January 1, 2013

Abstract: 

The use of exterior insulation is an effective means to increase the overall thermal resistance of wall assemblies that also has other advantages of improved water management and often increased air tightness of the building. However, the engineering basis and support for this work has not been conducted, resulting in obstacles for building official and building code acceptance. Additionally, the water management and integration of window systems, door systems, decks, balconies, and roof-wall intersections have not been adequately developed. This research project developed baseline engineering analysis to support the installation of thick layers of exterior insulation (2” to 8”) on existing masonry walls and wood framed walls and as well as relevant water management details.

Executive Summary

Exterior insulation is an effective means for increasing the overall thermal resistance of wall assemblies. It also has other advantages including improved water management and often increased airtightness of the building. The engineering basis and support work for exterior insulation, however, has not been conducted, resulting in obstacles for building official and building code acceptance. Additionally, water management strategies and integration practices for window systems, door systems, decks, balconies, and roof wall intersections have not been adequately developed. This gap also stands in the way of wider deployment.

In this research project, the Building Science Corporation (BSC) developed baseline engineering analysis to support the installation of thick layers of exterior insulation (2 in. to 8 in.) on existing masonry walls and wood framed walls. Wood furring strips (fastened through the insulation back to the structure) were used as a cladding attachment location. Water management details necessary to connect the exterior insulated wall assemblies to roofs, balconies, decks, and windows were created as guidance for integrating exterior insulation strategies with other enclosure elements.

Wind load withdrawal resistance capacities were determined based on guidance outlined in the National Design Specification for Wood Construction (American Forest & Paper Association 2005, Chapter 11, “Dowel Type Fasteners”). In all cases, the withdrawal capacity is independent of the thickness of the exterior insulation.

Analysis of gravity load capacity is more complex and has multiple variables that needed to be considered for the cladding attachment. BSC completed a numerical analysis for insulation thicknesses from 1 in. to 8 in. (in 1-in. increments). The laboratory testing was limited to 4-in.-thick installations and 8-in.-thick installations. The intent was that the results from the 4-in. test could be applied to installations up to 4 in. and the 8-in. test results could be applied to installations between 4 in. and 8 in.

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. BSC recommends, then, that the deflection be limited to 1/16 in. in service unless it is demonstrated that larger deflections can be tolerated.

For brittle claddings (such as stucco and cultured stone), movement could lead to cracking and potentially spalling of the material. For these systems BSC recommends that the in-service deflection limit be set to prevent deflection that may damage the cladding or impair its function. A limit of 1/64 in. is proposed for brittle claddings after initial deflection.

Most common residential cladding systems (metal, vinyl, wood, and fiber cement) are lightweight enough (<5 psf) that attachment to furring over any thickness of insulation does not create an issue. For these cladding systems, the predicted deflection based on a reasonable horizontal spacing (16-in. to 24-in. on center) and vertical fastener spacing (up to 24-in. on center), is so slight (1/200 in.), and creep effects are so minimal, that the deflection does not approach the proposed 1/16-in. maximum in-service deflection limit.

For heavier cladding systems (>10 psf) initial deflection is within the proposed deflection limit. There is, however, inadequate information about potential thermal and moisture expansion and contraction movements, as well as creep effects of certain insulation materials in exposed environments, to predict long-term service deflection. Additional research into the long-term deflection movement of heavier claddings in exposed environments is needed.

Integrating exterior insulation into the water management strategy of the building takes careful detailing at interfaces with other enclosure elements.

For the most part, placing the water resistive barrier to the exterior of the insulation has been the easiest because the details are largely similar to standard construction practices. Concern is often raised about how to support elements that were once positioned in the structural frame wall, which are now “pushed” outward into the plane of the exterior insulation (e.g., windows and step flashings). Careful use of blocking or box extensions can be integrated into the design to address these concerns.

Conversely, placing the water resistive barrier inboard of the exterior insulation has been more difficult for contractors to adopt because of some significant departures from standard construction details and common construction sequences. These concerns increased when these techniques were applied to a building retrofit. This does have, however, the benefits of placing the water resistive barrier in a more protected location (increasing durability), and locating the window in the plane of the existing framing.

BSC developed details to serve as guidance on how to effectively maintain the continuity of the water management. These details are presented in Appendix A of this report.

1 Problem Statement

1.1 Introduction

The underlying concept of insulating the exterior of existing masonry walls and wood framed walls is simple; it has a variety of advantages for durability and air barrier continuity (Lstiburek 2007; Hutcheon 1964). Even though the practice should be simple, several problems stand in the way of widespread implementation. For example, manufacturers of cladding systems and exterior insulation materials often limit thicknesses to 1½ in. with their warranties; the cladding attachment, then, becomes an issue. This problem has been tackled by various practitioners (Crandell 2010; Ueno 2010; Joyce 2009; Pettit 2009; Straube and Smegal 2009). Demonstrations by members of the Building Science Corporation (BSC) research team, which carried out the work described in this report, have shown that up to 8 in. of exterior insulation over the exterior of wood framed buildings is possible (Lstiburek 2009). The  engineering basis and support work, however, has not been conducted, resulting in obstacles for building official and building code acceptance. Additionally, water management strategies and procedures for integrating roofs, balconies, decks, and window systems have not been adequately developed. This gap also stands in the way of wider deployment.

In this research project, BSC developed baseline engineering analysis to support the installation of thick layers of exterior insulation (2 in. to 8 in.) on existing masonry walls and wood framed walls. Wood furring strips (fastened through the insulation back to the structure) were used as a cladding attachment location. Water management details necessary to connect the exterior insulated wall assemblies to roofs, balconies, decks, and windows were also created, resulting in guidance on integrating exterior insulation strategies with other enclosure elements. The details give consideration to both complete retrofit and phased retrofit approaches, furnishing connection details that allow for future integration with other high performance enclosure system elements.

1.2 Background

The existing residential building stock represents a significant portion of U.S. energy consumption. The residential and commercial building sectors consumed roughly 40% of the primary energy used in the United States in 2008. The residential sector consumed 21% and the commercial sector consumed 18% (U.S. Department of Energy, Energy Information Administration 2008). New construction represents only a small fraction of the total building stock in the country. The adoption of energy codes in many states has helped drive a move toward lower energy use buildings, but the existing building stock remains, for the most part, untouched.

In the past, retrofits of existing residential buildings typically involved the filling of framed cavity walls with insulation. The amount of effective thermal resistance that could be added, though, 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.

Adding insulation to the exterior of existing buildings has been a method used by retrofit contractors to overcome these limitations and achieve higher effective R-values for wall assemblies. The benefits of this approach extend beyond added thermal resistance; increased building durability and airtightness are often also realized.

BSC has been involved with numerous new construction and building retrofit projects that have used exterior insulation as part of the building energy use reduction strategy. Experience has shown that two primary questions are often raised:

  • How will the cladding be attached?
  • How will the water management of the assembly be accomplished?

1.3 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 the need (or desire) to re-clad or overclad the building. The driving force behind installing new cladding can include existing water management problems, comfort or durability concerns, end of service life for the cladding, or aesthetic issues. The need to replace the cladding gives the designer or contractor an opportunity to include exterior insulation as a way 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 the new cladding installation.

BSC completed a preliminary evaluation that looked 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 were taken from RSMeans Construction Data (Reed Construction Data 2011). Costs included in the analysis were the installed cost of the insulation material, 1 × 4 wood furring strips spaced at 16-in.on center (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 $100.00 per window was used in the reference model as an estimate of the additional cost for trim extensions that would be needed to account for the additional thickness of the exterior insulation. This value was estimated because actual costs can be highly variable. This variability results from 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. These items are associated with re-cladding and water management, and would be part of the retrofit project regardless of the addition of exterior insulation.

BSC ran simulations using Building Energy Optimization (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 1950’s era two-story slab on grade construction. Table 1 gives its basic characteristics.

Table 1. Benchmark House Characteristics

House Characteristicsft2
Finished floor area2,312
Ceiling area1,156
Slab area1,156
Wall area2,799
Window area410 (17.7% glazing ratio)

To examine the effectiveness of this single strategy, the wall conductance performance was isolated from all other aspects of the home. 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).1 The parametrics listed in Table 2 were run to see the effectiveness of the added thermal resistance in regard to the energy performance and utility cost.

Table 2. Parametric Steps and Cost

Parametric StepCost/ft2
Benchmark (uninsulated 2x4 wall)N/A
R-13 cavity fill insulation$2.20
R-13 cavity fill insulation + 1-in. exterior insulation (R-6.5)$3.55
R-13 cavity fill insulation + 1-in. exterior insulation (R-9.75)$3.76
R-13 cavity fill insulation + 2-in. exterior insulation (R-13) +
1x4 wood furring
$5.73
R-13 cavity fill insulation + two layers of 1.5-in. exterior insulation (R-19.5) + 1x4 wood furring

$7.19

R-13 cavity fill insulation + two layers of 2-in. exterior insulation (R-26) + 1x4 wood furring$7.58
R-13 cavity fill insulation + four layers of 2-in. exterior insulation (R-52) + 1x4 wood furring$11.07

Results indicated that for cold-climate zones (4 and higher), insulation up to 1.5 in. was a cost-optimized solution. This was mainly because this was 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 thicknesses up to 4 in. were demonstrated to be cost neutral as part of this simplified analysis in all cities except for Dallas, Texas (see Table 3 for reference cities). Insulation thicknesses up to 8 in. were demonstrated to be cost neutral, but only in cold-climate zones such as Boston, Massachusetts, and Duluth, Minnesota (see Appendix B for the results).

Although the analysis focused on conductance improvements only, some argument can be made that adding exterior insulation would likely also improve the overall airtightness of the assemblies (Ueno 2010). The benefits of 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.

Table 3. Reference Cities

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

1.4 Other Benefits

Using exterior insulation has many additional benefits other than 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 building, the potential for freeze thaw is practically eliminated because this approach not only keeps the masonry warmer, but also addresses exterior rain water 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 increase in drainage and drying that results from the ¾-in. gap created by the furring strips offers additional protection against water infiltration problems (Lstiburek 2010). The benefit is significant enough that the use of furring strips is a base recommendation for all cladding installations whether exterior insulation is used or not. The fact that the furring strips are an intrinsic component of this system adds a significant benefit to the long-term durability of these wall assemblies.

2 Cladding Attachment Design

Attaching the cladding over exterior insulation encounters two common barriers:

  • Cladding manufacturers that limit their warranties for installations of their cladding systems over only 1 in. to 1½ in. of insulation.
  • Availability of fasteners that are long enough to fasten through the cladding and insulation, while still maintaining the required embedment depth into the structure, is limited.2

To overcome these constraints, furring strips have been added as a cladding fastening location for assemblies when thicker levels of exterior insulation are used (2 in. and greater). This addresses the cladding manufacturer’s warranty and allows readily available fasteners and common cladding fastening procedures to be used.

For wood framed walls, long screws are used to attach the furring strips through the insulation back to the wood structure. For mass masonry walls an interim step is needed. To allow for an attachment point for the furring, wood 2×4 members (installed on the flat) are first attached to the masonry wall structure. The furring is then fastened back through the insulation to the 2×4 framing members with screws (see Figure 1).

Figure 1: Recommended cladding attachment design

Attaching cladding to furring strips that are fastened back through the exterior insulation has been used on numerous Building America test homes and communities in both new and retrofit applications. This strategy has been proven to be an effective and durable way to attach cladding (BSC 2010; BSC 2009a; BSC 2009b). The lack of engineering data, though, has been a problem for many designers, contractors, and code officials. Concerns about sagging of the cladding from rotation of the fasteners and compression of the insulating sheathing are often raised.

2.1 Previous Research

Recently, studies undertaken by the Foam Sheathing Coalition (FSC), along with a joint research project by the New York State Energy Research and Development Authority (NYSERDA) and the Steel Framing Alliance (SFA) completed some testing and analysis to develop prescriptive code tables for attaching cladding to framing over continuous insulation. This work included conducting some laboratory testing of lateral load resistance for various configurations of cladding and furring types fastened through exterior insulation into wood or steel framed wall assemblies. Two criteria were evaluated when examining the connection performance: (1) overall strength of the connection and (2) acceptable deflection performance.

The acceptable deflection limit is a performance requirement to limit the amount of vertical deflection that the installed weight of the cladding will induce on the furring strips. Excessive deflection could lead to concerns about gaps developing between the siding and other enclosure elements (such as windows, window trim, or other trim materials).

As part of the FSC and NYSERDA/SFA research, the acceptable deflection limit was set to a maximum of 0.015 in. (or 1/64 in.; Crandell 2010). The 0.015-in. deflection limit has a longstanding basis for wood connection design values used in the National Design Specification for Wood Construction (known as the NDS; American Forest & Paper Association [AF&PA] 2005). The FSC and NYSERDA/SFA research determined that in all cases the 0.015-in. deflection limit, not the average shear strength, controlled the design values for the capacities of the systems.

A secondary aspect of the FSC and NYSERDA/SFA research was to verify the accuracy of applying current engineering knowledge about wood to wood connections using the NDS Yield Theory (as detailed in General Dowel Equations for Calculating Lateral Connection Values: AF&PA Technical Report 12 [TR-12]; AF&PA 1999) in predicting connection capacities. The researchers discovered that the 5% offset yield prediction as calculated using the TR-12 resulted in a reasonably accurate prediction of the shear load at a deflection of 0.015 in. Although there is no mathematical connection between these values, the investigators considered this an adequate basis for designing to a 0.015-in. deflection limit given the limited amount of research and funding that had been available to that point. In addition, a safety factor of 1.5 was added to the calculated results to address potential concerns of creep of materials under sustained loads. The choice of the 1.5 safety factor was based on several factors including precedence in the NDS and limited long-term deflection testing; however, a significant amount of uncertainty still surrounds the actual amount of predicted creep. Additional research is needed in this area. . .

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Footnotes:

  1. More information about Building America can be found at www.buildingamerica.gov.
  2. Most pneumatic nail guns have a maximum fastener length limit of 3 in. to 3.5 in. This limits the amount of
    insulation that can be placed between the siding and the substrate in a direct siding application.