July 15, 2013

Abstract: 

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. For thick layers of exterior insulation (levels > 1.5 in.), the use of wood furring strips attached through the insulation back to the structure has been used by many contractors and designers as a means to provide a convenient cladding attachment location. The research presented in this report is intended to help develop a better understanding of the system mechanics involved and the potential for environmental exposure induced movement between the furring strip and the framing.

Executive Summary

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. For thick layers of exterior insulation (levels > 1.5 in.), the use of wood furring strips attached through the insulation back to the structure has been used by many contractors and designers as a means to provide a convenient cladding attachment location (Straube and Smegal 2009; Pettit 2009; Joyce 2009; Ueno 2010).

The research presented in this report is intended to help develop a better understanding of the system mechanics involved and the potential for environmental exposure induced movement between the furring strip and the framing. Building Science Corporation sought to address the following research questions:

  1. What are the relative roles of the mechanisms and the magnitudes of the force that influence the vertical displacement resistance of the system?
  2. Can the capacity at a specified deflection be reliably calculated using mechanics based equations?
  3. What are the impacts of environmental exposure on the vertical displacement of furring strips attached directly through insulation back to a wood structure?

The system mechanics portion of the research examined some of the discrete load components that help develop the vertical load resistance capacity of furring strips attached directly through insulation back to a wood structure. It was theorized that the capacity of the system is developed from several sources, including the moment resistance of the fasteners (including both bending strength of the fastener and the bearing strength of the furring and framing members), the compressive strength of the rigid insulation, as well as the static friction between the layers.

The system mechanics research provided some useful insights into the magnitude of the various load components, even if many of the exact mechanisms cannot be accurately predicted. The research was designed to focus on three mechanisms for resisting vertical gravity loads: (1) screw bending; (2) friction; and (3) a strut and tie effect. The bending capacities of the screw fasteners were noted to contribute a much lower amount to the system total vertical deflection resistance capacity when compared to the other studied mechanisms. From the results it appears that friction forces in the assembly may be significant, particularly at initial and small vertical deflections. While the presence of friction in the assembly may be significant, there is not enough information yet available to determine how to best account for, and make use of, the friction in the assemblies from a design perspective. The amount of friction due to precompression1 can be quite variable, 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 affecting the total capacity.

It was found that the theorized load components that were modeled do not provide a sufficiently accurate prediction of the measured load components to be used in a reliable design model. There were several factors to this, including sensitivity of the inputs, potential changes to the load resistance model depending on the amount of deflection, variability in the boundary conditions, as well as some additional system effects that are still not understood, identified, or quantified. Further study of the load component mechanisms may help to further refine our understanding and help us develop more accurate models that could be used for assembly design.

The second part of the testing work completed was a study on 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 lb/fastener, 15 lb/fastener, 30 lb/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.

The results of the long-term exposure tests reinforced much of the industry experience with this approach to cladding attachment over 4 in. of exterior insulation. Lightweight claddings (such as wood, fiber cement, and vinyl siding) represent the majority of the cladding that has been, and is currently being used with this type of attachment system. These claddings coupled with a fastener spacing of 16–24 in. o.c. are representative of low load per fastener assemblies. To date, no known problems have occurred with these systems. The low measured movement and apparent resistance to creep is in line with this experience.

For heavier claddings such as traditional stucco and adhered stone veneers, the per-fastener load would be expected to be higher. Under medium load (15 lb/fastener), assemblies installed over 4 in. of insulation seem to be demonstrating good performance, though more data are recommended to be collected. Under heavy load (30 lb/fastener), there appears to be a potential for long-term creep of the assemblies. More study is needed for these assemblies.

All of the test assemblies had notable movement within a range of deflections. With a daily movements on the order of ± 1/64 in. to ± 1/32 in. being measured for one of the assemblies. In service deflection limits for the assemblies should be set to account for this movement.

1 Problem Statement

1.1 Introduction

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 better effective R-value from reduced thermal bridging, better condensation resistance, reduced thermal stress on the structure, as well as other commonly associated improvements such as increased airtightness and improved water management (Hutcheon 1964; Lstiburek 2007). For thick layers of exterior insulation (levels > 1.5 in.), the use of wood furring strips attached through the insulation back to the structure has been used by many contractors and designers as a means to provide a convenient cladding attachment location (Straube and Smegal 2009; Pettit 2009; Joyce 2009; Ueno 2010). While the approach has been demonstrated to be effective, 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 the effects of varying environmental conditions.

1.2 Background

The residential building sector consumes approximately 21% of the primary energy used in the United States (DOE/EIA 2008). While new code standards are pushing for more energy-efficient buildings, there are a significant amount of existing buildings that are in great need of energy retrofits. In the past, retrofits of existing residential buildings typically involved the filling of framed cavity walls with insulation; however, the amount of effective thermal resistance that could be added was limited by the existing stud cavity depth (wood-framed walls) or strapping depth (common for mass masonry walls), the insulation material used (commonly fiberglass/mineral fiber or cellulose), and the amount of thermal bridging present from the wood framing.

The addition of insulation to the exterior of existing buildings has been demonstrated to be an effective means to overcome these limitations and provide higher effective R-values for building wall assemblies. The benefits of this approach extend beyond just added thermal resistance; benefits of increased building durability and airtightness are often also realized.

The use of exterior insulation has been common practice for many decades on buildings, particularly behind brick or masonry veneer claddings. Exterior insulation and finish systems use exterior insulation as a composite cladding assembly, providing the support structure for a lightweight finish coat.

Building Science Corporation (BSC) has been at the forefront of using exterior insulation approaches on residential buildings for several decades. The use of furring strips as the primary cladding attachment location is a strategy that has been used on many private as well as Building America supported projects (Pettit 2009; Ueno 2010).

he push for lower energy buildings has resulted in an increase of projects that are looking to use thick layers (> 1.5 inches) of exterior insulation on their buildings. This increase resulted in an increase in questions regarding the effectiveness of using furring strips attached through the insulation back to the structure.

In reaction to this, research into the performance of these systems has been funded by several groups such as the Foam Sheathing Coalition (FSC), the New York State Energy Research and Development Authority (NYSERDA), and the Steel Framing Alliance (SFA). The focus of this past research by FSC and NYSERDA/SFA was to try to develop prescriptive code tables for attaching cladding to framing over continuous insulation (Bowles 2010). The tables that were developed used an initial deflection limit of 0.015 in. as a basis for design. By limiting the initial deflection to 0.015 in., the intent was to keep long-term deflection due to potential creep of the system within acceptable limits, though these acceptable limits were not defined.

Research conducted by BSC was aimed at expanding on this previous research to include several types of insulation as well as to examine both short-term (initial loading) and long-term (sustained loading) performance of the system (Baker 2013).

While short-term capacities of the system were measured, the understanding of the system mechanics that help to develop the capacities were not well identified. The capacity of the system was theorized to be developed from several sources, including the bending strength of the fastener, the bearing strength of the furring and framing members, the compressive strength of the rigid insulation, as well as the static friction between the layers.

Figure 1. Theorized forces providing vertical displacement resistance: Shear and rotational resistance provided by fastener to wood connections (above left); rotational resistance provided by tension in fastener and compression of the insulation (above middle); vertical movement resistance provided by friction between layers (above right)

The long-term (sustained loading) tests that were completed were intended to examine the long- term creep effects of the system under sustained gravity load in relatively stable environmental conditions. The results of the initial testing raised some questions as to the impacts of changing environmental conditions on the long-term performance of the system. The analysis of the data did not show much movement in the systems over the course of the test period (July 2011 through January 2012); however, the movement that was noted seemed to indicate that the system deflections were influenced by even small changes in environmental conditions, and that hese changes may have greater impacts on the vertical movement of the furring strips than the effects of sustained gravity dead loads imposed by the cladding.

The research presented in this report is intended to help develop a better understanding of the system mechanics involved and the potential for environmental exposure to induce movement in the system. BSC sought to address the following research questions:

  1. What are the relative roles of the mechanisms and the magnitudes of the force that influence the vertical displacement resistance of the system?
  2. Can the capacity at a specified deflection be reliably calculated using mechanics based equations?
  3. What are the impacts of environmental exposure on the vertical displacement of furring strips attached directly through insulation back to a wood structure?

1.3 Relevance to Building America’s Goals

The use of exterior insulation on wall assemblies is an effective means to provide additional thermal resistance to enclosure assemblies. 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.

The results of the research is intended to provide specific guidance for cladding attachment over thicker layers of exterior insulation and evaluate the potential of developing mechanics based equations as a means of evaluation and design of the system capacities. It will also provide some preliminary data on field performance of the systems. The applicability of this research will extend to all climate zones and housing types.

1.4 Cost Effectiveness

In most circumstances, the exterior retrofit of a home with exterior insulation comes as part of a larger scope of work for a building retrofit. The choice to add exterior insulation is usually triggered by a need (or desire) to reclad or overclad the building. The driving force behind installing new cladding can be from any number of sources, including existing water management problems, comfort or durability concerns, end of service life for the cladding, or aesthetic concerns. The need to replace the cladding provides an opportunity for the designer or contractor to include exterior insulation as a means to increase the energy performance of the building at the same time. The cost effectiveness of this from an energy perspective is therefore dependent on the cost of the insulation as well any associated components above and beyond new cladding installation.

A preliminary evaluation was completed looking at the incremental cost of the varying thicknesses 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 was taken from RS Means Construction Data (Reed Construction Data 2011). 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/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 following basic characteristics (Table 1).

Table 1. Benchmark House Characteristics

House CharacteristicsSquare Footage
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 effectiveness of this single strategy. Given the assumed age of the home, the benchmark home had an uninsulated wall cavity (as per guidance from the 2011 Building America Benchmark Protocol). The following parametrics were run to see the effectiveness of the added thermal resistance to the energy performance and utility cost (Table 2). The analysis assumed that the cost of the measure is financed over a 5-year period at a 7% interest rate. An additional fuel escalation rate of 2% was also included in the analysis.

Table 2. Parametric Steps and Cost

Parametric StepCost/ft2
Benchmark (Uninsulated 2 × 4 Wall)N/A
R-13 Cavity Fill Insulation$2.20
R-13 Cavity Fill + 1 in. Exterior Insulation (R-6.5)$3.55
R-13 Cavity Fill + 1.5 in. Exterior Insulation (R- 9.75)$3.76
R-13 Cavity Fill + 2 In. Exterior Insulation (R-13) +
1 × 4 Wood Furring
$5.73
R-13 Cavity Fill + 2 Layers of 1.5-in. Exterior Insulation  (R-19.5) +
1 × 4 Wood Furring
$7.19
R-13 Cavity Fill + 2 Layers of 2-In. Exterior Insulation (R-26) +
1 × 4 Wood Furring
$7.58

Simulations were run for the following cities (Table 3):

Table 3. Reference Cities

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

Results indicated that for cold climate zones (4 and higher), insulation up to 1.5 in. was shown to be a cost-optimized solution. This was mainly due to this being the tipping point before which additional costs associated with the furring strips and additional screw fasteners required for cladding attachment needed to be added to the system. Insulation thickness above 2 in. was still demonstrated to be cost neutral as part of this simplified analysis in all cities except for Dallas.

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 as well (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.

1.5 Tradeoffs and 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. 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 buildings, the potential for freeze- thaw is practically eliminated, since this approach not only keeps the masonry warmer, but also addresses 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 so much additional protection against water infiltration problems (Lstiburek 2009) that the use of a drainage gap is a base recommendation for most cladding installations regardless of whether or not exterior insulation is used. 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 Research

Several groups such as the FSC, NYSERDA, and SFA have 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 this past research by FSC and NYSERDA/SFA was to develop prescriptive code tables for attaching cladding to framing over continuous insulation (Bowles 2010). The research methodology adopted used wood joint connection theory as the basis for the analysis, in particular the European Yield Theory that examined the performance of gapped wood to wood connections.

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 (AFPA) 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. A visual representation of the potential failure modes (AFPA 1999) is included . . .

Download the complete report here.

Footnotes:

  1. Precompression forces are the clamping forces generated in the assembly by attachment of the furring strips with the screw fasteners.