Expert Meeting Report: 

Cladding Attachment Over 

Exterior Insulation

P. Baker

Building Science Corporation

October 2013
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Expert Meeting Report: Cladding Attachment Over Exterior Insulation 

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-0-40337-03 

October 2013 
iv

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v

Contents

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

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

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

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

1 Introduction........................................................................................................................................... 1

2 Logistics................................................................................................................................................ 2

3 Research Questions............................................................................................................................. 4

3.1 Gravity Load Resistance......................................................................................................4

3.2 Wind Load Resistance .........................................................................................................4

4 Objectives.............................................................................................................................................. 5

5 Agenda................................................................................................................................................... 6

6 Presentation Summaries ..................................................................................................................... 7

6.1 Cladding Attachments of Exterior Insulation: History, Principles, and Codes ...................7

6.1.1 Questions and Discussion ......................................................................................11

6.1.2 Current Code Proposals.........................................................................................12

6.2 Exterior Cladding Attachment Research ...........................................................................12

6.2.1 Questions and Discussion ......................................................................................26

6.2.2 Proposed Deflection Limits and Potential Deflection ...........................................27

6.3 Structural Field Monitoring and Modeling of Thick Continuously Insulated Wall 

Assemblies.........................................................................................................................27

6.3.1 Questions and Discussion ......................................................................................36

6.3.2 Case Study Monitoring Setup:...............................................................................36

6.4 Wind Performance Testing of Walls with Exterior Rigid Foam .......................................37

6.3.1 Questions and Discussion ......................................................................................41

6.4 Recent Wind Loading Research.........................................................................................41

6.4.1 Questions and Discussion ......................................................................................46

6.5 Installing James Hardie Siding Over Exterior Rigid Foam ...............................................46

6.5.1 Questions and Discussion ......................................................................................47

6.6 Open Discussion Period.....................................................................................................47

7 Answers to Research Questions ...................................................................................................... 49

7.1 Gravity Load Resistance....................................................................................................49

7.2 Wind Load Resistance .......................................................................................................49

8 Action Items ........................................................................................................................................ 51

References................................................................................................................................................. 52
vi

List of Figures

Figure 1. Typical ASTM D1761 test setup................................................................................................. 9

Figure 2. Example of measured load versus deflection........................................................................ 10

Figure 3. Example of a single Z furring installation .............................................................................. 13

Figure 4. Examples of double Z furring installations (the one on the left has the same 

performance as a single furring, the one on the right improves performance by 

covering the first Z furring layer with insulation)............................................................................ 13

Figure 5. Example of a clip and furring cladding attachment system................................................. 14

Figure 6. Example of a furring strip attached directly back through the insulation to 

the structure........................................................................................................................................ 14

Figure 7. BSC staff test setup (4-in. gap)................................................................................................ 15

Figure 8. BSC staff test setup (4-in. mineral fiber insulation) .............................................................. 15

Figure 9. Load versus deflection (4-in. gap and 4 in. of insulation) .................................................... 16

Figure 10. Short-term gravity load response test setup ....................................................................... 18

Figure 11. Short-term load versus deflection for assemblies with 4 in. of exterior insulation......... 19

Figure 12. Short-term load versus deflection for assemblies with 8 in. of exterior insulation......... 19

Figure 13. Long-term gravity load response test setup....................................................................... 20

Figure 14. Long-term load versus deflection for assemblies with 4 in. of exterior insulation ......... 20

Figure 15. Discrete load components..................................................................................................... 22

Figure 16. Pre-compression force test setup........................................................................................ 22

Figure 17. Example of friction component test setup (weights placed on top of the furring 

replicate pre-compression clamping forces)................................................................................... 23

Figure 18. Exposed wall assemblies before the furring strips are loaded.......................................... 24

Figure 19. Exposed wall assemblies loaded to representative cladding weights.............................. 24

Figure 20. Exposed wall assemblies covered with a corrugated metal cladding panel.................... 25

Figure 21. Preliminary deflection over time of 4 in. of XPS insulation loaded to 8, 15, and 

30 lb/ft2

................................................................................................................................................. 25

Figure 22. Preliminary deflection over time of 4 in. of EPS insulation loaded to 8, 15, and 

30 lb/ft2

................................................................................................................................................. 26

Figure 23. Graphical representation of FEM .......................................................................................... 28

Figure 24. Example of 1D beam element modeling approaches.......................................................... 29

Figure 25. Results of 1D beam element modeling and system testing............................................... 29

Figure 26. Actual test panel (after testing and elastic recovery) and FEM (showing displacement 

under load) .......................................................................................................................................... 30

Figure 27. Load versus deflection plot of actual test and FEM prediction ......................................... 30

Figure 28. Graphical representation of the full-scale wall assembly with stucco cladding.............. 31

Figure 29. Full-scale wall test assembly (left) and deflection over time (right).................................. 32

Figure 30. Exterior retrofit wall assembly............................................................................................... 33

Figure 31. Wind and gravity load transfer through the wall assembly................................................ 34

Figure 32. Vertical displacement data (2nd floor, south), mm ............................................................. 35

Figure 33. Example of a displacement versus temperature plot ......................................................... 35

Figure 34. Schematic of pressure distribution across wall assemblies ............................................. 37

Figure 35. Example pressure load actuator test setup ......................................................................... 38

Figure 36. Results of wind testing on walls with rigid foam plastic insulation.................................. 39

Figure 37. IBHS full-scale test facility..................................................................................................... 40

Figure 38. Example of scale test house with an air cavity connected to the inside (under the 

wind tunnel) to deal with scaling problems..................................................................................... 42

Figure 39. Example of roof wind pressure coefficients on a gable ended test house....................... 42

Figure 40. Example of air box test setup and grid pattern on a roof assembly ................................. 43

Figure 41. Example of a building with sheathing failure at the gable end wall of the attic space.... 44

Figure 42. Schematic of pressure moderation test rig with idealized cladding ................................. 45

Unless otherwise indicated, all figures were created by Building Science Corporation.
vii

List of Tables

Table 1. Expert Meeting Participants ........................................................................................................ 3

Table 2. Expert Meeting Agenda................................................................................................................ 6

Table 3. Yield Modes From American Forest and Paper Association TR-12........................................ 7

Table 4. Common Cladding Weights (lb/ft2) ........................................................................................... 16

Table 5. Cladding Weight per Fastener (lb)............................................................................................ 17

Table 6. Observed Failure Based on Test Wind Speed and Wall Configuration ................................ 41

Unless otherwise indicated, all tables were created by Building Science Corporation.
viii

Definitions

ANSI American National Standards Institute

ASTM American Society for Testing and Materials

BSC Building Science Corporation

CFD Computational fluid dynamics

DOE U.S. Department of Energy

EPS Expanded polystyrene

FEM Finite element modeling

IBC International Building Code

IBHS Institute for Business and Home Safety

IECC International Energy Conservation Code

IRC International Residential Code

lb/ft2 Pounds per square foot

LVDT Linear voltage displacement transducer

NAHBRC National Association of Home Builders Research Center

NYS New York State

NYSERDA New York State Energy Research and Development Authority

ORNL Oak Ridge National Laboratory

OSB Oriented strand board

PEF Pressure equalization factor

XPS Extruded polystyrene
ix

Executive Summary

Building Science Corporation (BSC) held an expert meeting on Cladding Attachment Over 

Exterior Insulation on Saturday, July 28, 2012 at the Westford Regency Hotel in Westford, 

Massachusetts. Featured speakers included Jay Crandell of ARES Consulting, Peter Baker of 

BSC, Gary Parsons of DOW Chemical Company, Vladimir Kochkin of the National Association 

of Home Builders Research Center, Randy Van Straaten of BSC, and Dawn Cole of James 

Hardie Building Products (presented on her behalf by Jay Crandell). This was followed by an

open question-and-answer discussion period.

The Building America expert meeting focused directly on supporting the Building America 

Standing Technical Committee identified Critical Milestone #1:

By end of 2015, have adopted code language defining the requirements for attaching 

cladding over typical thicknesses of insulating sheathing (i.e., 1”, 1.5”, 2” and 4”) for 

both 16” and 24” o.c. framing.

The meeting focused on issues surrounding cladding attachment and performance of walls with 

exterior insulating sheathing. The topics of discussion were split into two1 separate categories:

1. Gravity load resistance

2. Wind load resistance.

The presentations explored these topics from the perspectives of engineering design, laboratory 

testing, field monitoring, and practical construction. Key results of this meeting were:

Gravity Load Resistance

There was general agreement that the proposed deflection limits (1/16 in. for board/panel 

sidings, and 1/64 in. for brittle claddings) were reasonable given the current state of knowledge.

Most comments from attendees were that, in reality, these limits are still conservative, and that 

greater movements in both cladding types could very likely be tolerated. 

The current engineering cladding attachment design approach has many limitations, as it is 

largely based on fitting research to past datasets and does not account for other aspects that may 

contribute to the development of capacity (friction and compression of insulation). This is being 

weighed against a critical industry need for code guidance on attachment over exterior insulation.

Long-term plans are to work with the America Wood Council to develop the design method to 

enable engineers to calculate solutions.

Creep is currently accounted for in design with safety factors applied to the design values for 

cladding attachment. Current research looking at exposed long-term deflection movement of 

 

1 Seismic load resistance should also be acknowledged here, but no work in this area is currently known and it was 

not discussed at the meeting.
x

cladding assemblies over exterior insulation will help to determine if these safety factors are 

appropriate.

Wind Load Resistance

Research has demonstrated that pressure moderation across the wall assemblies does occur.

Recent full-scale wall tests showed no damage to exposed (no cladding installed) exterior foam 

insulation when tested to extremely high wind loads. This is an encouraging result, as most 

cladding systems will reduce the pressure equalization factor on the insulation and will help 

clamp the insulation in place. Additional testing across a wider range of insulation types with 

various water resistive barriers is recommended. 

The newly available full-scale testing provides differing results about the pressure equalization 

factor of the siding. Further refinement to test methodologies and development of new 

methodologies could help to further refine the design requirements for wall layers.

Research is underway to approximate full-scale test methods using small-scale component 

testing or predictive modeling. The method proposed is to construct a small-scale idealized 

cladding test rig and compare the results to existing 3D flow models. If needed, a new 3D flow 

model will be developed and validated using data from full-scale wall assembly tests.
1

1 Introduction

Adding insulation to building exteriors effectively increases the thermal resistance of woodframed walls and mass masonry wall assemblies. The location of the insulation to the exterior of 

the structure has many direct benefits, including better effective R-value from reduced thermal 

bridging, better condensation resistance, reduced thermal stress, and other commonly associated 

improvements such as increased airtightness and improved water management (Hutcheon 1964;

Lstiburek 2007).

There is significant resistance to the widespread implementation of this approach, because

research and understanding about the performance of cladding systems installed either directly 

over the exterior insulation or to a cladding support system installed over the exterior insulation

is lacking. Performance questions have arisen about the gravity load resistance and the wind load 

resistance of these wall assemblies.

The expert meeting supported the Building America Standing Technical Committee Critical 

Milestone #1:

By end of 2015, have adopted code language defining the requirements for attaching 

cladding over typical thicknesses of insulating sheathing (i.e., 1”, 1.5”, 2” and 4”) for 

both 16” and 24” o.c. framing.

The intent of the meeting was to review the current state of industry knowledge about cladding 

attachment over exterior insulation with a specific focus on:

1. Gravity load resistance

2. Wind load resistance.

The presentations explore these topics from the perspectives of engineering design, laboratory 

testing, field monitoring, and practical construction. By bringing various groups together (who 

have conducted research or have experience in this area), a more holistic review of the design 

limits and current code language proposals can be completed and additional gaps identified. The 

results will help inform design standards and criteria.
2

2 Logistics

Building Science Corporation (BSC) held an expert meeting on Cladding Attachment over 

Exterior Insulation on Saturday July 28, 2012 at the Westford Regency Hotel and Conference

Center in Westford, Massachusetts. Thirty-three people (Table 1), including the six invited 

speakers, attended. Presentations and discussion topics included code development, laboratory 

testing, field testing, in-situ monitoring, and manufacturers’ recommendations. The presentations 

were followed by an open discussion period with all attendees.
3

Table 1. Expert Meeting Participants

Name Organization Email Address

1 Arne Anderson DOW aganderson@dow.com

2 Peter Baker BSC pbaker@buildingscience.com

3 Gary Bergeron Synergy gary@synergy-companies.com

4 Katie Boucher BSC katie@buildingscience.com

5 Rockford Boyer Roxul Inc. rockford.boyer@roxul.com

6 Anne Cope IBHSa acope@ibhs.org

7 Jay Crandell ARES jcrandell@aresconsulting.biz

8 Ken Eklund Washington State 

University eklundk@energy.wsu.edu

9 Paul Eldrenkamp Byggmeister paul@byggmeister.com

10 Ali Fallahi Fraunhofer CSE afallahi@fraunhofer.org

11 Doug Fast Product Testing 

Laboratories doug.fast@jm.com

12 Marquam George U. Alaska Southeast marquam.george@uas.alaska.edu

13 Michael Gestwick NRELb michael.gestwick@nrel.gov

14 Anthony Grisolia IBACOS agrisolia@ibacos.com

15 Brice Hereford Fastenmaster bhereford@olyfast.com

16 Martin Houston Walsh Construction mhouston@walshconstructionco.com

17 Pat Huelman University of Minnesota phuelman@umn.edu

18 David Johnston Vinyl Siding Institute djohnston@vinylsiding.org

19 David Joyce Synergy djoyce75@gmail.com

20 Phil Kaluza Alsaska pkaluza@gmail.com

21 Manfred Kehrer ORNLc kehrerm@ornl.gov

22 Vladimir Kochkin NAHBRCd vkochkin@nahbrc.com

23 Robert LePage BSC robert@buildingscience.com

24 Brian Lieburn Dow wblieburn@dow.com

25 Joseph Lstiburek BSC joe@buildingscience.com

26 Alan Mitchell Analysis North tabb99@gmail.com

27 Gary Parsons DOW gdparsons@dow.com

28 Betsy Pettit BSC betsy@buildingscience.com

29 Chris Schumacher BSC chris@buildingscience.com

30 Sam Taylor DOEe samuel.taylor.sr@gmail.com

31 Kohta Ueno BSC kohta@buildingscience.com

32 Randy Van 

Straaten BSC randy@buildingscience.com

33 Eric Werling DOE eric.werling@ee.doe.gov a Institute for Building Home Safety

b National Renewable Energy Laboratory

c Oak Ridge National Laboratory d National Association of Home Builders Research Center

e U.S. Department of Energy
4

3 Research Questions

The objective of this session was to examine the current state of the industry of cladding 

attachment over exterior insulation. The Standing Technical Committee Critical Milestone #1 is 

intended to support the adoption of code language by the end of 2015 that will allow cladding 

systems to be attached over exterior insulation for a range of thicknesses. BSC identified several 

key questions that needed to be reviewed to inform this process.

3.1 Gravity Load Resistance

What should be the in-service vertical deflection limits? There is a current understanding as to 

the “expected” performance of many of these assemblies (based on research completed to date); 

however, it is important that this information is not confused with “acceptable” performance.

Setting a reasonable acceptable performance metric is key to ensure limitations are not placed on 

emerging or alternate design approaches.

Is the current engineering cladding attachment design approach appropriate? The research and 

design methodology completed to date was built off past research and accepted limit state 

designs of wood-to-wood connections. Much of this past work was completed with the intent to 

remove several factors (such as friction and compression resistance of insulation) from the 

design. Current understanding of the mechanics of these assemblies suggests that these factors 

may be more significant, and possibly dominant, in the development of the system capacity and 

should possibly be included in the design methodology.

How should creep be accounted for in design? A large unknown is still the potential for longterm creep in the assembly. This has traditionally been accounted for by the application of safety 

factors to the design, which may result in overly conservative design values.

3.2 Wind Load Resistance

Are cladding systems and underlying building elements at risk of damage under high wind 

loads? A very conservative assumption is that the cladding elements are loaded to 100% of the 

incident wind load. In reality, the load is shared over many enclosure elements because of

pressure moderation. This can place underlying components, such as taped joints in exterior 

insulation, at risk of being damaged.

Do current test methods properly account for pressure moderation and load sharing across 

enclosure assemblies? The amount of pressure moderation across the assemblies is not 

necessarily well understood. Different testing programs have yielded significantly different 

results. There is still some question about whether the current test protocols accurately capture

the load distribution.

Can full-scale testing of assemblies be replicated or approximated with smaller laboratory testing 

of assemblies? Full-scale building test facilities can examine the performance of cladding 

systems in a full-scale test scenario. Given the scale, the cost of testing would likely be 

prohibitive if large sample sets of various scenarios need to be studied. The intent is to examine 

means of repeating or approximating full-scale test results and data using smaller test protocols.
5

4 Objectives

The expert meeting was designed to bring in experts to discuss issues relating to vertical load 

deflection issues and wind loading issues. The primary objectives were to:

1. Review the current code proposals for issues of approach, results, and potential impacts 

on future work or code adoption.

2. Review gaps in the current knowledge of load resistance mechanisms that are important 

to the development of system capacity.

3. Review the issues around creep in the enclosure assemblies and its impacts on long-term 

performance.

4. Review research into wind load resistance of cladding assemblies attached over exterior 

insulation.
6

5 Agenda

Table 2 includes the agenda that was followed for the expert meeting.

Table 2. Expert Meeting Agenda

Time Speaker Topic

8:30 to 9:00 Mingling and refreshments

9:00 to 9:15 Dr. Joseph Lstiburek

BSC Introduction

9:15 to 10:15 Jay Crandell

ARES Consulting

Cladding Attachments of Exterior Insulation: 

History, Principles, and Codes

10:15 to 10:30 Break

10:30 to 11:15 Peter Baker

BSC Exterior Cladding Attachment Research

11:15 to 12:00 Gary Parsons

DOW Building Solutions R&D

Structural Field Monitoring and Modeling of 

Thick Continuously Insulated Wall Assemblies

12:00 to 1:00 Lunch

1:00 to 1:45 Vladimir Kochkin

NAHBRC

Wind Performance Testing of Walls with 

Exterior Rigid Foam

1:45 to 2:30 Randy Van Straaten

BSC Recent Wind Loading Research

2:30 to 2:45 Break

2:45 to 3:30

Dawn Cole (presented on her 

behalf by Jay Crandell)

James Hardie Building Products

Installing James Hardie Siding over Exterior 

Rigid Foam

3:30 to 4:15 Open discussion

4:15 to 4:30 Peter Baker

BSC Final remarks
7

6 Presentation Summaries

Six main presentations described history, research, and current practice in the area of cladding 

attachment over exterior insulation.

6.1 Cladding Attachments of Exterior Insulation: History, Principles, and Codes

(Jay Crandell – ARES Consulting)

Mr. Crandell provided an overview of the wood connection design. He said that in the United 

States much wood connection design (withdrawal as well as shear) was based on equations fit to 

large sets of empirical data. However, much unexplained variability appeared in the data, 

resulting in the use of large safety factors to account for the variability.

In the 1940s the European Yield Theory was first conceived. This approach is based on an 

equilibrium of forces caused by rotation of fasteners in wood members and predicts performance 

of the connection at the point where yielding of materials (wood or fastener) has developed. The 

equations predict performance of a multitude of failure modes, with the governing mode being 

the one with the lowest capacity. A visual representation of the potential failure modes (AFPA 

1999) is included in Table 3.

Table 3. Yield Modes From American Forest and Paper Association TR-12

Yield Mode Description Graphic

Im Main member bearing failure

Is Side member bearing

II Side and main member bearing
8

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 members

The yield equations do predict reasonably well the joint yield strength based on the yield strength 

of wood in bearing and a fastener in bending. He stated that it can also be used with joints having 

gaps that are filled with 1⁄2–1 in. of Type II expanded polystyrene (EPS) insulation. The equations 

do not, however, predict deflection, nor do they take into account other effects such as friction, 

tension-compression strut (fastener tension to insulation compression), or head effects of the 

fastener (head diameter providing additional rotational resistance).

The European yield equations were calibrated back to old U.S. empirical-based shear design 

values at a rough prediction of performance at 0.015 in. joint slip, by dividing the values by 2.2.

In theory, cladding or furring attachment over continuous insulation was an extension of the 

yield equations. Several studies have examined the capacity of system to resist vertical 

displacement:

FPL RP 469 “Lateral Load-Bearing Capacity of Nailed Joints Based on the Yield Theory” (Aune 

and Patton-Mallory 1986a)

FPL RP47“Lateral Load-Bearing Capacity of Nailed Joints Based on the Yield Theory 

Experimental Verification” (Aune and Patton-Mallory 1986b)

“Fastening Systems for Continuous Insulation” (Bowles 2010)

“External Insulation of Masonry Walls and Wood Framed Walls” (Baker 2013)
9

The intent of the NYSERDA testing was to test a representative range of conditions to verify and 

calibrate the application of the yield equations, and based on this testing derive easy-to-use and 

reliable connection requirements for use by the building industry for common conditions. The 

scope of the testing followed ASTM Test Method D1761 (Figure 1 and Figure 2) and examined:

• Side member (furring/cladding)

• 3⁄4-in. and 3⁄8-in. pine

• 33 mil (20 g) steel hat channel

• Main member (studs/substrate)

• 2x studs (spruce pine fir worst-case 

density)

• 33 mil (20 g) and 54 mil (16 g) studs

• 7/16-in. and 3⁄4-in. oriented strand board 

(OSB)

• Fasteners

• Nails

• Wood screws

• Lag screws

• Self-drilling/tapping screws (steel 

connections).

Figure 1. Typical ASTM D1761 test setup
10

Figure 2. Example of measured load versus deflection

The measured data were compared to the predicted performance of the yield equations (based on 

actual properties of the materials used in the testing). The yield equations predicted tested joint 

load at about 0.015 in. deflection. Based on this result the following design approach was 

developed:

Step 1: Calculate using National Design Specification for Wood Construction yield equations

Step 2: Divide calculate results by 1.5

Step 3: Use calculated per fastener shear strength, cladding weight, and insulation thickness to 

determine fastener spacing

The procedure provided reliable and “workable” solutions and established a framework for 

future improvements.

This work was packaged into several code proposals. The first was submitted to the New York 

State (NYS) building code council and was approved and adopted in the current NYS Energy 
11

Code. It was also submitted for the 2012 International Energy Conservation Code (IECC), and 

2015 International Building Code (IBC), though the proposals did not pass. Future work will 

include public comments, the 2015 International Residential Code (IRC), and the International 

Conservation Code (ICC) 600.

Long-term plans are to work with the America Wood Council (to develop the design method to 

enable engineers to calculate solutions:

• Use the design method to calculate pre-engineered (prescriptive solutions).

• Get the methodology into design standards first, then into codes.

• Alternatively, develop a code compliance report (technical evaluation report).

6.1.1 Questions and Discussion

Validation of the European Yield Equations:

Peter Baker (BSC) question: Were the European yield equations fit for the data?

Jay Crandell (ARES) answer: Yes, the equations came from testing/verification.

Joe Lstiburek (BSC) comment: If you do all the tests with no foam, it is predictable. This is not 

useful for systems with foam.

Jay Crandell (ARES) comment: We’re taking baby steps; things go slowly, especially in code 

hearings. 

Peter Baker (BSC) question: If they tested with 1⁄2 in. to 1 in. of foam insulation, how did they 

“discount” the effect of friction and foam compression?

Jay Crandell (ARES) answer: Basically the test is designed to avoid friction effects. Thin shims 

were used in wood-to-wood connections to avoid friction. The same was likely true for the foam 

insulation tests.

2

Peter Baker (BSC) question: If you gap everything, why do they say that it was tested with 

foam? This effectively decoupled it.

Jay Crandell (ARES) answer: Agreed. They wanted a generalized equation to allow for 

shrinkage of wood members.

 2 After the expert meeting the yield equation validation testing was further reviewed. The test setups that included 

1⁄2-in. to 1-in. EPS insulation between the wood members were not shimmed to discount the friction and 

compression effects of the insulation. The test setups also used very large 40d nails (0.225-in. shank diameter). 

Large shank diameter fasteners would have significantly higher bending resistance than more common smaller 

shank diameter fasteners. The fasteners heads were also not driven flush with the wood furring (to avoid head fixity 

issues). With the head being held away, the furring would be able to slide along the shank reducing the potential for 

friction and compression forces on the insulation to be developed in the system. The theory is that the combination 

of the gap width, the fastener size, and the fastener not being driven flush, would help to reduce the impacts of 

friction and foam compression on the measured capacity of the system and allow for closer alignment with the 

predictions from the yield equations.
12

Jay Crandell (ARES) comment: For foam to reach the plastic limit, you need 14 degrees of 

fastener deflection. We need to get a model to explain it, to help the design. To increase joint 

stiffness/strength, you can increase fastener stiffness/diameter, or increase wood density.

Dividing the yield equation results by 2.2 takes us back to about the same answer as in previous 

codes. This method gives more “intelligence”; for example, you can design a seismic joint that 

has good ductility.

6.1.2 Current Code Proposals

Jay Crandell (ARES) comment: The wood industry has been concerned with the practicality of 

solutions derived by relying on the testing and the yield equations. There has been some 

resistance to a prescriptive table without some type of engineering to ensure that hidden 

conditions will be met.

Jay Crandell (ARES) comment: At the code hearings the wood industry wanted to modify the 

table from 4 in. to 2 in. thickness for the smallest diameter fastener. The industry felt having the 

table approved was a worthwhile and practical move.

Joe Lstiburek (BSC) comment: I agree with the approach for the code; it is the only way to pass 

through this code cycle. However, it’s the wrong approach. Experience in the field and larger 

scale testing show we are conservative. But that won’t matter in the code process. This is a 

political process. I am worried that once it is in the code, it will never come out. If we built a 

basement according to structural codes, we would be far overbuilt compared to an 8 in. wall 

conventional basement. Are we headed down this road?

6.2 Exterior Cladding Attachment Research

Peter Baker (BSC) briefly reviewed the benefits of exterior insulation on the thermal 

performance and condensation resistance benefit of framed wall assemblies. The means of 

attaching the cladding system over the exterior insulation has been a stumbling block for many 

practitioners.

There is a practical limit of installation methods based on the thickness of exterior insulation:

For insulation ≤ 11⁄2 in., attaching the cladding directly through the insulation back to the 

structure is often practical.

For insulation > 11⁄2 in. a secondary cladding support system is often needed.
13

Historically, the secondary cladding support 

system was poorly executed. A common means 

was through the use of a single ZZ-furring that 

extended completely through the insulation

(Figure 3). This single Z furring functions in 

essence like a steel stud, thereby eliminating 

much of the benefit of the exterior insulation.

Other means were though the use of a double Z

furring (Figure 4); however, this was also often 

poorly done as the first Z furring extended 

through the insulation layer and the second Z

furring formed a gap between the cladding and 

the insulation. For a double Z furring system to 

be at all effective, it must be coupled with at 

least two layers of insulation and the first Z

furring completely must be covered by the 

second layer of insulation.

Figure 3. Example of a single Z furring 

installation

Figure 4. Examples of double Z furring installations (the one on the left has the same performance

as a single furring, the one on the right improves performance by covering the first Z furring layer 

with insulation)
14

A further improvement is through the use of a 

clip and Z furring or hat channel approach 

(Figure 5). This approach reduces the thermal 

bridging to small clips (metal or fiberglass) 

that penetrate the insulation layer. The 

concerns with this approach have been the 

resistance by structural engineers to design the 

system without defaulting to larger steel 

support brackets, or higher system costs 

associated with pre-engineered proprietary 

systems.

To help reduce costs and simplify installation, 

a method of directly attaching the furring strip 

through the insulation back to the structure has 

also been used (Figure 6).

Figure 5. Example of a clip and furring cladding 

attachment system

This approach has been met with significant resistance from the industry, which lacks confidence 

that the connection will provide adequate structural resistance to the gravity loads imposed on 

the furring from the weight of the cladding. In reality, this approach has been used for several 

decades on numerous projects, and has demonstrated very good performance. Unfortunately, 

without engineering data, wide acceptance of the approach in industry has been limited.

One of the most common concerns is that the 

insulation provides no capacity for the system.

A simple test conducted by a BSC staff 

member on his own home demonstrated that 

the insulation provides significant capacity and

is critical to the assembly performance.

Two test iterations were conducted. The first 

test loaded two furring strips, gapped 4 in.

from the backup wall, with succession weight 

increments up to approximately 4.5 lb/ft2

(Figure 7). The second test followed the same 

protocol; however, 4 in. of rigid mineral fiber 

insulation was installed between the furring 

strips and the backup wall (Figure 8).

Figure 6. Example of a furring strip attached 

directly back through the insulation to the 

structure
15

Figure 7. BSC staff test setup (4-in. gap)

Figure 8. BSC staff test setup (4-in. mineral fiber insulation)

The test results (Figure 9) clearly demonstrate the additional capacity the insulation layer 

provided to the system, as considerable deflection was noted without the insulation, and almost 

none with the insulation in place.
16

Figure 9. Load versus deflection (4-in. gap and 4 in. of insulation)

Because the insulation is a significant factor in system capacity development, concerns are often 

raised about its structural capacity. Often it is stated that the insulation will crush under load.

This statement is true; however, the context is wrong. Loading the system until failure (500–

1000 lb or more per screw fastener) will crush most rigid insulation materials. This type of 

loading is at least an order of magnitude greater than the expected in-service loads (Table 4 and 

Table 5) of even the heaviest of cladding materials. Under expected in-service loads, the 

insulation materials have adequate capacity. 

Table 4. Common Cladding Weights (lb/ft2)

Low High

Vinyl 0.6 1.0

Wood 1.0 1.5

Fiber Cement 3.0 5.0

Stucco 10.0 12.0

Adhered Stone Veneers 17.0 25.0
17

Table 5. Cladding Weight per Fastener (lb)

Fastener Spacing (in.) 16 × 16 16 × 24 24 × 24

Area/Fastener (ft

2

) 1.78 2.67 4.00

Vinyl 1.8 2.7 4.0

Wood 2.7 4.0 6.0

Fiber Cement 8.9 13.3 20.0

Stucco 21.3 32.0 48.0

Adhered Stone Veneers 44.4 66.7 100.0

The design of the cladding attachment system is governed by acceptable deflection and not 

ultimate capacity. A proposed definition of acceptable deflection follows:

Movement a cladding system can accommodate without causing physical 

damage or exceeding aesthetic tolerances.

With this some limits were proposed:

• Lap sidings and panel cladding ~ 1/16 in.

• Brittle claddings ~ 1/64 in. (after initial deflection).

For lap siding and panel cladding BSC felt that minor deflection would not have the potential for 

physical damage to the siding, so the tolerance should be based on the expected aesthetic limit.

Given normal construction tolerances, most contractors would not be expected to achieve 

anything more precise than 1/16 in. (or even 1⁄8 in.) for a cladding installation.

For brittle claddings (such as stucco and cultured stone), the tolerance was set to a lower 

threshold, as BSC felt that movement could lead to cracking. This is likely a conservative limit,

which was chosen based on a lack of information about how much movement stucco claddings

are subjected to in a normal installation.
18

BSC has researched this cladding attachment approach 

for several years. In 2011, research conducted under the 

Building America program examined short- and longterm loading of wall assemblies using furring strips 

fastened back through the insulation as the primary 

cladding support structure (Baker 2012). The testing 

covered multiple insulation types:

• EPS

• Extruded polystyrene (XPS)

• Foil faced polyisocyanurate

• Rigid mineral fiber.

The short-term load testing was conducted on full 4-ft ×

8-ft wall panels with two 1 × 3 wood furring strips 

spaced 24 in. o.c. (Figure 10). The furring strips were 

attached with #10 wood screws spaced 16 in. o.c.

vertically. Tests were conducted at 4 in. thickness of 

insulation (Figure 11) and at 8 in. thickness (Figure 12).

Figure 10. Short-term gravity load 

response test setup
19

Figure 11. Short-term load versus deflection for assemblies with 4 in. of exterior insulation

Figure 12. Short-term load versus deflection for assemblies with 8 in. of exterior insulation

Adhered stone 

veneers

Stucco

Fiber cement

Wood

Vinyl

Adhered stone 

veneers

Stucco

Fiber cement

Wood

Vinyl

1/64”

1/32”

1/16” 1/8”

1/64”

1/32”

1/16” 1/8”

~ 1/200 in.

~ 1/64 in.

????

~ 1/200 in.

~ 1/64–1/16 in.

????
20

The long-term load testing was conducted on 

2-ft × 8-ft wall panels with a single 1 × 3 wood 

furring (Figure 13). The furring strip was 

attached with #10 wood screws spaced 16 in.

o.c. vertically. Tests were conducted at 4 in.

thickness of insulation only. The test panels 

were loaded to the following levels:

• 13 lb/ft2 (if furring is spaced at 24 in.

o.c.)

• 20 lb/ft2 (if furring is spaced at 16 in.

o.c.)

• 30 lb/fastener.

A fifth test panel was constructed with XPS as 

the insulation material; however, this panel 

was loaded to only 5 lb/ft2 (based on furring 

spaced 16 in. o.c.). Figure 13. Long-term gravity load 

response test setup

The test results (Figure 14) indicate that lightweight claddings (vinyl, wood, and fiber cement) 

have very little movement under either initial or long-term loading (~1/200 in.). The deflection 

for these claddings does not even approach the proposed limit (1/16 in.). The results were in line 

with a long history of performance of buildings constructed with this assembly.

Figure 14. Long-term load versus deflection for assemblies with 4 in. of exterior insulation

1/16 in.

1/32 in.

1/64 in.

Jumps in deflection measurements 

correlating to construction activities 

next to the polyisocyanurate test setup
21

For heavier brittle claddings (stucco and adhered stone veneers), initial deflection is less

important than long-term deflection. For stucco claddings (10 lb/ft2

), long-term deflection after 

initial deflection is within the proposed limit under stable environmental conditions. For adhered 

stone veneer (17–25 lb/ft2), there is a wide range of deflection potential; however, the capacity 

could be increased with increased fastener spacing.

The assemblies in the laboratory seemed to react to even slight temperature and relative humidity 

changes. This could indicate a greater sensitivity to environmental factors than to gravity 

loading. The EPS assembly seemed to have the inverse reaction compared to the other 

assemblies. It moves upward based on changing environmental conditions when other assemblies 

recorded downward movements. The research team could not explain these movements.

During the testing, work being conducted next to the test assemblies resulted in small (< 1/64 in.) 

but notable movement of the polyisocyanurate test setup (as indicated by the dashed red boxes 

on Figure 14). The first jump noted in the test data coincided with construction of a short-term 

wall setup test. During the construction an impact driver was being used; its vibration may have 

disturbed the assembly. The second noted jump occurred when a furring strip moved onto the 

test assembly. This raised questions about the “set in” potential caused by construction activities.

The completion of the 2011 testing raised more questions and highlighted gaps that needed 

additional research. Fundamental to this was the issue of system creep, which was still not well 

understood or quantified, because it is affected by multiple factors:

• Expansion and contraction of wood

• Expansion and contraction of insulation

• Relaxation of wood fibers

• Plastic deformation of insulation.

Many of these are affected by temperature and relative humidity, so the performance of these 

systems needs to be examined in exposed environments.

The exact mechanisms of the load deflection resistance were also not well quantified. Discrete 

load components (such as fastener bending/bearing, insulation compression, and friction forces 

between layers) may have impacted the system capacity, but had not been measured. BSC felt 

that understanding the factors that affect the development of system capacity would be important 

to examine means to engineer the attachment systems.

The Building America research continued into 2012. It focused on testing of the discrete system 

load components and on conduct long-term deflection testing of the assemblies in an exposed 

(outdoor) environment. As in 2011, the testing examined multiple insulation types.
22

• EPS

• XPS

• Foil faced polyisocyanurate

• Rigid mineral fiber.

For the discrete load component testing a series of tests was being completed to measure material 

properties (such as coefficients of friction and the compression modulus of the insulation 

materials), as well as the discrete load components (Figure 15):

• Fastener bending/bearing

• Insulation compression

• Friction forces between layers.

Figure 15. Discrete load components

Another aspect of the assembly that needed to be 

considered and examined was the pre-compression

(clamping forces) imposed on the system from 

driving the wood screw into the structural framing 

member (Figure 16). Previous testing showed that 

the failure mechanism for this assembly with a 

common wood screw was from the head pull of the 

fastener through the furring and not from deformed

insulation. Testing the system until the fastener head 

became overdriven would help to define the upper 

limit of pre-compression forces in the system. A 

small bench top test was completed that placed a 

load cell between a 2 × 4 wood stud and a 1 × 3 

wood furring strip.

Figure 16. Pre-compression force 

test setup
23

The two were fastened together using a #10 wood screw and the load was measured. Preliminary 

results indicated consistent force magnitudes:

• ~150 lb per fastener with the screw head flush with the furring surface

• ~180 lb per fastener with the screw overdriven.

Further testing is planned to examine the relaxation in the load over time.

A custom-built test apparatus is being used for the small-scale discrete load component tests to 

evaluate the individual force resistance components (Figure 17).

Figure 17. Example of friction component test setup 

(weights placed on top of the furring replicate pre-compression clamping forces)

The climate exposure testing is being completed using full-scale wall assemblies loaded to three 

representative cladding weights (Figure 18 through Figure 20):

• Fiber cement

• Stucco

• Cultured stone.

The assemblies are instrumented to measure temperature and relative humidity in the space 

created by the furring strip. The intent is to measure deflection of the assemblies for an entire 

year and compare the measurements to fluctuations in the environmental conditions (Figure 21

and Figure 22). On a few discrete days, the daily fluctuations will also be captured and 

compared.
24

Figure 18. Exposed wall assemblies before the furring strips are loaded

Figure 19. Exposed wall assemblies loaded to representative cladding weights
25

Figure 20. Exposed wall assemblies covered with a corrugated metal cladding panel

Figure 21. Preliminary deflection over time of 4 in. of XPS insulation loaded to 8, 15, and 30 lb/ft2

0”

1/64”

1/32”

1/16”

1/8”
26

Figure 22. Preliminary deflection over time of 4 in. of EPS insulation loaded to 8, 15, and 30 lb/ft2

Testing is currently underway, and the results have not been fully analyzed.

6.2.1 Questions and Discussion

Creep of assemblies:

Joe Lstiburek (BSC) comment: This long-term creep will be irrelevant compared to the 

movement caused by seasonal changes. But we need to go through this exercise to demonstrate 

that point.

System alternates:

Arne Anderson (DOW) question: Wouldn’t there be a benefit for using other types of cladding 

attachment (e.g., Z girts or ripped plywood)? 

Peter Baker (BSC) answer: Sure, but there was a limitation on what we could look at within our 

budget. Perhaps we should just say that if we don’t have a concern, we don’t need to worry about 

other options.

Test program:

Eric Werling (DOE): I am a bit worried about the sequence of testing. Shouldn’t we go for more 

extreme testing, temperature, wind, etc.?

Joe Lstiburek (BSC) answer: We need to do this as baby steps, to fundamentally understand.

Peter Baker (BSC) answer: We have a pretty extreme climate. The testing is done in a very cold

climate zone, with south-facing solar exposure. Also, thick insulation is most appropriate for 

colder climates, so Waterloo is a good representative location.

0”

1/64”

1/32”

1/16”

1/8”
27

Jay Crandell (ARES) comment: Temperature can have a big effect, on creep especially. This 

type of test gives us basic but important insight. Big temperature swings in foam material itself.

Vladimir Kochkin (NAHB) question: You used screws in all these tests; how did you control 

tightness?

Peter Baker (BSC) answer: We tried to mimic construction practice; drive screw heads to flush.

Tension is in the fastener. Measured force on the load cell was 150 lb—the area of distribution 

was not identified.

6.2.2 Proposed Deflection Limits and Potential Deflection

Pat Huelman (University of Minnesota) comment: These are really conservative deflection 

limits, compared to what we see in buildings in reality (e.g., band joist in a stucco building with 

wood lumber framing). Goes well beyond the deflections that we’re seeing in the testing.

Jay Crandell (ARES) comment: We’re looking at all the negatives, but there are positives to 

being able to move. It isolates the movement of the building from the cladding. It’s like resilient 

channels with drywall. The insurance industry pays a lot for stucco and plaster cracks after 

earthquakes, but not for failed buildings.

6.3 Structural Field Monitoring and Modeling of Thick Continuously Insulated 

Wall Assemblies

(Gary Parsons – DOW Building Solutions R&D)

Mr. Parsons began his presentation with a brief overview of finite element modeling (FEM), and 

how the model was validated through testing of discrete components, small-scale system tests up 

to full-scale wall tests (Figure 23) (Parsons et al. 2009).
28

Figure 23. Graphical representation of FEM

Initial modeling focused on the performance of a single fastener. The modeling and testing 

results were then used as inputs for a simple 1D beam element model (Figure 24 and Figure 25).

These connections were modeled in two configurations; one using a washer at the screw head 

(washer contact model) and one using a rigid sheet connecting the fasteners (direct connection 

model).
29

Figure 24. Example of 1D beam element modeling approaches

Figure 25. Results of 1D beam element modeling and system testing

Expanding from this, full-scale wall assemblies were then modeled (3D finite element analysis) 

and tested. The testing examined wind pressure resistance and gravity load resistance.

For the wind load resistance, the pressure drops when air leakage begins; however, it would be 

complex to model such failure criteria in an FEM. Based on test results, a maximum displacement of about 1 in. anywhere in the panel approximately corresponds to failure (air leakage). 

This criterion is used in FEM to predict the pressure at failure (Figure 26 and Figure 27).
30

Figure 26. Actual test panel (after testing and elastic recovery) and FEM 

(showing displacement under load)

Figure 27. Load versus deflection plot of actual test and FEM prediction

A stucco clad wall assembly was used for the gravity load resistance analysis. Finite element 

analyses were completed using both the washer contact model and direct connection 1D beam 

models with various edge and field spacings of the stucco lath attachment. A 4-ft × 8-ft full-scale 

wall assembly was also constructed and instrumented with linear voltage displacement 

transducers (LVDTs) to measure the gravity-induced creep over time (Figure 28).
31

Figure 28. Graphical representation of the full-scale wall assembly with stucco cladding

The FEM results had the following maximum vertical displacement values:

• Washer contact model = 2.25 mm

• Direct connection model = 0.5 mm.

The maximum stress in the foam insulation was indicated to be 0.05 MPa, which is one third the 

foam yield stress (one third maximum allowable stress ~ safety factor of 3).

By comparison, the magnitude of creep displacement of the stucco with respect to foam is about

0.04 mm after about two months, when the displacement appears to level off (Figure 29). The 

magnitude of the displacement confirms that the fastener layout based on FEM prediction of 

gravity loading is on the conservative side.
32

Figure 29. Full-scale wall test assembly (left) and deflection over time (right)

Mr. Parsons then discussed a case study of a residential building retrofit that was constructed in 

Vancouver, BC in Canada, where the movement of the cladding was monitored over time

(Parsons et al. 2012).

The original building was construction with the following wall assembly:

• Cement stucco with wire lath

• Semi-rigid fiberglass insulation (~ 1 in. thick)

• 31⁄2-in. steel studs with fiberglass batt insulation infill

• Polyethylene air/vapor barrier

• 1⁄2-in. interior drywall.

The retrofitted wall assembly was constructed as follows (Figure 30):

• 7⁄8-in. acrylic stucco on paper backed lath

• 7⁄8-in. Z-girts at 16 in. o.c. fastened with self-tapping screw fasteners at 6 in. o.c.

• 3-in. Type 4 rigid insulation (R-15) with peel and stick flashing at joints

• Self-adhered membrane

• 1⁄2-in. fiberglass faced exterior gypsum sheathing

• Existing 31⁄2-in. steel studs

• Existing 1⁄2-in. interior drywall.
33

Figure 30. Exterior retrofit wall assembly

The structural design of the cladding attachment functioned based on the following elements:
34

Wind and gravity loads are transferred 

through the exterior vertical Z girts to the 

insulation and backup wall.

The rigid girt spreads gravity and wind 

load onto rigid insulation.

The gravity load puts a tension load on 

the fastener because rotation is 

constrained by insulation (the fastener 

cannot rotate unless the foam compresses) 

and a shear load.

Wind and gravity put a compression load 

on the rigid insulation or tension load on 

fastener (Figure 31).

Figure 31. Wind and gravity load transfer through the wall assembly

LVDTs with an accuracy of 0.085 mm ± 5% were used to measure displacement. The LVDTs 

were placed in six locations in a test block of insulation. The sensors were placed to measure 

movement in the vertical y direction (three sensors at the bottom of the panel), in the lateral or x 

direction (two sensors in either edge of the panel), and in the out-of-plane or z direction (one

sensor placed near the middle of the panel). These test panels were placed at multiple floor 

heights on various elevations.

Displacement data were collected for more than one year (Figure 32). The results indicated 

stable performance and evidenced no creep over time.
35

Figure 32. Vertical displacement data (2nd floor, south), mm

The data were also analyzed to examine the thermal expansion and contraction correlation to the 

measured displacement (Figure 33). No correlation could be seen in the data collected.

Figure 33. Example of a displacement versus temperature plot
36

The results of the Vancouver monitoring project showed that displacement ranges (x, y, and z 

directions) are negligible and do not depend on measurement location. Also, there was a poor 

correlation between the temperature and displacement measurements. Overall the building 

performs well; no stucco performance problems have been reported to date.

It was also demonstrated that the FEM correlates reasonably well to connection scale tests;

however, FEM generally overpredicts movement of full-scale test assemblies.

6.3.1 Questions and Discussion

Finite element analysis:

Mike Gestwick (NREL) question: if finite element analysis consistently overpredicts, that should

suggest something is wrong with the model, right?

Gary Parsons (DOW) answer: Part of the mismatch may be the complexity of the building 

(corners, windows, etc.).

Mike Gestwick (NREL) comment: But you see the same thing on the 4 × 8 sections.

Modeling compared to case study:

Arne Anderson (DOW) question: What were the differences between the modeling and the test 

site?

Gary Parsons (DOW) answer: There was no Z-furring in the model and effectively fewer 

fasteners at the test site. We did not create more models to do this actual wall system.

6.3.2 Case Study Monitoring Setup:

Vladimir Kochkin (NAHB) question: Could you describe the y deflection—what was being 

measured relative to what?

Gary Parsons (DOW) answer: Sensors were placed as outboard as possible in the foam so as not 

interfere with stucco install, but still within the foam. Measurements were taken relative to 

brackets, through the DensGlass sheathing, attached to steel. The intent was to find a reference 

that will not move—ideally a post in the ground—but that won’t happen.

Anthony Grisolia (IBACOS) question: Any movement at the control joints?

Gary Parsons (DOW) answer: Not sure.

Anthony Grisolia (IBACOS) question: With the three-coat stucco, are there any data on each 

applied layer creeping?

Gary Parsons (DOW) answer: Sensors were on the building when stucco was being applied, but I 

don’t think the measurement could have made out the deflection from installers applying stucco

on the building.

Anthony Grisolia (IBACOS) question: How much total data?

Gary Parsons (DOW) answer: Probably will run another three years.
37

6.4 Wind Performance Testing of Walls with Exterior Rigid Foam

Vladimir Kochkin (NAHBRC) began his presentation by reviewing the issues surrounding wind 

pressure resistance of walls with exterior rigid foam insulation. Its importance is related to 

market implications, as the 2012 IECC will require exterior rigid insulation for walls in climate 

zones 3 and higher.

Exterior wall systems are loaded by positive wind pressure forces and leeward suction forces. A 

simple model would assume that 100% of the pressure is registered across the exterior sheathing 

layer only. In reality, the pressure is distributed across all the wall layers (Figure 34).

Figure 34. Schematic of pressure distribution across wall assemblies

The pressure distribution can be described by determining the pressure equalization factors 

(PEFs) for each layer of the wall assembly. The PEFs are evaluated based on the following 

equation:

PEF layer = ∆P layer/∆P total assembly

Understanding the pressure distributions is key to optimizing assembly designs to properly 

manage wind loads.

Two test programs that looked at these issues:

? lb

? lb

? lb
38

• High-R Walls for New Construction Structural Performance: Wind Pressure Testing 

(DeRenzis and Kochkin 2013)

• Wind Loads on Components of Multi-Layer Wall Systems with Air-Permeable Exterior 

Cladding (Cope et al. 2012).

The results from these test programs were used in the development of ANSI/SBCA FS100 –

2012 Standard Requirements for Wind Pressure Resistance of Foam Plastic Insulating Sheathing 

Used in Exterior Wall Covering Assemblies (SBCA 2012).

The NAHBRC testing used nominal wall specimens that were 4–9 ft tall and 4–12 ft long. The 

wall assemblies were instrumented with pressure sensors and deformation sensors. The loads 

were applied using pressure load actuators that can deliver a very quick response to modulate 

pressure loads on the test samples (Figure 35).

Figure 35. Example pressure load actuator test setup

Several types of wall assemblies were tested, including:

• Group 1 (exterior foam only)

• Group 2 (exterior foam + interior gypsum board) untaped

• Group 2 (exterior foam + interior gypsum board) taped

• Group 3 (vinyl siding, exterior foam + interior gypsum board)

• Group 4 (vinyl siding + exterior foam).
39

The assemblies were tested to failure with the peak pressures recorded. Vinyl siding effectively 

acts as a plate washer for the foam sheathing. PEF measurements, however, indicated vinyl 

siding does not pick up a significant load (< 0.1). The PEF was split roughly 50/50 foam versus

gypsum (Figure 36).

Figure 36. Results of wind testing on walls with rigid foam plastic insulation

The second series of test was conducted at the IBHS full-scale testing facility (Figure 37). This 

testing was conducted through a large project team, including:

• IBHS

• Foam Sheathing Coalition

• NAHBRC

• Vinyl Siding Institute

• American Chemistry Council

• ORNL/DOE
40

Figure 37. IBHS full-scale test facility

Testing was completed on two 2 × 4 wall systems with interior gypsum finish.

1. Vinyl siding over 1 in. of XPS

2. Vinyl siding over 7/16 in. of OSB.

Measurements were taken via 32 pressure tap locations on each test wall. The objectives were to 

measure the PEFs, measure the ultimate capacity of the wall assemblies, and observe the failure 

modes (Table 6).
41

Table 6. Observed Failure Based on Test Wind Speed and Wall Configuration

Test 

Seq. # Siding Wind Speed(mph at 16 ft)

Pressure

(lb/ft2) Angles Failure

1 Vinyl 43–46 < 10 360° at 

every 10° No damage

2 Vinyl 58–79 < 25 10 different 

orientations No damage

3 Vinyl 87–103 < 30 0° and 180° Damage to vinyl 

(both XPS and OSB)

4 No siding 105–107 < 35 0° No damage to foam

The results indicate that the vinyl siding resists as much as 75%–80% of the exterior negative 

design pressures. These results were greater than the results from the NAHBRC air box tests.

The differences between the pressure box and wind tunnel test are probably due to differences in 

uniform pressure compared to pressure gradients. Air can move behind vinyl siding, so that the 

pressure profile of the exterior is different than pressure profile behind vinyl siding. With 

increased pressures, more layers pick up the load. In runs with no vinyl siding, the load is mostly 

on the foam, and little is on the gypsum.

As long as the siding remains in place, the sheathing experiences 55%–60% of the peak negative 

and positive pressure acting on the wall system. The peak pressures across the gypsum wall 

board were generally 50%–60% of the pressure across the wall system. The vinyl siding products 

tested still had very good performance (failed at 120 mph, 33-ft elevation). A question was raised 

about whether the siding component should be considered when designing the foam sheathing.

The PEF for foam was greater without the siding.

Further research is needed to understand the actual force (rather than pressure at a single 

infinitely small location) on the vinyl siding fasteners. Other issues that need to be considered are 

the impacts on capacity of vinyl attached to OSB only (not stud penetration), or vinyl siding over 

thicker foam attached to vertical 1x framing (gap between siding and sheathing).

6.3.1 Questions and Discussion

Rating of vinyl siding:

Question: What was the siding rated for in terms of mph?

David Johnson (Vinyl Siding Institute) answer: It is not a direct relationship, but ASTM D5206 

testing provided values in the same ballpark of pressures where it failed. The point where it 

failed was comparable to where you would expect this siding to fail.

6.4 Recent Wind Loading Research

Randy Van Straaten (BSC/University of Western Ontario) began his presentation by reviewing 

some basics of wind testing. Unlike the IBHS full-scale testing laboratory, most wind testing 

needs to be completed using either scale models or pressure boxes installed over specific areas of 

a building or enclosure assembly.
42

With scale models, pressure relationships between areas and volumes are not directly scalable

(Figure 38). Certain adjustments need to be made to compensate for the differences.

Figure 38. Example of scale test house with an air cavity connected to the inside 

(under the wind tunnel) to deal with scaling problems

The National Institute of Standards and Technology hired the University of Western Ontario to 

put together time series of pressures on the outsides of residential-scale buildings. The work 

provided a database of pressure coefficients around the building areas (see Figure 39).

Figure 39. Example of roof wind pressure coefficients on a gable ended test house
43

Another example of testing looked to evaluate the pull off/pull through of fasteners attaching 

wood sheathing to roof framing. For this testing, an array of air boxes was attached to a roof in a 

grid to achieve spatial variation of pressure (Figure 40). This testing compared fluctuating 

loading versus ramp loading. The results indicated most nail fasteners demonstrated similar

performance, while staples demonstrated only 50% capacity compared to the nails Hurriquake 

nails have the highest resistance. Staples do, however, have some benefits, in that installers 

typically use many to increase resistance.

Figure 40. Example of air box test setup and grid pattern on a roof assembly

Testing wall claddings presents more difficult problems. Wall claddings are typically leaky (air 

porous), which requires more fan power. It is also difficult to attach an array of air boxes to 

flexible cladding systems without restricting movement.

There is some question about the significance of pressure moderation. Historically there have 

been numerous failures of wall sheathing at gable end walls (Figure 41). Dr. Kopp studied gable 

end failures and determined that they could not be explained solely by pressure differences, 

aerodynamics (more wind hitting it), or spacing of studs. He determined that drywall must take a 

significant portion of the pressure drop.
44

Figure 41. Example of a building with sheathing failure at the gable end wall of the attic space

The challenge is developing a method to predict the PEF. Several options are available; each has

advantages and disadvantages.

1. Empirical model from IBHS testing.

a. Cumbersome and expensive

2. Wind tunnel testing

a. Scale issues (small-scale siding)

b. Reynolds number effects (e.g. for flow through vinyl siding gaps)

c. Having only a full scale section in wind tunnel would not capture the bluff body

aerodynamic of the overall house structure

3. Computational fluid dynamics (CFD)

a. A wide range of length and time scales that need to be considered and could make 

CFD modeling difficult

b. Difficulties with modeling flow separation and turbulence

4. Apply/adapt an available model 

5. Develop full-scale component testing.

Currently, the most viable options appear to be apply/adapt an existing model, or assembly-level 

testing (which includes spatial and temporal variations).

Earlier flow network models examined the aerodynamic performance of roof pavers. There are 

two models to look to: the Sun and Bienkiewicz Model and the Gerhardt Model about the roof 
45

pavers. Some insights that can be gleaned from examining these models are that pressure 

moderation improves as the ratio of opening size to cavity depth increases, and greater flow 

resistance underneath. Also increasing the permeability of the pavers increases the pressure 

equalization (Figure 42).

Figure 42. Schematic of pressure moderation test rig with idealized cladding

Flow network models:

• 3D models

• Apply Darcy’s law for flow through openings—dominated by wall friction losses

• Apply Darcy’s law for cavity flow—assuming laminar flow (no Reynolds effects)

• Quasi steady state assumptions—no inertial or compressibility effects.

The current research plan is to:

• Analyze IBHS data.

• Build a test rig—start with idealized claddings.
46

• Compare to existing models.

• Develop an adequate model.

• Test the model with IBHS data.

The intent is to develop a means to test cladding systems using small-scale component tests that 

will provide results that can be reasonably correlated with full-scale test results. This will allow a 

wider range of cladding types to be tested in shorter periods and at much lower cost than fullscale building tests.

6.4.1 Questions and Discussion

None.

6.5 Installing James Hardie Siding Over Exterior Rigid Foam

(Dawn Cole – James Hardie Building Products, Inc.)

This presentation was given by Jay Crandell on behalf of Ms. Cole.

The current James Hardie installation requirements allow fiber cement siding to be installed 

directly over foam insulation up to 11⁄2 in. thick. This is a change from past literature that limited 

the thickness to 1 in. For foam sheathing thicker than 11⁄2 in., and up to 4 in., the fiber cement 

siding can be installed to the furring/strapping installed atop the insulation.

Wood furring:

• Shall have a specific gravity consistent with fastener holding capacity (typically 0.36 or 

greater).

• Shall be nominally 4 in. wide and thick enough for full engagement with fastener.

• James Hardie cautions the use of pressure-treated lumber and stainless steel fasteners.

Steel strapping:

20 gauge thickness minimum fastened with pins.

For wind load resistance, the fastener must have a solid connection of siding to a nailable 

substrate with a minimum 11⁄4 in. of fastener penetration into framing or full net penetration of 

furring connected to the framing. The shank diameter must also remain consistent for the 

required wind load.

James Hardie is concerned about the cantilevering effect, but has not seen a catastrophic 

response. Some issues in siding applications are nail shine (diversion of the fastener to miss the 

framing) and in rare circumstances, water migration has followed the fastener, through the foam, 

and into the framing.

Dimpling of siding has been noted, caused by the compressible nature of the foam insulation.

Field inspection of these occurrences revealed poor quality practices of the siding installation.
47

ASTM E330 Standard Test Method for Structural Performance of Exterior Windows, Doors, 

Skylights and Curtain Walls by Uniform Static Air Pressure Difference evaluations identified the 

following minimum limits:

• Type II (EPS)

• Type X (XPS) per ASTM C578 (2012)

• Type 1 (polyisocyanurate) per ASTM C1289 (2013).

• The recommendations are summarized in James Hardie Building Products Technical 

Bulletin 19.

6.5.1 Questions and Discussion

None.

6.6 Open Discussion Period

At the end of the presentations an open discussion period was held. A summary of the 

discussions follows.

Attachment of wood furring over exterior insulation:

Anthony Grisolia (IBACOS) question: Does this research apply to retrofits, in addition to new 

construction? Particularly attaching to brick?

Peter Baker (BSC) answer: Assuming that the baseline is wood frame construction, going back 

to wood frame makes sense. Overclad of masonry is a similar concept. Use 2 × 4 wood members 

attached to the masonry with insulation between the wood studs as the base layer, with the 

furring strips attached back through subsequent insulation layers into the wood studs.

Pat Huelman (University of Minnesota) comment: Fastening a furring strip to the building or 

substrate has been addressed. Nailing through foam can cause blowouts or problems; but I have 

not seen it with screws. All fasteners are not the same in terms of interactions (as opposed to 

structural requirements).

Jay Crandell (ARES) comment: I have not seen blowout, but have seen head pull-through, which 

is predictable. But many washers are available to improve pull-through resistance. Remember, 

Vladimir’s work at 100 lb/ft2 exceeds Florida’s wind code requirements.

Use of spray polyurethane foam:

Anthony Grisolia (IBACOS) question: We are doing nine homes with NYSERDA on retrofits.

Does anyone know anything about applying spray foam to exterior walls? (General reference 

back to John Straube’s work.)

Anthony Grisolia (IBACOS) comment: On the retrofit side, one problem is old siding with lead 

paint, as it is expensive to remove the lead paint. Going over the existing siding with spray 

polyurethane foam encapsulates the lead paint.

Acceptable cladding movement and deflection limits:
48

Peter Baker (BSC) question: In regards to the proposed acceptable deflection limits for cladding 

products, are we in the right range, or should we reconsider?

Brian Lieburn (DOW) comment: In terms of lightweight claddings, the 1/16 in. limit makes 

perfect sense. There is considerably more variation in practice. But if the entire wall surface 

deflects/sags uniformly, there is no real problem from a consumer acceptance standpoint.

Uneven settlement (lap siding) results in uneven instead of straight lines in laps; this could be a 

problem. A surprisingly small tolerance there, but I can’t put a number on it. It might take some 

consumer focus groups.

Peter Baker (BSC) comment: I think the main concern is at window and door openings as they 

are rigidly attached to the structure, and the differential movement between the cladding and the 

fenestration can result in problems. We have solutions to these problems by providing a 

movement joint at the interface (such as a sealant joint between the window and the stucco Jbead).

Published work:

Vladimir Kochkin (NAHB) question: Is BSC’s and DOW’s work published?

Gary Parsons (DOW) answer: Mostly.

Peter Baker (BSC) answer: 2011 research has a final technical report that has been published.

Builder experience:

Dave Joyce (Synergy Construction) comment: We have done 30 buildings with 4 in. of foam, 

and have not seen any creep yet.

Kohta Ueno (BSC) question: Does anyone have any good solutions to shingles on the outsides of 

foam walls? Paul Eldrenkamp with Byggmeister built a house in the 1980s (?) with horizontal 

strapping, 6 in. of exposure. Ended up being a huge amount of strapping, and thus cost. Didn’t 

bother kerfing the back of the strapping for drainage, but it is still on its original coat of paint and 

looks fine (coated six sides). In later cases, he used the prefabricated shingle panel products.

Peter Baker (BSC) answer: Use a second layer of wood sheathing and building wrap over the 

furring as a nail base for the shingles.
49

7 Answers to Research Questions

7.1 Gravity Load Resistance

What should be the in-service vertical deflection limits?

There was general agreement that the proposed deflection limits (1/16 in. for board/panel 

sidings, and 1/64 in. for brittle claddings) were reasonable given the current state of knowledge.

Most of the comments from attendees were that in reality these limits are still conservative, and 

that greater movements in both cladding types could very likely be tolerated. 

Is the current engineering cladding attachment design approach appropriate?

The current design approach has many limitations, as it is based to a great degree on fitting 

research to past datasets and does not account for other aspects that may contribute to the 

development of capacity (friction and compression of insulation). This is being weighed against 

a current critical industry need for code guidance on attachment over exterior insulation. Long 

term plans are to work with the AWC to develop the design method to enable engineers to 

calculate solutions.

How should creep be accounted for in design?

Creep is currently accounted for with safety factors applied to the design values for cladding 

attachment. Current research looking at exposed long-term deflection movement of cladding 

assemblies over exterior insulation will help to determine if these safety factors are appropriate.

7.2 Wind Load Resistance

Are cladding systems and underlying building elements at risk of damage under high wind 

loads? 

Research has demonstrated that pressure moderation across the wall assemblies does occur.

Recent full-scale wall tests showed no damage to exposed (no cladding installed) exterior foam 

insulation when tested to extremely high wind loads. This is an encouraging result, as most 

cladding systems reduce the PEF on the insulation and help clamp the insulation in place.

Additional testing across a wider range of insulation types with various water resistive barriers is 

recommended. 

Do current test methods properly account for pressure moderation and load sharing across 

enclosure assemblies?

The newly available full-scale testing provides differing results about the PEF of the siding.

Further refinement to test methodologies, and development of new methodologies, could help to 

further refine the design requirements for wall layers.

Can full-scale testing of assemblies be replicated or approximated with smaller laboratory 

testing?
50

Research is underway to try to approximate full-scale test methods using small-scale component 

testing or predictive modeling. The method proposed is to construct a small-scale idealized 

cladding test rig and compare the results to existing 3D flow models. If needed, a new 3D flow 

model will be developed and validated using data from full-scale wall assembly tests.
51

8 Action Items

Draft code language has been developed; SNY has already adopted it. Proposals submitted to the 

IBC code hearings, however, failed to be adopted. The language is to be further reviewed and 

modified based on responses from the IBC code hearings, and results of this meeting to refine 

the proposals for the IRC hearings in January 2013. Jay Crandell of ARES Consulting has been 

leading this effort.

BSC is to continue conducting research in the area of gravity load resistance. The focus is on 

trying to better understand and quantify the potential impacts of long-term creep on the vertical 

displacement of cladding systems.

The University of Western Ontario will continue to research the pressure moderation effects and 

wind resistance of wall cladding systems. The intent is to help develop test methods and analysis 

tools to allow for cost-effective wind resistance analysis of a wide range of cladding materials.
52

References

American Forest and Paper Association. (2005). “National Design Specification for Wood 

Construction, ANSI/AF&PA NDS-2005.” Washington, DC: American Forest and Paper 

Association.

American Forest and Paper Association. (1999). “General Dowel Equations for Calculating 

Lateral Connection Values. Technical Report 12.” Washington, DC: American Forest and Paper 

Association.

American Society for Testing and Materials. (2010). “ASTM E330 – 02(2010) Standard Test 

Method for Structural Performance of Exterior Windows, Doors, Skylights and Curtain Walls by 

Uniform Static Air Pressure Difference.” West Conshohocken, PA: ASTM International.

American Society for Testing and Materials. (2012). “ASTM C578 - 12b Standard Specification 

for Rigid, Cellular Polystyrene Thermal Insulation”, West Conshohocken, PA: ASTM 

International.

American Society for Testing and Materials. (2013). “ASTM C1289 - 13e1 Standard 

Specification for Faced Rigid Cellular Polyisocyanurate Thermal Insulation Board.” West

Conshohocken, PA: ASTM International.

Aune, P; Patton-Mallory, M. (1986a). “Lateral Load-Bearing Capacity of Nailed Joints Based on 

the Yield Theory.” Forest Products Laboratory Research Paper FPL RP 469, Madison, WI: U.S. 

Department of Agriculture, Forest Service Forest Products Laboratory.

Aune, P; Patton-Mallory, M. (1986b). “Lateral Load-Bearing Capacity of Nailed Joints Based on 

the Yield Theory Experimental Verification.” Forest Products Laboratory Research Paper FPL 

RP 470, Madison, WI: U.S. Department of Agriculture, Forest Service Forest Products 

Laboratory.

Baker, P. (2013). “External Insulation of Masonry Walls and Wood Framed Walls”, NREL 

Report No. SR-5500-54643, Golden, CO: National Renewable Energy Laboratory. Retrieved 

from http://www.nrel.gov/docs/fy13osti/54643.pdf

Bowles, L. (2010). “Fastening Systems for Continuous Insulation” Albany, NY: New York 

State Energy Research and Development Authority. Retrieved from www.nyserda.ny.gov/-

/media/Files/Publications/Research/Other%20Technical%20Reports/fastening-systems-forcontinuous-insulation.pdf.

Cope, A.D.; Crandell, J.H.; Johnston, D., Kochkin, V.; Liu, Z., Stevig, L.; Reinhold, T.A. (2012). 

“Wind Loads on Components of Multi-Layer Wall Systems with Air-Permeable Exterior 

Cladding.” Proceedings of the ATC/SEI Advances in Hurricane Engineering Conference, Miami, 

FL October 24-26, 2012. 
53

DeRenzis and Kochkin. (2013). “High-R Walls for New Construction Structural Performance 

Wind Pressure Testing.” Upper Marlboro, MD: National Association of Home Builders Research 

Center.

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.

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

pp. 74–78.

Parsons, G.; Hansboro, J.; Buck, C.; Croasdale, S.; Schwartz, J. (2012). “Structural and 

Hygrothermal Field Monitoring of Thick Continuously Insulated Wall Assemblies Utilized in a 

Multi-Story Residential Building.” Presented at the BEST3 Conference, Atlanta, GA.

Parsons G.; Hansboro, J.; Lapham, M.; Mazor, M.; Shembekar, P.; Maurer, M. (2011). “ThreeCoat Stucco Veneer Cladding Attachment Schemes for Thick Continuous Insulation (ci) Foam 

based on Experimentally Validated Finite Element (FE) Modeling” Presented at RCI 

International Convention and Trade Show, Reno, NV.

Structural Building Components Association. (2012). “ANSI/FS100 – 2012 Standard 

Requirements for Wind Pressure Resistance of Foam Plastic Insulating Sheathing Used in

Exterior Wall Covering Assemblies”, Madison, WI: Structural Building Components 

Association.
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