April 14, 2014

Abstract: 

Changes in the International Energy Conservation Code (IECC) from 2009 to 2012 have resulted in an increase in minimum insulation levels required for residential building. Not only are the levels increased, but the use of exterior rigid insulation has become part of the prescriptive code requirements. With more jurisdictions adopting the 2012 IECC builders are going to find themselves required to incorporate exterior insulation in the construction of their exterior wall assemblies. This research is an extension on the previous research that has provided significant insight into the mechanics as well as long term performance of exposed assemblies that use wood furring strips attached through the insulation back to the structure to provide a cladding attachment location.

Executive Summary

Changes in the International Energy Conservation Code (IECC) from 2009 to 2012 have resulted in an increase in minimum insulation levels required for residential buildings. Not only are the levels increased, but the use of exterior rigid insulation has become part of the prescriptive code requirements. With more jurisdictions adopting the 2012 IECC, builders will be required to incorporate exterior insulation in the construction of their exterior wall assemblies.

For thick layers of exterior insulation (levels greater than 1 ½ in.), many contractors and designers use wood furring strips attached through the insulation back to the structure as a means to provide a convenient cladding attachment location (Straube and Smegal 2009; Pettit 2009; Joyce 2009; Ueno 2010). However, there has been a significant resistance to its widespread implementation due to a lack of research and understanding of the mechanisms involved in the development of the vertical displacement resistance capacity. In addition, the long-term inservice performance of the system has been questioned due to potential creep effects of the assembly under the sustained dead load of the cladding and effects of varying environmental conditions.

Previous research has provided significant insight into the mechanics as well as long-term performance of exposed assemblies that use wood furring strips attached through the insulation back to the structure to provide a cladding attachment location. However, several key research questions still remain:

  • What are the impacts of different fastener types in the system capacity?
  • What is the impact of screw shaft bearing on the insulation material?
  • What are the impacts of material expansion and contraction on the pre- and postcompression forces in the assemblies?
  • Can deflection movement for heavier weight claddings be mitigated by denser fastener
    spacing?

This research was an extension of previous research conducted by BSC in 2011, and 2012 (Baker 2013a; Baker and Lepage 2014). Each year the understanding of the system discrete load component interactions, as well as impacts of environmental loading, has increased.

From the research, it was determined that using larger fasteners can increase the system capacity; however, simple cantilever bending tests significantly underestimate the actual capacity of the screw in the system. With the inclusion of the wood furring strip as the screw shaft bearing on the furring strips results in a double bending action. The portion of vertical load resistance capacity provided by the screw fasteners in double bending was determined to be around 4 lbf per fastener based on a standard #10 wood screw and 7 lbf per fastener based on a Headlok screw at 1/16 in. deflection (based on a 4 in. spacing between the oriented strand board sheathing and the furring).

The impact of the screw shaft bearing on the insulation is not insignificant, though it is difficult to accurately quantify. Measurements indicated a doubling of capacity based on a simple cantilever test, assuming only the base layer of insulation is rigidly attached to the structure and up to 8 times the capacity if both layers of insulation are rigidly attached.

Thermal expansion and contraction of materials can have a significant effect on the compression forces in the assembly. Measured results indicated a 100 lbf change over a temperature range of 130°F to –30°F. The changes in compression forces will impact the friction resistance component of the assemblies and may also play a role in the resistance provided by the compression strut. These fluctuations are also theorized to be the cause of the diurnal movements measured in the long-term exterior exposure testing (Baker and Lepage 2014).

There is a direct correlation between the number of fasteners used in the assembly and the system capacity. Dividing the total load resistance by the number of fasteners for each of the three tests yielded almost identical load versus deflection plots. This is important from a design perspective, as the results would indicate that the system capacity can be modified by increasing or decreasing the number of fasteners used in the assembly.

The long-term exterior exposure testing provided significant insight into the actual in service performance of the cladding attachment systems. Cladding weight resulting in 30 lbf per fastener load was too great for the assembly, and unacceptable creep of the system was clearly observed. By contrast limiting the cladding weight to 8lbf per fastener demonstrated very stable performance. The assemblies loaded to 15 lbf per fastener showed pretty stable performance as well, however, there may be a slight indication of system creep occurring with these assemblies. Based on current information to date, it is recommended to use a maximum load per fastener of no more than 10 lbf based on a standard #10 wood screw installed through up to 4 in. of insulation (Table 1). Higher capacities would be expected with larger screws or reduced insulation thickness.

Table 1. Recommended vertical fastner spaciing (minimum #10 wood screw) based on cladding

Cladding weight
(psf)
16 in. o.c.
furring
24 in. o.c.
furring
51812
1096
1564
2043
2532

1 Problem Statement

1.1 Introduction

Energy consumption reduction is increasing in importance in our society. The building industry is reacting by focusing on designing and building lower energy use buildings. The trend has been reinforced in building codes. Changes in the International Energy Conservation Code (IECC) from 2009 to 2012 have resulted in an increase in minimum insulation levels required for residential buildings. Not only are the levels increased, but the use of exterior rigid insulation has become part of the prescriptive code requirements. With more jurisdictions adopting the 2012 IECC, builders will be required to incorporate exterior insulation in the construction of their exterior wall assemblies. This is not surprising, as the addition of insulation to the exterior of buildings is an effective means of increasing the thermal resistance of both wood-framed walls as well as mass masonry wall assemblies. The location of the insulation to the exterior of the structure has many direct benefits, including: (1) higher effective R-value from reduced thermal bridging; (2) higher condensation resistance; (3) reduced thermal stress on the structure; as well as other (4) commonly associated improvements such as increased air tightness; and (5) improved water management (Hutcheon 1964; Lstiburek 2007).

The current prescriptive thermal resistance values for exterior rigid insulation required on the exterior walls as outlined in Table R402.1.1 of the 2012 IECC can be achieved for most climate zones without significant changes to current building practices, as the levels typically will require less than 1½ in. of insulation (IECC 2012). For insulation up to 1½ in. in thickness, direct attachment of cladding assemblies through the insulation back to the structure is a practical construction technique and one that is currently address in Table R703.4 of the International Residential Code (IRC 2012). Beyond 1 ½ in. of thickness, alternate means for cladding attachment are generally required due to current market availability of fastener lengths for cladding nail guns. This has created a problem for projects that are looking to exceed this 1½ in. practical limit. For thick layers of exterior insulation (levels greater than 1½ in.), many contractors and designers use wood furring strips attached through the insulation back to the structure as a means to provide a convenient cladding attachment location (Straube and Smegal 2009; Pettit 2009; Joyce 2009, Ueno 2010).

The technique is particularly well suited to retrofit projects that might otherwise be limited (in terms of space conditioning energy use reductions) due to existing construction dimensional constraints. This fits directly into the Building America goals of substantial reductions in energy consumption. While the energy benefits are apparent and easy to understand, the practical implementation has run into barriers that have slowed widespread adoption.

There is significant resistance to its widespread implementation due to a lack of research and understanding of the mechanisms involved in the development of the vertical displacement resistance capacity. In addition, the long-term in-service performance of the system has been questioned due to potential creep effects of the assembly under the sustained dead load of the cladding and effects of varying environmental conditions.

Previous research has provided significant insight into the mechanics as well as long-term performance of exposed assemblies that use wood furring strips attached through the insulation as a cladding attachment location (Baker 2013a; Baker and Lepage 2014). However, several key research questions still remain:

  • What are the impacts of different fastener types in the system capacity?
  • What is the impact of screw shaft bearing on the insulation material?
  • What are the impacts of material expansion and contraction on the pre and post compression forces in the assemblies?
  • Can deflection movement for heavier weight claddings be mitigated by denser fastener spacing?

The research completed was aimed at addressing these questions with the ultimate goal to help further the understanding of the mechanics involved. It was understood by the research team that detailed development of a full matrix of recommendations or specific design methodology was not going to be possible given the number of possible factors and testing that would have been involved. The primary intent was to focus on examining the relative magnitude of the discrete load components in the system to help further the general understanding and provide some preliminary guidance. Further refinements to the implementation of this strategy will be possible with additional testing and research.

1.2 Cost-Effectiveness

A preliminary evaluation was completed looking at the incremental cost of the varying thickness of insulation installed to the exterior of the wall assemblies. This preliminary cost analysis used foil-faced polyisocyanurate (PIC) as the baseline exterior insulation. Cost data for the exterior insulation were taken from RS Means Construction Data (2011 Reed Construction Data). Costs included in the analysis were the installed cost of the insulation material, 1 × 3 wood furring strips spaced at 16 in. o.c., and wood screws spaced at 24 in. o.c. vertically for the attachment of the furring back to the structure. A cost markup of $100 per window in the reference model was used as an estimate of the additional cost for trim extensions that would be needed to account for the additional thickness of foam added to the exterior of the home. This value is an estimate, as actual costs can be highly variable due to the many different design choices available for window placement, exterior window trim design, and attachment.

Other items such as house wrap or sheathing tape, self-adhered membrane flashings, metal flashings, siding, and siding fasteners were omitted from the analysis, as these items are associated with recladding and water management, and would be part of the retrofit project regardless of the addition of exterior insulation.

Simulations were run using BEopt simulation software developed by the National Renewable Energy Laboratory. An example home was used as the baseline to help demonstrate the benefits of using exterior insulation as part of a house energy retrofit. This benchmark home was assumed to be around 1950s era two-story slab-on-grade construction and had the basic characteristics listed in Table 2.

Table 2. Benchmark House Characteristics

House Characteristicsft2
Finished floor area2312
Ceiling area1156
Slab area1156
Wall area2799
Window area410 (17.7%
glazing ratio

The wall conductance performance was isolated from all other aspects of the home, to examine the cost effectiveness of this single strategy. The analysis was designed to examine the cost of the measure in conjunction with cost reductions due to lower energy use. The analysis combined the present worth of the cost of the measure (financed over a 5-year period at a 7% interest rate) and the cost of energy used (based on a 30-year period and a fuel escalation rate of 2%). A cost-optimized result has the lowest combined present worth of the both the cost of the measure and fuel cost over the period of the analysis. A cost-neutral result is one where the combined present worth of the measure and the fuel cost is lower than the present worth of the fuel cost of the benchmark home.

In this analysis, given the assumed age of the home, the benchmark home had an uninsulated wall cavity (as per guidance from the 2011 BA Benchmark Protocol). The benchmark house characteristics are listed in Table 3.

Table 3. Benchmark House Specifications

House CharacteristicSpecification
Ceilinguninsulated, vented (R-2)
Wallsuninsulated 2x4 @ 16” o.c. (R-3.8)
Windowsdouble clear, metal frames (U=0.45. SHGC=0.55)
Infiltration0.88 nACH

Table 4 illustrates the parametric steps that were run in the analysis. The analysis was completed for various climate zones ranging from 3A through 7A (as defined by the 2012 IECC) with the associated reference cities listed in Table 5.

Table 4. Parametric Steps and Cost

Parametric
step
DescriptionCost/ft2
RBenchmark (uninsulated 2x4 wall)N/A
1R-13 cavity fill insulation$2.20
2R-13 cavity fill + 1 in. exterior
insulation (R-6.5)
$3.55
3R-13 cavity fill + 1.5 in. exterior
insulation (R- 9.75)
$3.76
4R-13 cavity fill + 2 in. exterior
insulation (R-13) + 1x4 wood furring
$5.73
5R-13 cavity fill + 2 layers of 1.5 in. exterior
insulation (R-19.5) + 1x4 wood furring
$7.19
6R-13 cavity fill + 2 layers of 2 in. exterior
insulation (R-26) + 1x4 wood furring
$7.58

Table 5. Reference Cities

CityClimate
Zone
Dallas, TX3A
Kansas City, MO4A
Boston, MA5A
Duluth, MN7A

Results of the analysis are provided in Appendix A. Results indicated that for cold climate zones (4 and higher), insulation up to 1 ½ in. (parametric step 3) was shown to be a cost-optimized solution. This resulted mainly due to a jump in the cost of the measure with the addition of wood furring strips and screw fasteners when thicknesses of exterior insulation of 2 in. or more were used. Even with the jump in costs, insulation thickness ranging from 2 in. to 4 in. (parametric steps 4, 5, and 6) were still demonstrated to be cost neutral as part of this simplified analysis in all cities except for Dallas, Texas.

While the analysis run focused on conductance improvements only, there is some argument to be made that the addition of exterior insulation would likely also improve the overall airtightness of the assemblies (Ueno 2010). The benefits from increased airtightness are known to be very important in cold climate construction; however, it is also more difficult to isolate and apportion to individual measures.

1.3 Other Benefits

Using exterior insulation has many additional benefits other than simply increased thermal resistance. The single largest benefit is the increased condensation resistance that this strategy provides for cold climate buildings (Straube and Burnett 2005; Lstiburek 2007). The placement of the insulation to the exterior of the building acts to keep all of the structural elements at a much more even temperature throughout the year, reducing the risk of interstitial condensation. For wood structures, this can significantly reduce the potential for wood decay; an added benefit is that the seasonal thermal and moisture variations of the wood frame are greatly reduced. In masonry building, the potential for freeze thaw is practically eliminated, since this approach not only keeps the masonry warmer, but also address the exterior rainwater absorption into the masonry, which is the leading moisture source related to freeze thaw damage to buildings.

In addition to keeping the structure warm and preventing condensation, the use of the furring strips creates a significant upgrade in water management. The increase in drainage and drying that is provided by the ¾-in. gap created by the furring strips provides much additional protection against water infiltration problems. The use of a drainage gap is a base recommendation for most cladding installations regardless of whether or not exterior insulation is used (Lstiburek 2010). The fact that the furring strips are an intrinsic component of this system provides a significant added benefit to the long-term durability of these wall assemblies.

2 Previous Work

The earliest work that examined a wood-to-wood connection with rigid insulation installed in the joint was conducted by the U.S. Department of Agriculture Forest Products Laboratory (Aune and Patton-Mallory 1986a, 1986b). This researched looked to validate the European Yield Theory for wood-to-wood connections with gaps up to 1 in. The European Yield Theory (first conceived in the 1940s) is based on an equilibrium of forces caused by rotation of fasteners in wood members; this theory predicts performance of the connection at the point where yielding of materials (wood or fastener) has developed. The equations as set out in the American Forest and Paper Association Technical Report 12 General Dowel Equations for Calculating Lateral Connection Values predict performance of a multitude of failure modes, with the governing mode being the one with the lowest yield capacity (AFPA 1999).

The results from the Forest Products Laboratory, while similar in concept, did not provide much useful data when examining the attachment of furring strips over the insulation for cladding attachment purposes. A 1-in. gap may be considered large when looking at wood-to-wood structural connections, but it is small when looking at the application of furring strips installed over insulation where a minimum 2 in. would generally be expected, and thicknesses up to 8 in. or more being possible. Still, these tests being conducted with extruded polystyrene (EPS) insulation, gave way to the idea of possibly adopting the yield equations for the application of wood furring strips over exterior insulation.

Several groups such as the Foam Sheathing Coalition, the New York State Energy Research and Development Authority/Steel Framing Alliance funded research into the vertical load capacity of furring strips, installed over exterior insulation, that are fastened back to a wood or steel structure. The primary goal of the research by the Foam Sheathing Coalition and the New York State Energy Research and Development Authority /Steal Framing Alliance was to develop prescriptive code tables for attaching cladding to framing over continuous insulation (Bowles 2010). The research methodology adopted the European Yield Theory as the basis for the analysis.

For wood frame test specimens, the measured data were compared to the predicted performance of the yield equations as determined by the TR-12 (and calculated based on actual properties of the materials used in the testing). This research concluded that the 5% offset yield prediction as calculated using the TR-12 formulas, resulted in a reasonably accurate prediction of the shear load at a deflection of 0.01 in. While there was no mathematical connection between these values, the research team considered this to be an adequate basis for designing for a 0.01-in. deflection limit given the scope of the research. In addition, a divisor of 1.5 was applied to the calculated results to address potential concerns of assembly creep under sustained loads. The methodology was used to develop prescriptive code tables for attaching furring strips to framing over continuous insulation (Bowles 2010).

In 2011, Building Science Corporation (BSC) under the Building America Program began what turned out to be a multiyear research program that examined both short-term loading as well as long-term loading of wall assemblies using furring strips fastened back through the insulation as the primary cladding support structure (Baker 2013a, 2013b; Baker and Lepage 2014). The combined results of this research are discussed below.

During the course of reviewing previous research, BSC determined that acceptable deflection instead of ultimate capacity of the systems governed the design. For lap sidings and panel claddings with joints (metal, vinyl, wood, and fiber cement), movement is aesthetic in nature and not a health and safety issue. The acceptable amount of deflection will be a function of acceptable aesthetics for the cladding system chosen. For most lap siding or panel cladding systems, variations up to 1/16 in. or even 1/8 in. may be acceptable because the material and installation tolerances are easily greater than the potential gap development (Baker 2013a, 2013b). This analysis shaped the research plan, which intended to look at vertical movement of the furring strip with respect to the structural wall as the performance criteria for design.

The research conducted by BSC was developed around two principal topics: (1) system mechanics, and (2) long-term sustained loading. The test plan also expanded upon previous testing by the American Forest and Paper Association and the New York State Energy Research and Development Authority (which had typically been limited to EPS insulation only) to include the following insulation types:

  • EPS
  • Extruded polystyrene (XPS)
  • Foil-faced PIC
  • Rigid mineral fiber (MF).

Full-scale initial load response (or short-term) testing of assemblies using 4 in. of exterior insulation indicated a system capacity of approximately 45–50 lbf per fastener at 1/16 in. of vertical deflection. The results were consistent regardless of insulation type used except for one outlier test of rigid MF that demonstrated a capacity of 65 lbf at 1/16 in. of deflection (Baker 2013a).

Observations made during the initial load full-scale testing (such as slippage between material layers) raised questions regarding mechanisms that contributed to vertical displacement resistance. The small-scale load component tests were developed to evaluate specific mechanisms of load resistance with the hope to better understand the relative magnitude of each in the development of the full system capacity. The components of interest were: (1) rotational resistance of the fasteners; (2) strut and tie component of the compression of the insulation; and (3) friction between material layers. These three load components are illustrated in Figure 1 below.

Above left: shear and rotational resistane provided by fastener to wood connections; above middle: Rotational resistance provided by tension in fastener and compression of the insulation; above right: vertical movement resistance provided by friction between layers

Figure 1. Load component schematics (Baker and Lepage 2014)

The results of the research provided some useful insights into the magnitude of the various load components, even if many of the exact mechanisms could not be accurately predicted. From the results it appeared that friction forces in the assembly may be significant, particularly at initial and small vertical deflections. The amount of friction due to precompression could be quite variable, however, as measured precompression forces were noted to change dramatically over time and with changing environmental conditions. The strut and tie model was demonstrated to provide additional capacity; however, the results were not clear as other unanticipated factors appear to affect the total capacity such as additional resistance from screw shaft bearing on the insulation materials. The bending capacities of the screw fasteners were noted to contribute a much lower amount to the system total when compared to the other studied mechanisms; however again, the testing did not yield conclusive results (Baker and Lepage 2014).

Long-term tests were also completed to evaluate the performance of the systems under sustained gravity loads. The first series of tests were completed on full-scale assemblies with 4 in. of rigid insulation in a controlled laboratory environment. Four assemblies were tested using different insulation types (as listed previously). Each assembly was loaded to 30 lbf per fastener. A fifth assembly was also constructed using 4 in. of XPS insulation. This fifth assembly was loaded to only 8.6 lbf per fastener.

The test assemblies loaded to 30 lbf per fastener demonstrated very stable performance in the laboratory environment. Most assemblies did not record a deflection greater than 1/32 in., with the exception of the PIC sample, which had a deflection of approximately 3/32 in. The fifth assembly loaded to 8.6 lbf per fastener had very little observed movement (~1/200 in.) (Baker 2013a).

During the course of the research it was noted that movement of the assemblies appeared to have a stronger correlation with environmental changes that from sustained loading as both upward as well as downward movement was noted. From the test data collected, it was not possible to differentiate movements of the samples that result from prolonged loading (creep) or from environmental changes. Because of the suspected climate sensitivity, additional testing of exterior samples exposed to a variety of temperature and humidity conditions was recommended.

A second round of long-term testing was developed to study the impacts of climate exposure on the vertical movement of furring strips attached over exterior insulation. A total of 12 assemblies were constructed (four different insulation types loaded to three different levels, 8 lbf/fastener, 15 lbf/fastener, 30 lbf/fastener) in an outdoor exposed environment. Vertical deflection movements of the furring strip with respect to the framing were measured at various intervals between July 2012 and September 2012. Assemblies loaded 8 lbf per fastener had recorded movement on the order of 1/32 in. Similarly, assemblies loaded to 15 lbf per fastener had . . .

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