External Insulation of Masonry 

Walls and Wood Framed Walls 

P. Baker 

Building Science Corporation

January 2013
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iii

External Insulation of Masonry Walls and Wood Framed Walls

Prepared for: 

The National Renewable Energy Laboratory 

On behalf of the U.S. Department of Energy’s Building America Program 

Office of Energy Efficiency and Renewable Energy 

15013 Denver West Parkway 

Golden, CO 80401 

NREL Contract No. DE-AC36-08GO28308 

Prepared by: 

P. Baker 

Building Science Corporation 

30 Forest Street

Somerville, MA 02143

NREL Technical Monitor: Cheryn Metzger

Prepared under Subcontract No. KNDJ-0-40337-00 

January 2013 
iv

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v

Contents

List of Figures ............................................................................................................................................ vi

List of Tables ............................................................................................................................................. vii

Definitions................................................................................................................................................. viii

Executive Summary................................................................................................................................... ix

1 Problem Statement............................................................................................................................... 1

1.1 Introduction...................................................................................................................................... 1

1.2 Background...................................................................................................................................... 1

1.3 Cost Effectiveness............................................................................................................................ 2

1.4 Other Benefits.................................................................................................................................. 4

2 Cladding Attachment Design .............................................................................................................. 5

2.1 Previous Research............................................................................................................................ 6

2.2 Wind Load Resistance ..................................................................................................................... 8

2.3 Gravity Load Resistance ................................................................................................................ 10

3 Water Management Details................................................................................................................ 24

3.1 Exterior Insulation Materials ......................................................................................................... 26

3.2 Water Resistive Barrier.................................................................................................................. 27

3.3 Air Barrier...................................................................................................................................... 28

3.4 Cladding Attachment ..................................................................................................................... 28

3.5 Window Integration ....................................................................................................................... 29

3.6 Roof Integration............................................................................................................................. 31

3.7 Balcony Integration........................................................................................................................ 37

3.8 Deck Integration ............................................................................................................................ 37

4 Conclusions ........................................................................................................................................ 38

References................................................................................................................................................. 40

Appendix A: Water Management Details................................................................................................ 41

Appendix B: BEopt Simulation Graphs ................................................................................................ 107

B.1 Dallas, Texas................................................................................................................................ 107

B.2 Kansas City, Missouri.................................................................................................................. 108

B.3 Boston, Massachusetts................................................................................................................. 109

B.4 Duluth, Minnesota ....................................................................................................................... 110

Appendix C: Short-Term Deflection Test Protocol.............................................................................. 111

C.1 Support Structure ......................................................................................................................... 111

C.2 Insulation and Furring Strips ....................................................................................................... 111

C.3 Measuring Devices....................................................................................................................... 111

C.4 Loading Mechanism .................................................................................................................... 111

C.5 Testing Procedure ........................................................................................................................ 111

Appendix D: Long-Term Deflection Test Protocol .............................................................................. 113

D.1 Support Structure ......................................................................................................................... 113

D.2 Insulation and Furring Strips ....................................................................................................... 113

D.3 Measuring Devices....................................................................................................................... 113

D.4 Loading Mechanism .................................................................................................................... 113

D.5 Testing Procedure ........................................................................................................................ 113
vi

List of Figures

Figure 1. Recommended cladding attachment design ........................................................................... 5

Figure 2. Table 8 excerpt from Crandell (2010)........................................................................................ 7

Figure 3. Example of fastener yielding in Mode IV................................................................................ 12

Figure 4. Short-term deflection test panel.............................................................................................. 14

Figure 5. Example hysteresis test plot for short-term deflection testing ........................................... 15

Figure 6. Long-term deflection test panel .............................................................................................. 16

Figure 7. Dead weights attached to bottom edge of furring strips...................................................... 17

Figure 8. Short-term deflection testing results (4-in.-thick insulation) ............................................... 18

Figure 9. Short-term deflection testing results (8-in.-thick insulation) ............................................... 18

Figure 10. Forces providing vertical displacement resistance ............................................................ 19

Figure 11. Deflection of furring strips under sustained load ............................................................... 21

Figure 12. Typical exterior retrofit detail, showing furring strip cladding attachment...................... 25

Figure 13. Layering patterns of exterior insulation ............................................................................... 26

Figure 14. WRB options with exterior insulation................................................................................... 27

Figure 15. Recommended air barrier location for retrofit assemblies ................................................ 28

Figure 16. Recommended cladding attachment design ....................................................................... 29

Figure 17. Detail of masonry sill removal to allow for insulation to return to the window frame..... 31

Figure 18. Common roof to wall connections........................................................................................ 31

Figure 19. Different drainage plane placement for exterior insulation retrofits................................. 32

Figure 20. Roof to wall interface with WRB behind the insulation ...................................................... 34

Figure 21. Roof to wall interface with WRB at the face of the insulation............................................ 37

Figure 22. Annualized energy related costs versus average source energy savings for Dallas, 

Texas.................................................................................................................................................. 107

Figure 23. Average source energy savings reduction versus insulation level for Dallas, Texas... 108

Figure 24. Annualized energy related costs versus average source energy savings for Kansas 

City, Missouri .................................................................................................................................... 108

Figure 25. Average source energy savings reduction versus insulation level for Kansas City,

Missouri............................................................................................................................................. 108

Figure 26. Annualized energy related costs versus average source energy savings for Boston, 

Massachusetts.................................................................................................................................. 109

Figure 27. Average source energy savings reduction versus insulation level for Boston, 

Massachusetts.................................................................................................................................. 109

Figure 28. Annualized energy related costs versus average source energy savings for Duluth, 

Minnesota.......................................................................................................................................... 110

Figure 29. Average source energy savings reduction versus insulation level for Duluth,

Minnesota.......................................................................................................................................... 110

Unless otherwise noted, all figures were created by BSC.
vii

List of Tables

Table 1. Benchmark House Characteristics............................................................................................. 3

Table 2. Parametric Steps and Cost.......................................................................................................... 3

Table 3. Reference Cities............................................................................................................................ 4

Table 4. Insulation Materials ...................................................................................................................... 8

Table 5. Design Withdrawal Values for Various Common Screw Fasteners ........................................ 9

Table 6. Adjustment Factors...................................................................................................................... 9

Table 7. Allowable Design Wind Pressure (psf)..................................................................................... 10

Table 8. Yield Modes from AF&PA TR-12 ............................................................................................... 11

Table 9. Nominal 5% Offset Limit (lb) at Connection Yield Mode for #8 Wood Screws..................... 12

Table 10. Nominal 5% Offset Limit (lb) at Connection Yield Mode for #10 Wood Screws................. 13

Table 11. Nominal 5% Offset Limit (lb) at Connection Yield Mode for #12 Wood Screws................. 13

Table 12. Nominal 5% Offset Limit (lb) at Connection Yield Mode for 1⁄4-in. Lag Screws.................. 13

Table 13. Target Loads for Short-Term Deflection Testing .................................................................. 15

Unless otherwise noted, all tables were created by BSC.
viii

Definitions

AF&PA American Forest & Paper Association

BEopt Building Energy Optimization (software). More information 

about BEopt can be found at http://beopt.nrel.gov/

BSC Building Science Corporation. More information about BSC 

can be found at www.buildingscience.com 

CMU Concrete masonry unit

EPS Expanded polystyrene

FSC Foam Sheathing Coalition

ft2 Square foot

in. Inch

lb Pound

NDS National Design Specification for Wood Construction

NYSERDA New York State Energy Research and Development Authority

OSB Oriented strand board

PIC Polyisocyanurate

psf Pounds per square foot

RH Relative humidity

s Second

SFA Steel Framing Alliance

SPF Spruce-Pine-Fir

TR-12 General Dowel Equations for Calculating Lateral Connection 

Values: AF&PA Technical Report 12 (AF&PA 1999)

WRB Water resistive barrier

XPS Extruded polystyrene
ix

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 
x

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

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 11⁄2 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 
2

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.
3

Table 1. Benchmark House Characteristics

House Characteristics ft2

Finished Floor Area 2,312

Ceiling Area 1,156

Slab Area 1,156

Wall Area 2,799

Window Area 410 (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 Step Cost/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 + Two Layers of 1.5-in. Exterior Insulation 

(R-19.5) + 1 × 4 Wood Furring $7.19

R-13 Cavity Fill + Two Layers of 2-in. Exterior Insulation 

(R-26) + 1 × 4 Wood Furring $7.58

R-13 Cavity Fill + Four Layers of 2-in. Exterior Insulation 

(R-52) + 1 × 4 Wood Furring $11.07

Results indicated that for cold-climate zones (4 and higher), insulation up to 1.5 in. was a costoptimized 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 

 1 More information about Building America can be found at www.buildingamerica.gov.
4

cold-climate construction; however, it is also more difficult to isolate and apportion to individual 

measures.

Table 3. Reference Cities

City Climate Zone

Dallas, Texas 3A

Kansas City, Missouri 4A

Boston, Massachusetts 5A

Duluth, Minnesota 7A

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 3⁄4-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.
5

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 11⁄2 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

 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.

Furring strips attached back 

through the insulation to the wood 

frame structure

Furring strips attached back through the 

insulation to 2 × 4 wood framing that is 

attached to the masonry wall

2 × 4 studs installed on 

the flat

Wood structural 

framing
6

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.

This work resulted in the development of proposed code tables that set forth prescriptive 

requirements for attaching cladding over exterior insulating sheathing (see Figure 2). The table 

was developed using calculated results from the TR-12 supported by laboratory testing of a 
7

representative but limited selection of various cladding attachment and fastener configurations. 

The table prescribes a maximum amount of insulation that can be installed based on maximum 

cladding weight, stud spacing, and vertical fastener spacing. This testing addressed attachments 

to wood and steel framing in addition to wood sheathing. Attachment to other materials such as 

masonry was not investigated.

Figure 2. Table 8 excerpt from Crandell (2010)

(Used with permission)
8

The results of this earlier research laid a good foundation for guidance on cladding attachment. 

Several key questions were answered; however, the work has led to other questions about the 

deflection of the furring strips:

• What is the impact of different insulation materials?

• What is the impact of increased thicknesses beyond 4 in.?

• What is the impact of prolonged loading?

To answer these questions, BSC designed the research described in this report. The BSC research 

team examined the problem using engineering numerical analysis and laboratory testing of 

cladding attachment using furring strips. The analysis examined the ability of the system to resist 

wind withdrawal loads, initial (short-term) gravity loading, and prolonged (long-term) gravity 

loading of cladding systems.

The numerical analysis portions were completed following standards set out in the 2005 NDS 

(AF&PA 2005) and the TR-12 document (AF&PA 1999). The numerical analysis examined both 

wind withdrawal load and vertical gravity load resistance of the assemblies.

Laboratory testing was designed to examine short-term as well as long-term loading performance 

for several common types of exterior insulation materials. Table 4 lists the materials used in each 

of the tests conducted.

Table 4. Insulation Materials

Insulation Type Product Brand

Type II Expanded Polystyrene (EPS) Plastispan Plastifab

Type IV Extruded Polystyrene (XPS) C-200 Owens Corning

Foil-Faced PIC Thermax CI DOW Chemical

Rigid Mineral Fiber RB80 Roxul

The laboratory work was also designed to expand on insulation thicknesses using 4-in.-thick 

material (two layers, 2 in. thick) for the baseline tests. Additional testing of 8-in.-thick material 

(four layers, 2 in. thick) was conducted. Other variables such as stud spacing, furring strip 

dimensions, and fastener types were maintained across each test; however, different fastener 

types were needed for thicker insulation installations.

2.2 Wind Load Resistance

Wind load withdrawal resistance is a function of the fastener withdrawal capacity and is 

independent of the length of the fastener. As a result, insulation thickness has no bearing on the 

withdrawal capacity of the fastener. Withdrawal capacities of fasteners have been well studied 

and documented. Design capacities can be determined following design guidelines set out in the 

2005 NDS (AF&PA 2005). Because of this, no laboratory testing was deemed to be necessary.
9

2.2.1 Numerical Analysis

Fastener withdrawal resistance was evaluated under Chapter 11, “Dowel-Type Fasteners” of the 

2005 NDS (AF&PA 2005). The withdrawal strength is determined by the following equations:

W = 1800·G3/2·D3/4 (for lag screw attachment)

W = 2850·G2·D (for wood screw attachment)

W = 1380·G5/2·D (for nail and spike attachment)

where

W = Withdrawal strength (per inch of embedment)

G = Specific gravity of wood

D = Unthreaded diameter.

To be consistent with earlier research, Spruce-Pine-Fir (SPF) or any other softwood species with 

a specific gravity of 0.42 per the 2005 NDS was used as the minimum in the calculation. Wood 

members with higher specific gravity numbers will result in increased capacity. Design values 

were calculated for the common fastener types listed in Table 5.

Table 5. Design Withdrawal Values for Various Common Screw Fasteners

Fastener Type

Unthreaded Shank 

Diameter 

(in.)

Withdrawal Capacity per 

Inch of Thread Penetration

#8 Wood Screws 0.164 82 lb

#10 Wood Screws 0.190 96 lb

#12 Wood Screws 0.216 109 lb

1⁄4-in. Lag Screws 0.250 173 lb

The calculated withdrawal values were multiplied by the adjustment factors given in Table 6 as 

outlined in Table 103.1 Applicability of Adjustment Factors for Connections (NDS 2005) per the 

2005 NDS to determine the allowable stress design (ASD) adjusted design values.

Table 6. Adjustment Factors

Adjustment Factors

Load Duration Factor CD = 1.6

Wet Service Factor CM = 1.0

Temperature Factor Ct = 1.0

End Grain Factor Ceg = 1.0

Toe-Nail Factor Ctn = 1.0
10

The results were tabulated based on horizontal spacing of furring strips and vertical spacing of 

the fasteners (see Table 7).

Table 7. Allowable Design Wind Pressure (psf)

#8 Wood Screw #10 Wood Screw #12 Wood Screw 1⁄4-in. Lag Screw

Furring Spacing 

(in.)

Furring Spacing 

(in.)

Furring Spacing 

(in.)

Furring Spacing 

(in.)

Vertical 

Fastener 

Spacing

16 24 16 24 16 24 16 24

8 in. 148 99 172 115 195 130 301 200

12 in. 99 66 115 76 130 87 200 134

16 in. 74 49 86 57 98 65 150 100

24 in. 49 33 57 38 65 43 100 67

2.3 Gravity Load Resistance

Unlike the wind load resistance and the fastener withdrawal values, the lateral load capacity of 

wood furring installed over exterior insulating sheathing does not have well-defined guidance. 

To evaluate the lateral load capacity of furring strips installed over insulation as a cladding 

attachment system, laboratory testing and computational analyses were completed.

2.3.1 Numerical Analysis

Using the FSC and NYSERDA/SFA research as a starting point (Crandell 2010), the 5% offset 

yield values were calculated for various insulation thickness using the methodology set out in the 

TR-12 document (AF&PA 1999). Six modes of failure are evaluated in the general dowel 

equations (Table 8). The failure modes are functions of either crushing (bearing failure) in the 

wood members or bending (yielding) of the dowel fastener. An example of a mode IV failure 

(dowel yielding in the side and main member) can be seen in Figure 3.
11

Table 8. Yield Modes from AF&PA TR-12

Yield Mode Description Graphic

Im Main Member Bearing Failure

Is Side Member Bearing

II Side and Main Member Bearing

IIIm Main Member Bearing and Dowel Yielding in 

the Side Member

IIIs Side Member Bearing and Dowel Yielding in 

the Main Member

IV Dowel Yielding in the Side and Main Member
12

Figure 3. Example of fastener yielding in Mode IV

Four types of screw fasteners were used (#8 wood screw, #10 wood screw, #12 wood screw, and 

1⁄4-in. lag screw). These four are the most common fasteners expected to be used for attaching 

wood furring back to the structure.

A minimum penetration depth of 1.5 in. was assumed for all fasteners into the primary support 

member. A 0.75-in. bearing of the fastener in the furring was assumed. Similar to the withdrawal 

capacity calculations, the minimum specific gravity of both the wood stud and the wood furring 

strip was set to 0.42.

Nominal design values (values before any safety factors are applied) for each of the baseline 

fasteners used are given in Table 9 through Table 12. The design value is the lowest yield limit 

for all of the failure modes evaluated. For most cases, yield mode IIIs governs, except for #8 

wood screws where for larger gaps, yield mode IV governs (see dashed red lines in tables). 

These calculated results are consistent with observed yield failures noted during laboratory 

testing.

Table 9. Nominal 5% Offset Limit (lb) at Connection Yield Mode for #8 Wood Screws

Gap 

(in.) Im Is II IIIm IIIs IV

1 565 414 128 141 66 75

2 565 414 90 96 40 42

3 565 414 69 72 29 28

4 565 414 56 57 22 22

5 565 414 47 48 18 17

6 565 414 40 41 15 14

7 565 414 35 36 13 12

8 565 414 32 32 12 11
13

Table 10. Nominal 5% Offset Limit (lb) at Connection Yield Mode for #10 Wood Screws

Gap 

(in.) Im Is II IIIm IIIs IV

1 656 479 148 170 80 101

2 656 479 104 116 49 57

3 656 479 80 87 35 39

4 656 479 65 70 27 30

5 656 479 54 58 22 24

6 656 479 47 50 19 20

7 656 479 41 43 16 17

8 656 479 37 38 14 15

Table 11. Nominal 5% Offset Limit (lb) at Connection Yield Mode for #12 Wood Screws

Gap 

(in.) Im Is II IIIm IIIs IV

1 747 545 169 208 97 143

2 747 545 119 142 59 83

3 747 545 91 107 42 57

4 747 545 74 85 33 44

5 747 545 62 71 27 35

6 747 545 53 61 23 29

7 747 545 47 53 20 25

8 747 545 42 47 17 22

Table 12. Nominal 5% Offset Limit (lb) at Connection Yield Mode for 1⁄4-in. Lag Screws

Gap

(in.)

Im Is II IIIm IIIs IV

1 1221 882 275 322 151 201

2 1221 882 193 220 92 115

3 1221 882 148 165 66 79

4 1221 882 120 132 51 60

5 1221 882 101 110 42 48

6 1221 882 87 94 35 40

7 1221 882 76 82 31 34

8 1221 882 68 73 27 30

2.3.2 Short-Term Deflection Testing Protocol

The short-term (or initial loading) test method was designed to emulate whole-wall system 

effects in that the tests were conducted on full height assemblies (see Appendix C for the full test 

protocol). The intent was to minimize variations in the installation of a single fastener by 
14

distributing the load over multiple fasteners (in these tests, 14 fasteners were used for each test 

panel). The test panels were 8-ft tall by 4-ft wide and anchored to a masonry block wall (see 

Figure 4). Each test panel consisted of the following:

• 2 × 4 wood studs at 24-in o.c.

• 3⁄8-in. oriented strand board (OSB) sheathing

• Building wrap

• 4-in. or 8-in. exterior insulation (two or four layers of 2-in.-thick material with joints 

offset)

• 1 × 3 wood furring fastened back to the studs with #10 wood screws at a 16-in. o.c. 

vertical spacing (14 fasteners total per panel).

Figure 4. Short-term deflection test panel

The furring strips were loaded with a hydraulic ram, bringing up the load on the wall assembly to 

a specific target load and recording the deflection. The load was then released to examine the 

amount of plastic deformation created in the system to that point. The system was then reloaded 

up to the next target load and the protocol repeated. Table 13 gives the target loads. The 

hysteresis plots created by the loading and unloading of the test panels were designed to examine 

the amount of plastic deformation induced into the assembly at each load increment (see Figure 

5).
15

Table 13. Target Loads for Short-Term Deflection Testing

Total Load (lb) Load/ft2 (lb) Load/Fastener (lb)

120 3.8 8.6

370 11.6 26.4

500 15.6 35.7

750 23.4 53.6

1,000 31.3 71.4

Figure 5. Example hysteresis test plot for short-term deflection testing

0

100

200

300

400

500

600

700

800

900

1000

0 50 100 150 200 250

Load (Lbs)

Deflection (1/1000")

Short-Term Deflection Testing - 4 in. Foil-Faced PIC (Thermax)

Test 1 Peak
16

2.3.3 Long-Term Test Protocol

For the long-term testing, the test setup was reduced to a single furring because of space 

constraints and issues with equal load share over two furring strips with fixed dead weights (see 

Appendix D for the full test protocol). The test panels were 8-ft tall by 16-in. wide and anchored 

to a masonry block wall (see Figure 6). Each test panel consisted of the following:

• 2 × 4 wood stud (single stud)

• 3⁄8-in. OSB sheathing

• Building wrap

• 4-in. exterior insulation (two layers of 2-in. material with joints offset)

• 1 × 3 wood furring fastened back to the stud with #10 wood screws at 16 in. o.c. 

vertically (7 fasteners total per panel).

Figure 6. Long-term deflection test panel

The furring strips were loaded with a 213-lb dead weight (see Figure 7). Deflection readings 

were taken every 30 s during the initial loading of the test panels and several times during the 

first day of loading. After the first day, readings were typically taken each day for the following 
17

month, and then every few days thereafter. Temperature and relative humidity (RH) readings 

were also recorded.

Figure 7. Dead weights attached to bottom edge of furring strips

2.3.4 Results and Discussion

2.3.4.1 Short-Term Deflection Testing

Figure 8 and Figure 9 summarize the results of the short-term deflection testing. The deflection 

recorded is the vertical movement differential between the OSB sheathing and the furring strips 

(averaged between the two furring strips). Overlaid on the charts are the ranges of weights by 

cladding types commonly used in the industry. In addition, the common deflection gap sizing is 

highlighted for reference.
18

Figure 8. Short-term deflection testing results (4-in.-thick insulation)

Figure 9. Short-term deflection testing results (8-in.-thick insulation)

1/64 

in.

1/32 

in.

1/16 

in.

1/8 

in.

Adhered Stone Veneers

Stucco

Fiber Cement Siding

Wood Siding

Vinyl/Metal Siding

1/64 

in.

1/32 

in.

1/16 

in.

1/8 

in.

Adhered Stone Veneers

Stucco

Fiber Cement Siding

Wood Siding

Vinyl/Metal Siding
19

The capacity of the system was developed from several sources including the bending strength of 

the fastener, the bearing strength of the furring and framing members, and the compressive 

strength of the rigid insulation, as well as other factors such as static friction between layers (see 

Figure 10).

Shear and rotational 

resistance provided by 

fastener to wood 

connections

Rotational resistance 

provided by tension in 

fastener and 

compression of the 

insulation

Vertical movement 

resistance provided by 

friction between layers

Figure 10. Forces providing vertical displacement resistance

Under initial loading, the load is taken up by the bending of the fasteners as well as the precompression forces induced on the insulation by the tightening of the furring. As the vertical load 

increases, a greater portion of the load will be placed on the insulation through increased 

compression. The increased compression load results from the bending and rotation of the 

fasteners, which creates a normal force on the insulation.

The deflection testing showed that friction between the various layers is reasonably significant in 

the development of the system capacity. During most of the tests conducted, slippage between 

layers occurred as the vertical load overcame the static friction between the layers. This resulted 

in jumps in the deflection readings. Slippage occurred between the layers in products with 

smoother surfaces; however, slippage was not seen for products with rougher surfaces such as 

the rigid mineral fiber insulation.

Because several factors are acting together to develop the system capacity, there is no linear 

relationship of system capacity to number of fasteners. In simpler terms, doubling the number of 

fasteners will not result in a doubling of system capacity. Also, because a portion of the capacity 

is based on compression forces on the exterior insulation from both rotational resistance as well 

as static friction resistance, variations in installation practices may have impacts on the initial 

capacity of the system until the insulation compression and static friction is developed.

Based on the system configuration used in the test setup (fasteners spaced 24-in. o.c. horizontally 

and 16-in. o.c. vertically), the deflection resulting from the dead weight for metal, vinyl, wood, 

and fiber cement siding would be approximately 0.005 in. (1/200 in.) or less for insulation 

thicknesses up to 8 in. For all practical purposes, deflections in this range could be considered as 

zero deflection because of other factors such as material tolerances, construction tolerances, and 

thermal expansion and contraction movement. In addition, moisture expansion and contraction 

movements are greater in magnitude for the materials under consideration.
20

For stucco cladding systems, the amount of deflection anticipated would still be small (less than 

1/32 in. for up to 4 in. of insulation and less than 1/16 in. for up to 8 in. of insulation). Note that 

this initial deflection would happen before the stucco mortar was hydrated and would not cause

cracking of the solid stucco systems later. For cracking concerns, long-term movement of the 

system would need to be reviewed.

For adhered stone veneers, the anticipated deflection given the stud spacing and fastener spacing 

used in the test begins to become more of a concern. Movement in excess of 1⁄8 in. for 4-in.-thick 

layers of insulation and 1⁄4 in. for up to 8 in. of insulation could be possible with very heavy stone 

and thick mortar layers. The capacity of the system could be increased by using closer stud 

spacing and closer vertical spacing of fasteners; however, a linear interpolation based on capacity 

per fastener may not yield the most accurate results because other factors are not considered in 

this simplification as discussed previously.

The effects of insulation type did not appear to have a significant effect on the developed 

capacity of the systems. The relative performances of the types of insulation changed between 

the 4-in. tests and the 8-in. test. For example, the PIC insulation had the second highest capacity 

for the 4-in. test, but the lowest measured capacity for the 8-in. test. It is probable that variances 

in installation practices have a greater impact on the performance than the type of insulation 

material used.

Examination of the data shows that at 0.015-in. deflection (1/64 in.), the capacities of the 

systems are around 10 psf (23 lb per fastener) for EPS and XPS and around 13 psf (29 lb per 

fastener) for PIC and mineral fiber insulation at a thickness of 4 in. As determined by the 

protocol in the TR-12 (AF&PA 1999), the predicted value for #10 wood screws with a 4-in. gap 

is 27 lb. The results correlate reasonably well with the findings of the FSC and NYSERDA/SFA 

research (Crandell 2010). For the 8-in. deflection testing there was a wider range of results at the 

0.015-in. deflection. Capacities were measured between 7 psf (16 lb per fastener) for mineral 

fiber and PIC, 9 psf (21 lb per fastener) for XPS, and 12 psf (27 lb per fastener) for EPS. As 

determined by the protocol in the TR-12, the predicted value for #10 wood screws with an 8-in. 

gap is 14 lb. The predicted value was at the low end of the measured data, and the actual 

measured capacity was always higher.

2.3.4.2 Long-Term Deflection Testing

Figure 11 highlights the results of the long-term deflection testing. Overlaid on the chart are 

common deflection measurements. The systems were loaded with an ultimate load of 213 lb (13 

psf at 24-in. o.c., 20 psf at 16 in.). The weight was chosen to be representative of heavier stucco 

(10 psf to 12 psf) or adhered stone veneer (17 psf to 25 psf) claddings. An additional test using 

XPS insulation was conducted with a dead weight of 53 lb (3 psf at 24-in. o.c., 5 psf at 16-in. 

o.c.) to see if the creep effects differed under small loading. This test was designed to simulate 

fiber cement cladding installation.

For all tests (other than the PIC sample) the long-term deflection values were less than or right 

around 1/32 in. after loading for 6 months. The deflection noted with the PIC sample seemed to 

be demonstrating more potential for ongoing creep. The maximum deflection reached 3/32 in. 

after 6 months with a relatively consistent movement trend. 
21

There is some concern with the results of the PIC test. Two spikes in the deflection of the PIC 

were correlated with construction activities on the wall immediately adjacent to the PIC test 

setup. These construction activities were assumed to cause these spikes. Because of this, 

additional deflection noted in the PIC setup may result from the proximity of the test setup to 

other laboratory equipment and testing activities and not purely from creep effects of the 

insulation.

Figure 11. Deflection of furring strips under sustained load

In the early stages of the testing (the first 3 weeks after the initial loading), very minor additional 

downward vertical movement was seen. The temperature and RH, however, were maintained at a 

more stable range. In all cases a very slight trend for additional deflection can be seen. The 

magnitude, though, was on the order of 0.0025 in. (1/400 in.), and it might not result from creep 

effects from sustained loading. More substantial movement seemed to occur shortly after the first 

3 weeks, when the temperature in the laboratory increased slightly (by approximately 5°F) and 

the RH dropped (from approximately 55% RH to 40% RH). Movements on the order of 0.01 in. 

or 1/100 in. were observed.

Looking at the complete data set, a slight trend in the movement appears to result from 

fluctuations in the temperature and RH. The temperature in the laboratory space fluctuated 

between 60°F and 75°F and the RH fluctuated between 60% and 30% over the course of the 

testing. Deflection movement in the test setups seems to track to these environmental changes. A 

Jumps in the deflection at these 

locations resulting from 

construction activities disturbing 

the test setup 

1/64 

in.

1/32 

in.

1/16 

in.
22

drop in the RH results in a general trend of an increase in the vertical downward deflection of the 

furring strips. It is interesting to note that the converse is true as well. An increase in the RH 

seems to correspond to an upward vertical movement of the furring strips. This was true for all 

insulations except for EPS. The movement of the EPS test panel demonstrated a reverse trend, 

where a drop in the RH resulted in an upward vertical movement of the furring strips.

The test conducted at 5 psf on the XPS sample demonstrated very stable performance with 

almost no movement seen in the sample even with changing temperature and RH.

From the test data, it is difficult to differentiate movements of the samples that result from 

prolonged loading (creep) or from environmental changes. Both positive as well as negative 

movements were noted. The movements from environmental changes are most likely caused by 

material expansion and contraction from moisture adsorption or thermal changes. Given the 

limited testing, the magnitude of this effect cannot be predicted at this point. In addition, material 

property changes may affect performance over the range of actual in-service temperatures. This 

was not accounted for in the testing. Additional testing of exterior samples exposed to a variety 

of temperature and humidity conditions is recommended.

2.3.5 Recommendations

The work conducted by NYSERDA/FSA used an initial deflection limit of 0.015 in. as a basis 

for design (Crandell 2010). By limiting the initial deflection to 0.015 in., the intent was to keep 

long-term deflection caused by potential creep of the system within acceptable limits, although 

these acceptable limits were not defined. As an actual ultimate design criteria, the initial 0.015-

in. deflection limit (short-term) should not be confused with the in-service acceptable deflection 

(initial and long-term combined). 

Based on experience, past research and testing, and the results of the laboratory work, a 1/64-in. 

deflection limit as an in-service standard is too conservative for most practical purposes. Such a 

small movement would not be able to be detected in board and siding installations. For other 

cladding such as adhered stone veneers and stucco, the initial deflection is not as significant an 

issue because the mortars have not been hydrated and the cladding is a viscous fluid (and not 

solid) when the initial movement takes place. Once the stucco and adhered stone veneers have 

cured, though, further movement may become a concern.

Acceptable deflection limits, then, need to be specific to the type of cladding. For lap sidings and 

panel claddings with joints (metal, vinyl, wood, and fiber cement), the 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. As a result, it is 

recommended 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 it is recommended that the service 

deflection limit be set to prevent deflection that may damage the cladding or impair its function. 

For brittle claddings, a limit of 1/64 in. is proposed after initial deflection.
23

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. o.c.) and vertical fastener spacing (up to 24-in. o.c.)—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 limit.

For heavier cladding systems (>10 psf), initial deflection is within the proposed deflection limit. 

Information about potential thermal and moisture expansion and contraction movements, as well 

as creep effects of certain insulation materials in exposed environments, is inadequate for 

predicting long-term in-service deflection. Additional research into the long-term deflection 

movement of heavier claddings in exposed environments is needed. 
24

3 Water Management Details

The use of exterior insulation has been a stumbling block for many designers and contractors. 

Even though the concept is simple, the details required to maintain continuity of the water 

management system can often be confusing.

At the most basic level there are two choices. The water management of the assembly is 

maintained by either placing the water resistive barrier (WRB) interior of the insulation or 

exterior of the insulation. The choice of where it will be applied is a complex one and requires 

consideration and weighing of many factors.

For the most part, placing the WRB to the exterior of the insulation has been the easier of the two 

approaches because the details are largely similar to standard construction practices. Concern is 

often raised about how to support elements that once were positioned in the structural frame 

wall, which are now “pushed” outward into the plane of the exterior insulation (e.g., windows 

and step flashings). 

Conversely, placing the WRB inboard of the exterior insulation (see Figure 12) 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 placement has benefits, however, in that the 

WRB is placed in a more protected location (increasing durability) and the window is located in 

the plane of the existing framing. With either choice, specific details are required to maintain the

continuity of the WRB at the connection of the wall assemblies with other building elements 

such as foundations, roofs, porches, decks, windows, and doors.
25

Figure 12. Typical exterior retrofit detail, showing furring strip cladding attachment

Details were developed to serve as guidance on effectively maintaining the continuity of the 

water management. The matrix of details is broken down into two primary categories:

Wood framed wall construction

• Lap siding

• Building paper

• Board sheathing

• 2 × 4 wood studs

• Interior lath and plaster.

Mass masonry construction

• 3-wythe mass masonry wall

• 3⁄4-in. vertical wood furring

• Interior lath and plaster.
26

A multitude of variations are possible but the BSC team felt that these two baseline assemblies 

were representative of housing structures that are currently targeted for energy retrofits. By 

extension, the details are not intended to cover all possible scenarios that a designer, contractor, 

or homeowner might encounter. Instead, they are intended to inform and guide choices by 

illustrating and discussing the intended goals of the retrofit.

3.1 Exterior Insulation Materials

The primary focus of this research is to examine the installation of exterior rigid insulation. 

Exterior rigid insulation material is generally separated into four product categories3

:

• Foil-faced PIC

• XPS

• EPS

• Rigid mineral fiber (fiberglass or rockwool).

Exterior insulation can be installed in a single or multiple layers (see Figure 13). The number of 

layers will depend on the overall amount of insulation in the design and the available product 

thicknesses. Installations 2 in. and less will typically be done in a single layer. Installations 

greater than 2 in. are more commonly done in multiple layers. When multiple layers are used, 

BSC recommends offsetting the joints both horizontally and vertically to minimize the effects 

that gaps at the board edges may have on the thermal performance of the insulation.

Figure 13. Layering patterns of exterior insulation

 3 The authors of this research also acknowledge and support the use of exterior closed cell spray polyurethane foam 

as an exterior insulation approach. This approach is not discussed in this document because it was outside the scope 

of this research.

Single layer of 

insulation

Double layer of insulation (joints 

offset and staggered)
27

3.2 Water Resistive Barrier

The WRB is generally located either between the insulation and the wall structure or to the 

exterior of the insulation behind the cladding. A third option of placing the WRB in between two 

layers of insulation is also possible, but this is uncommon and can lead to significant confusion 

and coordination problems during construction. Although the third option can be used, it 

generally does not create any significant advantages that would justify the problems it might 

cause. The choice of the location of the WRB will affect many other enclosure connection 

details. The strategy should be clear and consistent throughout the entire project.

The most common WRBs in the residential market today are building or house wraps, which are 

mechanically attached using nails or staples. Although fully adhered air barrier membranes 

(which can often perform the functions of a WRB) have been widely used in the commercial 

market, they have only recently become more common in residential construction. 

Manufacturers are developing and marketing residential-grade versions (permeable and 

impermeable) of these self-adhered membranes. 

Another application that is becoming more prevalent is using liquid applied membranes that are 

sprayed, rollered, or brushed onto the exterior sheathing. Finally, using exterior insulation 

products as WRBs is becoming more and more popular. Many manufacturers currently have ICC 

Evaluation Service approvals for their products to be listed as WRBs. Of the insulation products 

mentioned previously, only foil-faced PIC and XPS are currently recommended for use as WRBs 

through taping and sealing of the joints with sheathing or other compatible construction tape or 

self-adhered membrane flashing. This is an important consideration when choosing an insulation 

product and water management strategy. With thinner insulation thicknesses, adding a building 

wrap over top of the insulation can allow the drainage plane of the assembly to be placed to the 

exterior of any of the insulation products listed previously. As the insulation thickness increases, 

the placement of the building wrap becomes less practical because long cap nails or staples 

would be required to attach the wrap before the furring can be installed. For these assemblies the 

use of the insulation as the WRB is usually more practical.

Figure 14. WRB options with exterior insulation

WRB behind 

insulation

WRB in front of 

insulation

Taped and sealed insulation 

functioning as the WRB
28

3.3 Air Barrier

As part of an exterior insulation retrofit of a building, some consideration should also be given to 

improving the airtightness of the building. With exterior retrofits, it is often convenient to 

integrate an exterior air barrier system as part of the design (see Figure 15). There are significant 

advantages to this placement of the air barrier, with the greatest being simplified continuity (no 

disruption at partition walls or floor separations). As such, the details developed also include air 

barrier continuity issues, though the exact strategy is kept generic. For the purposes of this 

research, the air barrier has been placed at the interface between the existing structural wall and 

the exterior insulation. This is the recommended placement of the air barrier for these types of 

wall assemblies, although BSC recognizes that other possibilities exist.

Figure 15. Recommended air barrier location for retrofit assemblies

3.4 Cladding Attachment

The cladding must always be attached by some means back to the primary structure. For thinner 

insulation thicknesses, it may be possible to directly fasten the cladding through the insulation 

back to the structure. The practical thickness limit is around 1.5 in. for most wood and fiber 

cement siding materials and 2 in. for vinyl siding. This limit is based on the availability of 

fasteners of sufficient length. Most siding guns will have an upper limit of around 3 to 31⁄2 in. for 

nails.

As a result, it has been a common practice for many years to use furring strips fastened back 

through the insulation with screws as the cladding attachment mechanism. Using furring strips 

has other enclosure benefits such as better drainage from behind the cladding and increased 

drying of the assembly.

Mass masonry walls need to be handled in a slightly different way because of practical 

limitations of available masonry anchors and anchor installations. Although it is not impossible, 

using masonry screws to attach the furring back to a masonry structure through thick levels of 

insulation is often considered to be impractical because most masonry screws require predrilling 

WRB in front of the 

insulation

Combined air barrier 

and WRB behind the 

insulation

Taped and sealed insulation 

functioning as the WRB

Air barrier behind the 

insulation
29

of a pilot hole in the masonry. Powder-actuated fasteners are not recommended for use with the 

insulation in place because they can damage the insulation material. The recommended approach 

is to attach 2 × 4 wood studs on the flat directly to the masonry wall before installing the 

insulation (see Figure 16). The 2 × 4 studs then serve as the anchor location for the furring strips.

Figure 16. Recommended cladding attachment design

3.5 Window Integration

Integrating the installation of exterior insulating with window assemblies will depend on the 

window details. The best case scenario is that the window systems are being replaced at the same 

time that the insulation is being installed on the exterior of the building. With this scenario, it can 

be fairly simple to integrate the water management details of the windows with the wall 

assemblies. In some cases, the windows are not intended to be replaced for some time, or not 

intended to be replaced at all. In either case, integrating the exterior insulation may pose a few 

more design challenges. 

3.5.1 Replacement Window Details

With window replacement, the location of the window in the plane of the wall is often driven by 

aesthetics. It is common to desire or require maintaining the exterior appearance of older wood 

sided homes. In this case, the placement of the window will tend to be out in the field of the 

insulation so that traditional exterior trim dimensions can be maintained. This approach also 

creates deeper interior sills that many people enjoy. This window placement strategy is 

colloquially called an “outie” window.

Furring strips attached back 

through the insulation to the wood 

frame structure

Furring strips attached back through the 

insulation to 2 × 4 wood framing that is 

attached to the masonry wall

2 × 4 studs installed 

on the flat

Wood structural 

framing
30

On the other end of the spectrum, windows may also be placed in line with the existing structure, 

an approach colloquially known as an “innie” window. This placement can help to maintain 

existing interior trim, but does result in deeper exterior trim returns.

The placement of the window does not need to dictate the placement of the WRB (or the other 

way around), but there are certain detailing benefits associated with maintaining simple planes of 

water management. With outie windows, it is easy to integrate the frame with the WRB when the 

WRB is located at the front face of the insulation. This combination of window location and 

WRB avoids the need for complicated wrapping and changes in plane of the WRB at a critical 

enclosure interface. By keeping the details simple, the overall risk of problems developing is 

greatly reduced. This does not mean that other combinations should not be undertaken, because 

proper detailing and appropriate material and product use can also reduce risk. In fact, with 

certain materials such as liquid applied membranes, the risks involved with complicated 

geometries of the substrate are practically eliminated, allowing for greater design flexibility.

3.5.2 Integration With Existing Windows

With existing windows,4 extant conditions can create some integration challenges. Older singleglazed wood windows are still common in many areas. These windows may be desired to be 

maintained for various reasons, and multiple strategies can be employed to improve their 

performance. Similarly, in some cases, windows have already been replaced during previous 

retrofits to the home and are not currently planned for replacement as part of the exterior 

insulation retrofit.

The integration details for the exterior insulation with existing windows were developed with the 

intention that the windows might be replaced in the future or that their replacement might 

planned as a separate phase to the exterior insulation retrofit. For this reason, the details were 

developed to allow the windows to be replaced at some point without disturbing the installation 

of the exterior insulation.

Mass masonry walls pose an additional challenge for maintaining existing windows. Most 

windows in mass masonry walls are set back in the first wythe of the masonry rough opening. 

This geometry creates a potential for flanking losses of the thermal insulation. To address this, 

insulation needs to be returned into the rough opening and connected as closely as possible to the 

window frame. Where the existing windows have wide jambs and head trim, the space may be 

readily available. Where the trim is thinner, installing insulation at the returns can be more 

difficult. Products that are designed to accommodate such concerns are currently on the market. 

High thermal resistance insulations with a rated R-value of R-10/in. (such as aerogel insulations) 

would be appropriate to use in instances where the limited space may compromise the desired 

thermal performance.

A secondary concern with mass masonry wall occurs at the window sills. For existing wood 

windows, there is inadequate space to allow even high thermal resistance products to be used. In 

 4 Note that without removal of the window system, existing water and air infiltration conditions may lead to 

durability concerns with the enclosure. Furthermore, when changes to the enclosure are made (such as increasing 

airtightness and adding thermal insulation), previous conditions that might not have degraded the enclosure’s 

performance can sometimes become a concern. The individuals integrating these details are responsible for ensuring 

that no other concerns are preexisting with the building.
31 

these locations, the masonry sill should be removed (see Figure 17). The exposed area can then 

be grouted to create a smooth surface that is positively sloped to the exterior. Subsequently, the 

area is covered with a membrane flashing that is integrated into the WRB of the wall assembly. 

The removal of the sill masonry or stone then provides adequate space to return the insulation to 

the window frame. 

Masonry sill removed to allow for the insulation 

to return back to the window frame

Figure 17. Detail of masonry sill removal to allow for insulation to return to the window frame

3.6 Roof Integration

Several common roof to wall conditions may be encountered (see Figure 18).

• Upper roof to lower wall

(all attic types)

• Upper wall to lower roof

(vented attic or porch)

• Upper wall to lower roof (unvented 

attic)

Figure 18. Common roof to wall connections
32

At the upper roof to lower wall condition, the termination of the exterior insulation does not 

change significantly whether the attic is vented or unvented. The differences would mostly be 

associated with the attic design and not the wall design.

Where an upper wall intersects a lower roof, the termination details are critical and will depend 

greatly on the design of the attic below. For vented attics or porch roofs, the intent is to maintain 

the continuity of the insulation past the roofline because the wall above and the continuation of 

the wall below the roof are both considered exterior wall assemblies.

From an airtightness perspective, what is known as a “chain-saw” retrofit is the most effective 

way to address this detail. In this case, the roof or porch are physically cut from the building, 

allowing the exterior insulation and air barrier (if part of the design) to run continuous past the 

roofline. The roof or porch structure is then reattached or independently supported and flashed 

back into the water management strategy of the building. Alternately, the roof structure may be 

left in place and the insulation installed from above and below the existing roofline. This creates 

greater risk of air leakage and requires more careful detailing of the air barrier system.

For unvented attics, the location of the insulation and the air barrier will also affect the details. 

For these roof to wall connections a chain-saw retrofit will not provide any additional benefit 

because the plane of the insulation and airtightness follows the roofline. 

In all cases, flashing of the roof to wall interface is critical for maintaining the continuity of the 

water management system. The location of the roof flashing will depend on the selected location 

of the WRB of the wall assembly (Figure 19).

Figure 19. Different drainage plane placement for exterior insulation retrofits

If the WRB is designed to be behind the exterior insulation, the step flashing and shingles must 

extend back to the plane of the exterior wood sheathing or masonry wall. A consideration for the 

detailing of this interface is the future need to replace the roof of the building. The roof covering 

will undoubtedly have a shorter service life than the wall cladding. For this reason, a way to 

access the roof to wall interface behind the exterior insulation should be provided so that future 

work can be completed without disrupting the primary siding installation. A minimum 8-in. band 
33

of siding and insulation is recommended for installation at the roof to wall interface. This band 

creates a removable termination to allow for future access to the flashing at the roof to wall 

interface (sequence shown in Figure 20). The band has a secondary benefit as an easy detail for 

installing a kick-out flashing if the end of the roof terminates in the field of the wall.

Step 1:

Install self-adhered membrane flashing at the 

roof to wall interface.

Install kick-out flashing at the edge of the roof.

Step 2:

Install the roof covering and step flashing 

following standard roofing practice.

Shingle lap the top edge of the step flashing with 

the wall WRB or strip in additional membrane 

flashing.
34

Step 3:

Install insulation over the wall area.

Construct an 8-in. gap along the roofline.

Step 4:

Fill the 8-in. gap with a strip of insulation that can 

be removed later to allow for access to the step 

flashing during replacement of the lower roof. 

Figure 20. Roof to wall interface with WRB behind the insulation

If the WRB is designed to be placed at the face of the exterior insulation, more standard roofing 

details can be used. BSC still recommends providing the trim band for access to the roof to wall 

interface, although it is no longer as critical. Attaching the step flashing can be accomplished 

using longer nails that can penetrate through the insulation back to the wall sheathing. As the 

insulation thicknesses increase, this becomes less practical. As an alternative, a strip of OSB or 
35

plywood can be added to the front of the insulation and covered with a self-adhered membrane 

flashing. This plywood or OSB strip would then serve as the nail base for the step flashing, 

resulting in a more conventional installation. The construction sequence is likely to be the single 

greatest challenge that a builder will face with this approach. Roof coverings are installed as 

soon as possible to protect the building from rain infiltration. The concern is that the detail 

requires the wall insulation to be in place before the roof is installed. This has created problems 

on several retrofit projects. A solution to this is to install a strip of wall insulation (that is 

temporarily flashed back to the wall sheathing) at the roof to wall connection to allow the roof to 

be installed independently of the wall system (sequence shown in Figure 21). This requires 

preplanning and coordination during construction.

Step 1:

Install an air barrier transition membrane and 

insulation at the roof to wall interface.
36

Step 2:

Install a self-adhered membrane that extends 

from the roof deck, up and over the insulation, 

and connects to the wall sheathing (this step 

temporarily waterproofs the roof to wall 

connection during the construction process).

Install kick-out flashing at the roof edge. 

Step 3:

Install the roof covering and step flashing 

following standard roofing practice.

Strip in the top edge of the step flashing with 

additional membrane flashing.
37

Step 4:

Install insulation over the wall area.

Shingle lap the joint at the curb with the wall 

WRB or strip in additional membrane flashing.

Figure 21. Roof to wall interface with WRB at the face of the insulation

3.7 Balcony Integration

Balconies (or decks where the drainage is on top of the structure) are most similar to upper wall 

to lower roof interfaces. Again, separating the balcony from the structure to allow the insulation 

and the air barrier to run continuous past the edge of the balcony will maintain the best 

continuity of the thermal insulation and the air barrier. In many cases, however, the balconies are 

part of the building structure, either through cantilevered wood framing members or cast 

concrete as is the case with many masonry buildings. In these situations, cutting and removing 

the balcony may not be feasible.

Wood framed balconies are more easily handled than concrete balconies. If the balcony cannot 

be separated from the building, insulating above and below can often sustain suitable continuity. 

Because of the high conductivity of concrete, the most challenging situation is likely to be the 

cast concrete balcony. 

3.8 Deck Integration

Similar to upper wall to lower roof interfaces with vented attics and porch roofs, the best 

approach from a thermal and air barrier continuity perspective is to use a chain-saw retrofit 

approach with framed decks. In this case, the deck is physically cut from the building, allowing 

the exterior insulation and air barrier (if part of the design) to run continuous past the deck line. 

The deck structure is then reattached or independently supported and flashed back into the water 

management strategy of the building. As an alternative, the deck structure may be left in place 

and the insulation installed from above and below the existing roofline. This creates greater risk 

of air leakage and requires more careful detailing of the air barrier system.
38 

4 Conclusions

In this research project, BSC developed baseline engineering analysis to support the installation 

of thick layers of exterior insulation on existing masonry and frame walls. Water management 

details necessary to integrate windows, doors, decks, balconies, and roofs were also created to 

serve as guidance for integrating exterior insulation strategies with other enclosure elements. The 

details give consideration to complete retrofit as well as phased retrofit approaches. These 

connection details allow for future integration with other high performance enclosure system 

elements.

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, therefore, 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 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 after initial deflection for brittle claddings.

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 is not 

problematic. For these cladding systems, the predicted deflection based on a reasonable 

horizontal spacing (16- to 24-in. o.c.) and vertical fastener spacing (up to 24-in. o.c.), is so slight 

(1/200 in.) and creep effects are so minimal that the predicted 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. This lack of information makes predicting long-term in-service deflections 

difficult. 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 WRB to the exterior of the insulation has been easiest because the 

details are largely similar to standard construction practices. Concern is often raised about a way 

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). 

Conversely, placing the WRB inboard of the exterior insulation has been more difficult for 

contractors to adopt because of some significant departures from standard construction details 
39

and common construction sequences. These concerns increased when these techniques were 

applied to a building retrofit. This WRB placement, however, has benefits. The WRB is placed 

in a more protected location (increasing durability), and the window can be located in the plane 

of the existing framing.

As part of this work, BSC developed details to serve as guidance on effectively maintaining the 

continuity of the water management strategy. These details are presented in Appendix A of this 

report.
40

References

AF&PA (2005). National Design Specification (NDS)for Wood Construction. American Forest 

& Paper Association, Washington, DC. www.awc.org/standards/nds.html

AF&PA (1999). General Dowel Equations for Calculating Lateral Connection Values: 

Technical Report 12. American Forest & Paper Association, Washington, 

D.C. www.awc.org/standards/nds.html 

BSC (2010). Building America Annual Report: 8.1 Westford Habitat for Humanity Prototype 

House, Westford, MA. Somerville, MA: BSC.

BSC (2009a). Building America Annual Report: D.2 HFH of Greater Lowell, Farmhouse 

Renovation, Bedford, MA. Somerville, MA: BSC.

BSC (2009b). Building America Annual Report: D.4 “Lot 26” – Meadows at Cumberland Ridge, 

Dowingtown, PA. Somerville, MA: BSC.

Crandell, J.H. (2010). Task 3 Report: Develop a Prescriptive Table for Attaching Cladding to 

Framing over Continuous Insulation and Develop a Code Proposal. SFA and NYSERDA. New 

York City, NY

Hutcheon, N.B. (1964). “Principles Applied to an Insulated Masonry Wall.” NRC-IRC (National 

Research Council of Canada-Institute for Research in Construction) Canadian Building Digest 

(CBD-50). Ottawa, ON: National Research Council of Canada.

Joyce, D. (November 2009). “Retrofitting Exterior Insulation.” Journal of Light Construction., 

Accessed August 10, 2011 from http://www.jlconline.com/building-envelope/retrofittingexterior-insulation.aspx

Lstiburek, J.W. (2007). “Building Sciences: The Perfect Wall.” ASHRAE Journal (50:May); pp. 

74–78.

Lstiburek, J.W. (2009). “Building Sciences: Building in Extreme Cold.” ASHRAE Journal

(51:February); pp. 56–59.

Lstiburek, J.W. (2010). “Building Sciences: Mind the Gap, Eh!” ASHRAE Journal (52:January); 

pp. 57–63.

Pettit, B. (2009). “Deep Energy Retrofit of a Sears Roebuck House: A Home for the Next 100 

Years.” High Performing Buildings (2:Spring).

Reed Construction Data (2011). RSMeans CostWorks 2011, 15th Annual Edition. Norwell, MA: 

Reed Construction Data.

Straube, J.F.; Smegal, J. (2009). Building America Special Research Project – High-R Walls 

Case Study Analysis. RR-0903. Accessed August 10, 2011 

http://www.buildingscience.com/documents/reports/resolveuid/6cf308d789f2d387127ad58c05ce

892b

Ueno, K. (2010). “Residential Exterior Wall Superinsulation Retrofit Details and Analysis.” 

Performance of the Exterior Envelopes of Whole Buildings XI. Atlanta, GA: ASHRAE.

U.S. Department of Energy, Energy Information Administration (2008). Annual Energy Review 

2007. DOE/EIA-0384(2007). Washington, DC: Energy Information Administration.
41

Appendix A: Water Management Details
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Appendix B: BEopt Simulation Graphs

B.1 Dallas, Texas

Utility Rates: $0.13/kWh, $1.09/therm

Figure 22. Annualized energy related costs versus average source energy savings for Dallas, 

Texas 
108

Figure 23. Average source energy savings reduction versus insulation level for Dallas, Texas 

B.2 Kansas City, Missouri

Utility Rates: $0.08/kWh, $1.23/therm

Figure 24. Annualized energy related costs versus average source energy savings for 

Kansas City, Missouri 

Figure 25. Average source energy savings reduction versus insulation level for 

Kansas City, Missouri
109

B.3 Boston, Massachusetts

Utility Rates: $0.18/kWh, $1.70/therm

Figure 26. Annualized energy related costs versus average source energy savings for 

Boston, Massachusetts

Figure 27. Average source energy savings reduction versus insulation level for 

Boston, Massachusetts
110

B.4 Duluth, Minnesota

Utility Rates: $0.10/kWh, $0.87/therm

Figure 28. Annualized energy related costs versus average source energy savings for 

Duluth, Minnesota

Figure 29. Average source energy savings reduction versus insulation level for Duluth, Minnesota
111

Appendix C: Short-Term Deflection Test Protocol

C.1 Support Structure

A wood framed wall was constructed and anchored to the concrete masonry unit (CMU) wall 

with metal brackets on the side and top to minimize movement of the framed wall relative to the 

CMU wall during testing. The wood framed wall is used to support the insulation and strapping 

for testing. The support frame consisted of 2 × 4 studs spaced at 24-in. o.c. and single top and 

bottom plates. Standard OSB sheathing was screwed into the frame. Plastic house wrap was 

installed to accurately represent the majority of enclosure wall systems in the field. 

C.2 Insulation and Furring Strips

The insulation was installed in either two layers of 2-in. material or four layers of 2-in. material 

depending on which test was being conducted. The insulation was installed with horizontal joints 

offset between adjacent layers, and no vertical joints. The furring strips used for this testing were 

nominal 1 × 3 SPF. The screws were installed at 16-in. o.c. with dimension priority given to the 

bottom of the furring strip. The top screw was placed 1 in. below the top of the furring strip. A 

ratchet setting was used to ensure uniform compression forces along the furring strips, and the 

setting was noted in the test spreadsheet. The two furring strips were offset 9/16 in. below the 

insulation to avoid movement of the loading steel angle.

C.3 Measuring Devices

Using magnetic bases, 21⁄4-in. deflection gauges were installed on a steel bar attached to the 

concrete block wall. Two metal clips were installed near the bottom of each furring strip; these 

were installed for the gauge readings. Two gauges were used to measure the deflection of the left 

and right furring strips; the third measured the displacement of the OSB. Note that any rotational 

deflection of the furring strips may influence the dial gauge reading.

C.4 Loading Mechanism

The loading mechanism used was a hydraulic jack rated to 4,000 lb. A 1,000-lb load cell was 

connected to the top of the hydraulic jack; the load cell was connected to a digital reader to 

convert voltage into mass readings. A 2.5 × 2.5 steel angle was used to transfer the load to the 

two furring strips. To avoid the steel angle hitting the wall, the furring strips were installed 0.5 

in. below the bottom of the supporting wall. 

C.5 Testing Procedure

The following procedure assumes that the framing, sheathing, and sheathing membrane have 

already been installed:

Install the support shelf using the 1⁄2-in. plywood spacers. The plywood spacers ensure 

that the insulation is installed flush with the bottom of the framed test wall, and that the 

furring strips are installed 1⁄2 in. below the edge of the insulation. Align the furring strips 

with the framing and install one screw near the bottom. Align the top of the furring strip 

with the stud marking at the top of the test wall, and finish installing the screws into the 

furring strip from the bottom to the top, inserting the insulation between the sheathing 

membrane and furring strip as required. 
112

Install the angle using two small wood screws to hold it in place temporarily. Ensure that 

the angle is spaced evenly between the two furring strips, so that the push point for the 

hydraulic ram is equidistant from each furring strip. Install the dial gauges and ensure 

that they are all vertical. Install the two angles used as measurement points for the dial 

gauges near the bottom of each furring strip, and ensure that they are level. Place the 

hydraulic jack in the center and ensure that it is vertical. There is a concave point on the 

metal angle in which the end of the hydraulic ram rests.

Apply a small load to the hydraulic ram (~40 lb) and remove the screws that were used to 

temporarily hold the metal angle in place. Previous testing showed that keeping this metal 

angle screwed in place did affect the readings.

The amount of load applied to the furring strips is dependent on the test method. For the first 

series of Building America short-term deflection tests, some hysteresis analysis was conducted 

by applying various loads, and then releasing the load. If hysteresis measurements are taken, the 

hydraulic ram should only be unloaded to approximately 20 lb (<1 lb/ft2

) so that the metal angle 

does not fall off the ram.

For the original set of 4-in. Building America short-term deflection testing, the testing protocol 

was as follows:

1. Load wall to 120 lb (4 lb/ft2

)

2. Unload wall to ~20 lb

3. Load wall to 360 lb (11 lb/ft2

)

4. Unload wall to ~20 lb

5. Load wall to 500 lb (16 lb/ft2)

6. Unload wall to ~20 lb

7. Load wall to 750 lb (23 lb/ft2)

8. Unload wall to ~20 lb

9. Load wall to 1,000 lb (31 lb/ft2)

To unload the wall, especially at high loads, unscrew the release valve very slowly because the 

load will drop very quickly. Make any notes as required in the Excel template for data collection 

and analysis.
113

Appendix D: Long-Term Deflection Test Protocol

D.1 Support Structure

A wood framed wall was constructed and anchored to the CMU wall with metal brackets on the 

side and top to minimize movement of the framed wall relative to the CMU wall during testing. 

The wood framed wall was used to support the insulation and strapping for testing. The support 

frame consisted of a single 2 × 6 stud with a single top and bottom plate. Sixteen-inch-wide 

standard OSB sheathing was screwed into the frame. Plastic house wrap was installed to 

accurately represent the majority of enclosure wall systems in the field.

D.2 Insulation and Furring Strips

The insulation was installed in two layers of 2 in. (16-in. wide). The insulation was installed with 

horizontal joints offset between adjacent layers, and no vertical joints. A single 1 × 3 furring strip 

was installed in the center of the insulation. The furring strips used for this testing were nominal 

1 × 3 SPF. The screws were installed at 16-in. o.c. with dimension priority given to the bottom of 

the furring strip. The top screw was placed 1 in. below the top of the furring strip. 

D.3 Measuring Devices

A reinforced metal angle was attached to the back of the OSB so that the horizontal leg of the 

angle extended past the front surface of the insulation. A deflection gauge was attached to a 

wood block securely fastened to the bottom of the strapping so that the needle on the deflection 

gauge rested on the metal horizontal leg of the metal angle. As the strapping deflected, the metal 

angle remained stationary relative to the OSB sheathing, and the deflection of the strapping 

relative to the OSB was measured.

 

D.4 Loading Mechanism

For the long-term testing, weight was hung from a bracket securely fastened to the outside 

surface of the strapping so that the load is carried in the same position as it would be if cladding 

were attached to the strapping. To simulate cladding weight, buckets were three-quarters filled 

with concrete. The remainder of each bucket was filled with gravel so that the total added weight 

was approximately 213 lb (20 lb/ft2

). These buckets were attached to the bracket with chain, and 

then slowly lowered to a hanging position using a hydraulic automotive jack.

Four of the test walls were loaded to 20 psf, and a fifth comparison wall was loaded to 50 psf.

D.5 Testing Procedure

The first reading was taken at the 30-s mark, which captured the immediate initial deflection 

once the weight was added. Several more readings were recorded during the first day to capture 

any deflection curve, should one exist, and then daily readings were taken Monday through 

Friday until the testing was complete.
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