact shown in Figure 5.

Unsurprisingly, assemblies C1, C2, and C3 exhibit an increase in effective R-value when compared to their respective counter­parts B1, B2, and B3, respectively.

Figures 2 to 5 present the evolution of the effective R-value as a function of exterior insulation (CI) R-value. Using fewer

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Figure 6 – Impact of cavity insulation on thermal performance of assemblies.

data points, F ig ure 6 shows the impact of nominal cavity insulation R-value on the assembly’s effective R-value. For simplicity, the data points using R-20 CI were chosen for the graph. The level of thermal bridging provided by different cladding attachments is responsible for the respective positioning of the curves, but the three series of data exhibit a similar slope.

Incremental Thermal Efficiency We have seen from the previous exam­

ples that the addition of insulation (whether it is added in the cavity or as CI) to an already-insulated assembly has a differ­ent impact on the effective R-value. In that respect, the slope of the curves in graphs of effective R-value (REFF) as a function of nominal R-value (RNOM) can be further examined.

The slopes can be defined as the incre ­mental thermal efficiency index (ITEI) as follows:

The ITEI will be a fractional number between 0 and 1 that can be used to rate the thermal efficiency of an insulation con­figuration. In other words, ITEI will provide a numerical assessment of thermal bridg­ing of an insulation configuration in an assembly. Assemblies where insulation is

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discontinuous and in the presence of sig­nificant thermal bridging will exhibit lower ITEI numbers, whereas better-performing assemblies will obtain higher ITEI. Tabl e 1 provides ITEI values for the base assemblies of the current study.

A distinct ITEI is obtained for the exte ­rior insulation (CI) and cavity insulation configuration. For assemblies with vertical z-girts (significant thermal bridging), it is not surprising to see the lowest ITEI from this study at 0.18 for CI, irrespective of the amount of cavity insulation (a virtually iden­tical exterior insulation ITEI is obtained for A1, B1, and C1). For assemblies with reduced impact of thermal bridges (aluminum rail and clips, brick ties), exterior insulation ITEI numbers are higher.

Code Compliance of Basic Assemblies

Effective R-values obtained from the nine basic assemblies in this study were compared to the require ­ments found in the 2015 edition of the National Energy Code for Buildings for Canada (NECB). Tabl e 2 indicates for each basic assembly the amount of CI required to meet the requirements of the code. All assemblies with required CI greater than R-30 for a given zone are “practically” not compliant, as thickness of CI would be in excess of 5 in. (127 mm) to meet the requirement.

HYGR O T H E R M A L S I M U L AT ION S In order to confirm whether or not the

code-compliant basic assemblies are viable from the standpoint of moisture manage­ment, hygrothermal simulations were per­formed on a subset of the basic assem­blies. Exterior-insulated assemblies were not modeled, as these pose virtually no risk of degradation or mold growth due to poor moisture management.

Parameters for hygrothermal simula­tions are identical to those presented in 2016 (Côté, 2016). Results presented here are steady-state numbers, obtained when consecutive years in the simulation pro­vided the same results throughout a 365-day period.

Assuming that the CI is the lowest per­meance material of the assembly (therefore that CI will dictate the wetting and drying behavior of the assembly), F ig ure 7 shows the steady-state yearly evolution of the moisture content of the sheathing behind CI for the B3 assembly, including brick clad­ding. Relative humidity never exceeds 90% in both scenarios. In addition, there is a low moisture content of sheathing, confirming a low risk of moisture accumulation. We also note that, although these simulations are realized in two different climate zones, the presence of an appropriate amount of CI maintains temperature at the exterior side of batt insulation approximately at the same level in both scenarios.

This is not true, however, in all sce­narios. For example, if CI is required above R-20, vapor-permeable CI will be required

Assembly

Incremental Thermal Efficiency Index

Exterior Insulation (CI)

Cavity Insulation

A1 0.18

0.31 B1 0.18

C1 0.19

A2 0.51

0.39 B2 0.49

C2 0.48

A3 0.63

0.42 B3 0.62

C3 0.78

Table 1 – Incremental Thermal Efficiency Index (ITEI) for all basic assemblies.

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to reduce the risk of moisture-related issues in the assembly.

CONCLUSION Innovative (hybrid) split-insulated wall

assemblies have shown to provide improve ­ments in thermal performance. However, the impact of the use of a combination of cellulose and SPF in the cavity with CI cannot compensate for the use of a poor-performing cladding attachment method (caused by major levels of thermal bridg­ing). Moisture management in such sys ­tems can easily be managed, as long as the understanding of the dynamics of wetting and drying of these assemblies (through all materials of these assemblies—not only the air barrier or WRB) is taken into con­sideration.

Assembly A1 A2 A3

Zone 4: R-18.1 Required CI All zones > R-30 Zone 4: R-23.3 Zone 5: R-22.4 for climate Zone 5: R-28.2 Zone 6: R-26.7 zones 4 to 8 Other zones > R-30 Z7 & Z8 > R-30

Assembly B1 B2 B3

Zone 4: R-8.2 Zone 4: R-7.1 Required CI Zone 4: R-22 Zone 5: R-12.6 Zone 5: R-10.4 for climate Other zones > R-30 Zone 6: R-17.2 Zone 6: R-14.5

zones 4 to 8 Zone 7: R-26.2 Zone 7: R-21.3 Zone 8: > R-30 Zone 8: R-27.9

Assembly C1 C2 C3

Zone 4: R-8.1 Zone 4: R-5.9 Required CI Zone 4: R-18.7 Zone 5: R-11.4 Zone 5: R-8.2 for climate Other zones > R-30 Zone 6: R-15.6 Zone 6: R-11.2

zones 4 to 8 Zone 7: R-23.1 Zone 7: R-16.5 Zone 8: > R-30 Zone 8: R-22.0

Table 2 – Amount of CI required on basic assemblies for compliance with NECB REFERENCES 2015 prescriptive U-Value requirements.

ASHR A E Handbook – Fundamentals, American Society of Heating, Refr igeration and Air-conditioning Engineers, Inc.. Table 1, p. 26.1.2009.

BC Hydro. Building Envelope Thermal Bridging Guide, Version 1.1. Availableonline at http://www. bchydro.com/constr uc ­tion. 2016.

J-F. Côté et al. “Condensation in Wall Assemblies: Can Vapor Diffusion Through Highly Permeable Air Barriers Increase the Risk?” Proceedings of the RCI Inc. 31st International Convention & Trade Show. 2016. the 14th Canadian Conference on Research Project 1365-RP, Thermal

Grin and J. Lstiburek. “Structural and Building Science and Technology. pp. Performance of Building Envelope Hygrothermal Analysis of Hybrid 267-275. 2014. Details for Mid- and High-Rise Wall Systems,” Proceedings of P. Roppel and M. Lawton. ASHRAE Construction. 2011.

Figure 7 – Hygrothermal simulations of assembly B3 for climate zones 4 and 6.

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