Creep Behavior of Structural 

Insulated Panels (SIPs) 

Results from a Pilot Study

Dwight McDonald

Marshall Begel

C. Adam Senalik

Robert J. Ross

Thomas D. Skaggs

Borjen Yeh

Thomas Williamson

Research Note

FPL–RN–0332

United States Department of Agriculture

Forest 

Service

Forest Products

Laboratory

December

2014
December 2014

McDonald Dwight; Begel, Marshall; Senalik, C. Adam; Ross, Robert J.; 

Skaggs, Thomas D.; Yeh, Borjen; Williamson, Thomas. 2014. Creep behavior of structural insulated panels (SIPs): results from a pilot study. Research 

Note FPL-RN-0332. Madison, WI: U.S. Department of Agriculture, Forest 

Service, Forest Products Laboratory. 12 p.

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Abstract

Structural insulated panels (SIPs) have been recognized as 

construction materials in the International Residential Code 

(IRC) since 2009. Although most SIPs are used in wall 

applications, they can also be used as roof or floor panels 

that are subjected to long-term transverse loading, for which 

SIP creep performance may be critical in design. However, 

limited information on creep performance of SIPs under 

transverse loading is available. Collaborative pilot studies 

were undertaken by the USDA Forest Products Laboratory 

and APA–The Engineered Wood Association to explore the 

creep behavior of SIPs under bending- and shear-critical 

configurations. Results from these pilot studies will serve as 

the basis for more comprehensive future studies. This paper 

provides detailed test results from these pilot studies.

Keywords: creep, structural insulated panels

Conversion Table

English unit Conversion factor SI unit

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

pounds per cubic foot 

(lb/ft3)

16.018 kilograms per cubic 

meter (kg/m3)

pounds per square 

foot (lb/ft2)

47.880 pascal (Pa)

pound force (lbf) 4.448 newton (N)

Temperature (°C) = [Temperature (°F) – 32] / 1.8
Creep Behavior of Structural Insulated 

Panels (SIPs)

Results from a Pilot Study

Dwight McDonald, Engineering Technician

Marshall Begel, General Engineer

C. Adam Senalik, Research General Engineer

Robert J. Ross, Supervisory Research General Engineer

Forest Products Laboratory, Madison, Wisconsin

Thomas D. Skaggs, Manager, Product Evaluation

Borjen Yeh, Technical Services Director

Thomas Williamson, Vice-President

APA–The Engineered Wood Association

Introduction

The USDA Forest Products Laboratory (FPL) and cooperators such as APA–The Engineered Wood Association have a 

long history of researching the properties and performance 

characteristics of sandwich-type products. Structural insulated panels (SIPs) are a commercially available product 

that has evolved from early FPL sandwich material research. 

SIPS are now in wide use throughout both residential and 

nonresidential construction industries, with over 70 million 

square feet produced on a yearly basis. Several in-service 

concerns have arisen related to the use of SIPs, and answers 

to these concerns would greatly enhance the growth of the 

SIP industry. For example, limited data are available for the 

creep performance of SIPs under in-service conditions. This 

paper summarizes results of a pilot study designed to investigate the creep characteristics of SIPs under indoor environmental conditions. 

Background

Structural insulated panels are a composite building material. They consist of a sandwich of two layers of structural 

facers with an insulating layer of foam plastic insulation 

adhered between. The facers can be sheet metal, oriented 

strandboard (OSB), or other materials, and the foam plastic 

insulation is usually expanded polystyrene (EPS), extruded 

polystyrene (XPS), or polyurethane foam.

Structural insulated panels have structural properties similar to those of an I-beam or I-column. The rigid insulation 

core of the SIP performs as a web, and the facers perform 

as flanges. Structural insulated panels replace several components of conventional buildings, such as studs and joists, 

insulation, vapor barrier, and air barrier. As such they can be 

used for many different applications, such as exterior wall, 

roof, and floor systems.

Structural insulated panels are most commonly made of 

OSB panels sandwiched around a foam core made of EPS, 

XPS, or rigid polyurethane, but other materials can be used, 

such as plywood, steel, aluminum, cementitious panels, and 

even exotic materials such as stainless steel, fiber-reinforced 

plastic, and magnesium oxide. Some SIPs use fiber-cement 

or plywood for the panels, and agricultural fiber, such as 

wheat straw, for the core. This study considered only SIPs 

made with OSB facings and an EPS core.

Creep is the tendency of a solid material to deform slowly 

under the influence of sustained load. It occurs as a result 

of long-term exposure to levels of stress that are below the 

yield strength of the material. Creep increases with higher 

temperatures.

The rate of creep deformation is a function of material properties, exposure time, exposure temperature, moisture, and 

applied structural load. Depending on the magnitude of the 

applied load and its duration, deformation may become so 

large that a component can no longer perform its intended 

function. 

Structural insulated panels are frequently used in both floor 

and roof application. Historically, determination of transverse load capacities of SIPs for these applications has been 

conducted using ASTM E 72, which is a short load duration test. Limited data are available describing the creep 

performance of SIPs under long-term loading conditions 

that are likely to occur during in-service use. The purpose of 

the pilot test program described here was to investigate the 

long-term creep performance characteristics of SIPs used in 

horizontal loading applications based on a relatively small 

sample size. This pilot test program is intended to lead to a 

larger scale test program based on ASTM D 6815 to develop 

specific design recommendations for SIPs with respect to 

creep deformation.
Research Note FPL–RN–0332

2

Materials and Methods

Material Selection and Specimen Preparation

The test specimens were sampled from a regular production run and are representative of the product under evaluation. Two matched paired test groups were selected, one 

for short-term bending tests, and one for long-term creeprupture bending tests. Forty-eight 12-in.-wide by 

12.25-in.-deep by 20-ft sections were manufactured by

 a Structural Insulated Panel Association (SIPA) member 

for testing, each with 11.25-in.-thick nominal 1.0-lb/ft3 density ASTM C 578, Type 1 expanded polystyrene (EPS) 

foam cores. Twenty-foot-long sections were delivered to 

the FPL, and matching 4-ft-long sections were delivered 

to APA. The panel facings consisted of APA-rated, 

7/16 Performance Category, Exposure 1, 24/16 span rated 

OSB sheathing on both sides. The OSB complied with the 

requirements contained in ANSI/APA PRS 610.1-2013. The 

test samples were constructed with the OSB strength axis 

parallel with the panel length. The core and the OSB facings 

were bonded with an approved adhesive conforming to ICCES AC05 requirements. Hem–Fir, No. 2 and better nominal 

2- by 12-in. lumber was purchased locally for end blocking. 

For the end blocking, the International Residential Code 

(IRC) detail was followed, and 8d common (0.131 by 

2.5 in.) nails were hand-driven at 6 in. on center.

Table 1 shows specimen and test configuration details, and 

Figure 1 shows the end conditions for SIP specimens, as 

noted in Table 1. The relatively long-span (span-to-depth ratio of 18:1) and short-span (span-to-depth ratio of 4:1) tests 

at the FPL and APA, respectively, are intended for evaluation of bending and shear creep performance.

Test Methods

All testing was conducted using ASTM D 6815 as the basis. 

Testing conducted at FPL was in a controlled environment 

of 70 °F and 50% relative humidity (RH). The testing conducted at APA was at indoor laboratory ambient conditions, 

which were monitored throughout the creep tests. All test 

specimens were simply supported and loaded by two equal 

concentrated forces spaced a distance of one-third the total 

span from the end supports. The loading rate for the shortterm test was such that the target failure load was achieved 

in approximately 1 min. The creep test specimens were 

loaded such that the average time to attain the preselected 

constant stress level did not exceed the average time to failure of the short-term tests. The specimens were subjected 

to three long-term test loads (33%, 22%, and 11% of the 

average maximum short-term failure load) for a minimum 

period of 90 days. During this period, midspan deflection 

readings were taken for each specimen until the 90-day time 

period has elapsed. At a minimum, deflection readings were 

taken at approximately once per second after the application 

of the constant load (initial deflection), and every minute 

for the first hour, then every 30 min for the next 120 days, 

including a 30-day creep recovery.

The test loads was removed after 90 days, and the midspan 

deflections continued to be monitored for the remaining 

30 days. After the creep recovery period, all specimens were 

tested in the same manner as the short-term (control) tests 

to determine the residual strength of each specimen after the 

90-day creep loading.

Test results obtained from the short-term (control) and 

90-day creep tests and residual strength after the creep 

tests, are provided in Tables 2 to 11.

Results and Discussion

The FPL testing was performed as outlined in Figures 2 to 5, 

and the APA testing was performed as outlined in Figures 6 

and 7. All test specimens from both FPL and APA survived 

the 90-day constant loads without a bending failure. Based 

on the equations in X2.3 of ASTM D 6815, the creep rate 

and fractional deflection were calculated and are summarized in Tables 10 and 11. Deflection plots and creep values 

are shown in Figures 8 to 11. Typical failure modes from the 

short-term (control) and creep recovery tests are shown in 

Figures 12 to 14. No significant strength loss was observed 

after 90 days of loading (see the average PMax in Tables 2 

to 9).

Conclusions and Recommendations

The next phase of testing will attempt to develop creep factors for this specific product with statistical factors in accordance with ASTM D 6815. A wood or wood-based product 

that meets the criteria of ASTM D 6815 would be one that 

exhibits duration of load performance that is characteristic 

of structural lumber in its dry-use condition.
Creep Behavior of Structural Insulated Panels (SIPs): Results from a Pilot Study

3

Table 1. Specimen and test configuration detailsa

Test 

number End condition Load level

Number of 

samples Duration

1 Foam flush Average maximum load of short term bending tests 3 1 min

1a Foam flush (Test no. 1 load) × 1/3 3 90 days

1b Foam flush (Test no. 1a load) × 2/3 3 90 days

1c Foam flush (Test no. 1a load) × 1/3 3 90 days

2 2× end block Average maximum load of short term bending tests 3 1 min

2a 2× end block (Test no. 2 load) × 1/3 3 90 days

2b 2× end block (Test no. 2a load) × 2/3 3 90 days

2c 2× end block (Test no. 2a load) × 1/3 3 90 days

aSpecimens were 12 in. wide by 12.25 in. deep. Test spans for FPL and APA were 18 ft and 4 ft, respectively.

Table 2. FPL short-term bending, foam flush endsa

Sample 

ID

EI

(×106 lbf-in2)

Max M

(×103 lbf-in.)

Slope 

(lbf/in.)

Ext PMax

(in.)

PMax

(lbf)

1A 150.5 72.4 841.70 2.610 2,011

13A 148.5 44.5 830.38 1.538 1,237

16A 144.4 72.7 807.60 2.755 2,021

22A 150.4 44.1 840.81 1.505 1,226

23A 161.3 45.5 901.65 1.476 1,264

24A 159.5 75.9 892.02 1.906 2,109

31A 149.8 40.9 837.34 1.421 1,136

38A 149.5 44.3 836.00 1.556 1,232

43A 147.6 44.4 825.52 1.533 1,233

Average 151.3 53.9 845.89 1.811 1,497

COVb 0.04 0.28 0.04 0.28 0.28

aSpecimens were 12 in. wide by 12.25 in. deep. Test span and shear leg length 

were 18 ft and 72 in., respectively. Two failure modes were observed, each with a 

consistent strength value: Specimen that had adhesion failure failed near 1,200 lbf. 

Specimen that had flange compression failed near 2,000 lbf.

bCOV, coefficient of variation.

Table 3. FPL short-term bending, 2× end blocka

Sample 

ID

EI

(×106 lbf-in2)

Max M

(×103 lbf-in.)

Slope 

(lbf/in.)

Ext PMax

(in.)

PMax

(lbf)

27A 144.7 39.9 809.15 1.407 1,109

28A 149.5 41.2 835.85 1.434 1,145

30A 146.9 46.5 821.33 1.627 1,291

34A 149.2 43.6 834.26 1.492 1,210

36A 154.6 47.4 864.59 1.625 1,317

40A 155.5 70.8 869.26 2.396 1,966

41A 148.5 71.8 830.05 2.640 1,996

45A 145.0 46.9 810.91 1.673 1,304

46A 145.9 45.9 815.55 1.631 1,276

Average 148.9 50.5 832.33 1.769 1,402

COV 0.03 0.24 0.03 0.25 0.24

aSpecimens were 12 in. wide by 12.25 in. deep. Test span and shear leg length 

were 18 ft and 72 in., respectively. Two failure modes were observed, each with a 

consistent strength value: Specimen that had adhesion failure failed near 1,200 lbf. 

Specimen that had flange compression failed near 2,000 lbf.

bCOV, coefficient of variation.
Research Note FPL–RN–0332

4

Table 4. FPL bending after creep, foam flush endsa

Sample 

ID

EI

(×106 lbf-in2)

Max M

(×103 lbf-in.)

Slope 

(lbf/in.)

Ext PMax

(in.)

PMax

(lbf)

3A 153.5 42.0 858.51 1.437 1,167

15A 153.7 44.1 859.37 1.558 1,225

20A 152.0 46.1 849.66 1.609 1,280

4A 152.6 43.5 853.49 1.473 1,209

9A 154.0 74.4 860.79 2.664 2,067

19A 146.2 44.9 817.24 1.609 1,248

2A 148.9 41.9 832.38 1.444 1,165

8A 151.9 79.4 849.17 2.936 2,205

18A 145.9 46.5 815.90 1.658 1,292

Average 151 51.4 844.06 1.821 1,429

COVb 0.02 0.28 0.02 0.31 0.28

aSpecimens were 12 in. wide by 12.25 in. deep. Test span and shear leg length 

were 18 ft and 72 in., respectively.

bCOV, coefficient of variation.

Table 5. FPL bending after creep, 2× end blocka

Sample 

ID

EI

(×106 lbf-in2)

Max M

(×103 lbf-in.)

Slope 

(lbf/in.)

Ext PMax

(in.)

PMax

(lbf)

37A 159.0 46.6 888.85 1.577 1,293

39A 152.4 47.6 852.27 1.620 1,323

48A 151.1 87.5 844.80 3.422 2,431

42A 145.2 38.4 812.02 1.348 1,065

47A 145.3 44.1 812.28 1.565 1,224

29A 145.9 47.0 815.80 1.668 1,306

26A 149.3 45.7 834.61 1.594 1,269

32A 190.7 81.2 1066.36 3.169 2,256

35A 202.5 36.4 1132.11 0.962 1,012

Average 160.2 52.7 895.46 1.881 1,464

COVb 0.13 0.35 0.13 0.44 0.35

aSpecimens were 12 in. wide by 12.25 in. deep. Test span and shear leg length 

were 18 ft and 72 in., respectively.

bCOV, coefficient of variation.

Table 6. APA short-term bending, foam flush end detaila

Sample 

ID

PMax 

(lbf)

Midspan deflection (in.) at four loads

PMax 695 lbf 463 lbf 232 lbf

17B 1,826 1.102 0.154 0.096 0.043

11B 2,045 1.375 0.114 0.070 0.029

14B 1,892 1.175 0.140 0.089 0.042

7B 2,164 1.245 0.096 0.049 0.009

24B 2,194 1.510 0.147 0.094 0.044

12B 2,051 1.312 0.129 0.080 0.036

23B 2,097 1.483 0.135 0.087 0.038

10B 2,160 1.505 0.120 0.075 0.033

6B 2,331 1.505 0.110 0.067 0.032

Average 2,084 1.357 0.127 0.079 0.034

COVb 0.07 0.12 0.15 0.19 0.32

aSpecimens were 12 in. wide by 12.25 in. deep. Test span was 4 ft.

bCOV, coefficient of variation.
Creep Behavior of Structural Insulated Panels (SIPs): Results from a Pilot Study

5

Table 7. APA short-term bending, 2× end block detaila

Sample 

ID

PMax 

(lbf)

Midspan deflection (in.) at four loads

PMax 469 lbf 313 lbf 156 lbf

45B 1,487 0.339 0.102 0.064 0.023

40B 1,578 0.233 0.055 0.033 0.009

27B 1,399 0.226 0.065 0.041 0.020

28B 1,493 0.245 0.054 0.027 0.005

36B 1,596 0.259 0.061 0.037 0.014

30B 1,129 0.184 0.064 0.041 0.019

46B 1,419 0.279 0.085 0.057 0.030

41B 1,387 0.341 0.137 0.104 0.063

34B 1,173 0.158 0.055 0.033 0.013

Average 1,407 0.252 0.075 0.049 0.022

COVb 0.12 0.25 0.37 0.49 0.79

aSpecimens were 12 in. wide by 12.25 in. deep. Test span was 4 ft.

bCOV, coefficient of variation.

Table 8. APA bending after creep, foam flush end detaila

Sample 

ID

PMax 

(lbf)

Midspan deflection (in.) at four loads

PMax 695 lbf 463 lbf 232 lbf

2B 2,029 1.172 0.159 0.103 0.043

8B 2,002 0.921 0.136 0.085 0.025

18B 2,133 1.489 0.162 0.104 0.049

4B 2,210 1.598 0.146 0.094 0.041

9B 1,986 1.132 0.160 0.104 0.032

19B 2,007 1.412 0.229 0.136 0.052

3B 1,972 1.365 0.136 0.078 0.030

15B 2,063 1.366 0.145 0.065 0.026

20B 1,887 1.151 0.146 0.087 0.033

Average 2,032 1.290 0.158 0.095 0.037

COVb 0.05 0.16 0.18 0.21 0.27

aSpecimens were 12 in. wide by 12.25 in. deep. Test span was 4 ft.

bCOV, coefficient of variation.

Table 9. APA bending after creep, 2× end block detaila

Sample 

ID

PMax 

(lbf)

Midspan deflection (in.) at four loads

PMax 469 lbf 313 lbf 156 lbf

26B 1,525 0.305 0.094 0.063 0.029

32B 1,525 0.279 0.072 0.045 0.019

35B 1,620 0.498 0.061 0.038 0.013

29B 1,644 0.330 0.080 0.050 0.021

42B 1,242 0.242 0.079 0.051 0.023

47B 1,382 0.266 0.078 0.051 0.021

37B 1,471 0.232 0.058 0.037 0.014

39B 1,575 0.253 0.060 0.025 0.007

48B 1,592 0.325 0.084 0.052 0.024

Average 1,508 0.303 0.074 0.046 0.019

COVb 0.08 0.27 0.17 0.24 0.35

aSpecimens were 12 in. wide by 12 1⁄4 in. deep. Test span was 4 ft.

bCOV, coefficient of variation.
Research Note FPL–RN–0332

6

Table 10. FPL creep and recovery summary

Sample 

ID Load Ends

Initial 

deflection 

under load 

(in.)

Group 

average 

(in.)

Final creep 

deflection 

(in.)

Group 

average 

(in.)

Fractional 

creep 

(%)

Group 

average 

(%)

Residual 

deflection 

after recovery 

(in.)

Group 

average 

(in.)

35A High Block 0.587 0.607 0.369 0.304 63 50 0.085 0.068

26A High Block 0.644 0.316 49 0.082

32A High Block 0.588 0.226 38 0.037

18A High Foam 0.697 0.714 0.349 0.195 50 27 0.124 0.086

2A High Foam 0.725 0.218 30 0.111

8A High Foam 0.720 0.017 2 0.023

47A Medium Block 0.684 0.615 0.103 0.051 15 8 0.045 0.033

29A Medium Block 0.575 0.006 1 0.025

42A Medium Block 0.585 0.056 10 0.028

19A Medium Foam 0.595 0.555 0.181 0.137 30 24 0.047 0.037

4A Medium Foam 0.549 0.136 25 0.040

9A Medium Foam 0.521 0.094 18 0.022

39A Low Block 0.293 0.266 0.104 0.074 36 27 0.011 0.014

48A Low Block 0.265 0.058 22 0.023

37A Low Block 0.241 0.059 25 0.010

20A Low Foam 0.232 0.230 0.091 0.110 39 48 0.020 0.021

3A Low Foam 0.249 0.127 51 0.009

15A Low Foam 0.210 0.113 54 0.036

Table 11. APA creep and recovery summary

Sample 

ID Load Ends

Initial 

deflection 

under load 

(in.)

Group 

average 

(in.)

Final 

creep 

deflection 

(in.)

Group 

average 

(in.)

Residual 

deflection after 

recovery 

(in.)

Group 

average 

(in.)

26B High Block 0.080 0.066 0.143 0.123 0.053 0.041

32B High Block 0.068 0.122 0.049

35B High Block 0.049 0.104 0.021

2B High Foam 0.100 0.113 0.246 0.288 0.066 0.077

8B High Foam 0.114 0.283 0.065

18B High Foam 0.125 0.334 0.099

29B Medium Block 0.039 0.044 0.091 0.086 0.029 0.040

42B Medium Block 0.047 0.096 0.045

47B Medium Block 0.045 0.072 0.047

4B Medium Foam 0.059 0.060 0.102 0.113 0.006 0.012

9B Medium Foam 0.067 0.118 0.004

19B Medium Foam 0.055 0.118 0.027

37B Low Block 0.004 0.011 0.016 0.029 0.005 0.016

39B Low Block 0.010 0.028 0.020

48B Low Block 0.018 0.042 0.022

3B Low Foam 0.020 0.016 0.035 0.031 0.010 0.012

15B Low Foam 0.019 0.032 0.002

20B Low Foam 0.009 0.026 0.024
Creep Behavior of Structural Insulated Panels (SIPs): Results from a Pilot Study

7

Figure 1. End conditions for blocked (top) 

and foam flush (bottom).

Figure 2. FPL static loading diagram.

Figure 3. Creep loading diagram (FPL).

Figure 4. Static bending test setup (FPL).

Figure 5. Creep test setup (FPL).

Figure 6. Static bending test setup (APA–The 

Engineered Wood Association).

Figure 7. Creep test setup (APA–The Engineered Wood Association).
Research Note FPL–RN–0332

8

Figure 8. Graphs of creep samples after 120 days (FPL). 
Creep Behavior of Structural Insulated Panels (SIPs): Results from a Pilot Study

9

Figure 9. Graphs of creep recovery (FPL).
Research Note FPL–RN–0332

10

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 20 40 60 80 100 120

3B

15B

20B

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 20 40 60 80 100 120

4B

9B

19B

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 20 40 60 80 100 120

2B

8B

18B

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 20 40 60 80 100 120

37B

39B

48B

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 20 40 60 80 100 120

Midspan deflection (in.)

29B

42B

47B

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 20 40 60 80 100 120

Time (days)

26B

32B

35B

Low load - block ends Low load - no blocks

Medium load - block ends

High load - block ends High load - no blocks

Medium load - no blocks 

Figure 10. Graphs of creep samples after 120 days (APA–The Engineered Wood Association). 
Creep Behavior of Structural Insulated Panels (SIPs): Results from a Pilot Study

11

Figure 11. Graphs of creep recovery (APA–The Engineered Wood Association).
Research Note FPL–RN–0332

12

Figure 14. Short-term bending, typical failure mode (OSB 

bending) of foam flush detail (top) and typical failure mode 

(tearing of EPS foam) of 2× end block detail (bottom) 

(APA–The Engineered Wood Association). Figure 13. Compression failure (FPL). 

Figure 12. Adhesion failure (FPL).