The Detection of Stud Line Cracking in Metal Stud Walls

STEVEN CRAIN 

The Dow Chemical Company 

200 Larkin Center 

1605 Joseph Drive Midland, MI 48674 

DALWINDER GILL, MEDHAT HANNA, 

DONALD LEBLANC 

National Research Council Canada 

Centre for Surface Transportation Technology 

2320 Lester Road 

Ottawa, Ontario K1V1S2 Canada

ABSTRACT 

Polyurethane spray foams have found widespread use in the construction industry as insulation, moisture and air barriers. 

Due to the in-field application of Spray-Applied Polyurethane Foam (SPF), the foam is applied under a wide variety of 

environmental conditions. Applying spray polyurethane foams to the intended substrates especially wooden or metal studs 

may not be feasible because of problems obtaining adhesion and/or cohesion to the studs under relatively low temperatures. 

At relatively low substrate temperatures curing polyurethane foam may exhibit cohesive or adhesive failure of the foam to 

the substrate. 

Adhesive failure at the stud-foam interface is commonly called stud line cracking and sometimes rifting. Stud line 

cracking is especially difficult to spot for metal studs of a wall due to the inability to see the foam-metal interface within the 

channel of the stud. The implication of an un-repaired crack may be that air and moisture vapor can enter the building 

envelope via the crack minimizing the intended performance of the spray foam. 

At the Center for Surface Transportation Technology’s (CSTT) Climatic Laboratory, mock metal and wood stud walls 

were built and instrumented with a thermocouple to measure bulk foam temperature rise. The metal stud walls were also 

instrumented with a modified miniature compression load cell to evaluate the forces applied to the studs of the wall in an 

attempt to understand the events prior to stud line cracking. A description with examples of the technique that was used to 

understand the effect of substrate temperature on stud line cracking will be discussed. 

INTRODUCTION

Closed cell medium density spray-applied polyurethane foam insulation (mSPF) is applied to a variety of substrates. A 

significant application of mSPF is to the exterior wall envelope particularly the interior cavities of the walls framed by 

wood or metal studs. An exterior wall envelope is a system or assembly of exterior wall components constructed to protect 

the conditioned interior space from the exterior environments [1]. mSPF is generally applied for its insulation value but also 

as an air and moisture vapor barrier. 

mSPF comes as a two component system designed to be spray-applied to substrates without sagging. One component is 

the polyisocyanate liquid and the other component is a complex liquid blend of physical and chemical blowing agents, 

polyols, diluents, foam stabilizing agents, catalysts and fire retardant additives. The two component system is separately 

heated while being transferred in pre-heated hoses at elevated pressures to a handheld impingement mix chamber (spray 

gun) before spraying out onto the substrate. The liquid blend begins to react quickly giving cream times between 0.5 and 3 

seconds and rise times between 4 and 30 seconds. As the foam expands, it is restricted in five of six directions (sheathing, 

header, footer and studs) of the cavity and expands both to the opening of the cavity and into any gaps or cracks resulting 

from the various construction practices. 

There is interest in both individual component construction materials and gaps between construction materials on the 

transport properties of heat, air and moisture [2]. Pollutants that may penetrate a building through gaps in between 

construction materials are also of concern [3]. mSPF is defined as >90% closed cell foam per ASTM C 1029 and is 

therefore an excellent choice when spray-applied to insulate and seal the exterior wall envelope. 
Figure 1. Stud line cracking after SPF system inappropriately applied below manufacturers recommended substrate 

conditions. 

Due to the in-field application of spray-applied polyurethane foam, the foam is applied under a wide variety of 

environmental conditions throughout the year. Under winter conditions, at relatively low substrate temperatures for the 

formulations intended design, the polyurethane foam may exhibit cohesive or adhesive failure of the foam to the substrate. 

Cohesive and adhesive failures at the stud-foam interface are commonly called stud line cracking and sometimes rifting. 

Much of the foam delamination known in the industry is generally avoidable if the substrate is kept clean [4]. From our 

experience delamination due to moisture, frost and other contaminants is typically an adhesion failure and stud line cracking 

at the foam-substrate interface due to inappropriately low substrate temperatures is via cohesive failure. mSPF system 

suppliers specify upper and lower substrate and ambient temperature levels that are acceptable for a particular formulation. 

An example of a foam system applied at an inappropriately low substrate temperature is shown in Figure 1. As the foam 

cooled and began to shrink, extensive cohesive failure occurred opening the exterior wall envelope to air and moisture 

intrusion. 

Stud line cracking for commercial walls can be difficult to detect when metal studs, typically in the shape of c-channels, 

are used in construction. The stud-foam interface at the footers and headers and in some cases along the studs themselves is 

within the c-channel itself. To determine if stud line cracking has occurred within the c-channels it has been necessary to 

deconstruct the walls. The Foam Deflection Force Gauge as an in-situ measurement device not only allowed confirmation 

as to whether a stud-foam delamination had occurred but also gave a real time pressure profile of the spray-applied foam 

within the cavity. In combination with bulk foam temperature measurements, formulations were found that optimized the 

foam performance when spray-applied in winter like conditions. 

EXPERIMENTAL

 Each residential 4X8 foot wall assembly was constructed with three cavities of different widths (see Figure 4). The 

widths were 8, 16 and 24 inch on center using 7/16 inch thick oriented strand board (OSB) assembled with nominal 2 x 4 

inch spruce wood studs using deck screws. Each commercial 4X8 foot wall assembly was also constructed with three 

cavities at 8, 16 and 24 inch on center. The commercial wall was constructed of 1 inch thick THERMAXTM HD brand 

isocyanurate insulation sheathing (THERMAX is a trademark of The Dow Chemical Company) attached to c-channel metal 

studs using metal screws in combination with plastic washers. The metal studs for the commercial wall assemblies were 16 

gauge Dietrich CSJ 3 5/8 inch steel studs with an extra c-channel at the top to simulate a larger thermal mass. 

The polyol system and polymeric isocyanate temperatures were recorded separately using a Type T temperature probe 

within an adapter, provided by Gaco. The adapter was located between the spray gun and the hose whip. The temperature 

was recorded at 1 HZ using the same data acquisition (DA) system used for bulk foam temperature. 

Type T Thermocouples were used to monitor and record the temperature of the bulk foam for 60 minutes from the time 

of spray-applying the mixed A and B materials. The thermocouple was inserted through the sheathing in the middle of the 

24 inch cavity to a height of 0.5 inch from the sheathing inner surface. The temperature was recorded at 1HZ. Type T 

thermocouples are made from Thermo Electric NN24T copper-constantan wire. The junction was made by twisting the 

bared wire ands and soldering. The thermocouple wire has an accuracy of 0.1°F. 

A foam deflection force gage (FDFG) was constructed at the Climatic Laboratory in Ottawa by attaching an Omega 

LCFA-5, miniature compression load cell with load capacity of 5 lb, for both compression and tension directions, to a 

bracket (see Figure 2). An aluminum cylinder with a diameter of 1 inch was bolted to the LCFA. A hole approximately 1.1 

inch in diameter was bored into the middle of the header of the 24 inch on center cavity. The bracket was bolted to the 
Figure 2. LCFA-5 (1) attached to bracket (2). Cylinder 

(3) attached to LCFA-5. 

Figure 3. FDFG sitting over an access hole in the top 

channel of a mock commercial wall. THERMAXTM 

brand insulation sheathing in the foreground. 

header (see Figure 3). The aluminum cylinder height was adjusted such that the bottom edge of the cylinder was flush with 

the inner wall of the cavity. Aluminum tape was placed over the entire hole such that the cylinder was adhered to the tape 

and the tape extended 0.25 inch around the perimeter of the hole. The load cell pressure was recorded at 1HZ. The load cell 

has an accuracy of 0.2% and maximum safe overload of 150% of rated capacity. 

A spray foam formulation is prepared by first combining all of the polyol component constituents, including the blowing 

agent. The formulation and polyisocyanate are then fed separately through a proportioning machine and a spray gun. For 

some of the spray-applied walls, the spray foam proportioner was the Gusmer FF 2500 and the gun used was the Glascraft 

Probler with 02 nozzle. For a number of the spray-applied walls the spray foam proportioner was the Glascraft MH and the 

gun used was the Glascraft Probler with a 02 nozzle. The hose length was 250 feet long (3/8” dia.) with a 10-foot whip 

hose (1/4” dia.). The rest of the spray-applied walls employed a GAMA Evolution G-250 H proportioner and the gun used 

was the Gaco D-gun with 70 mix chamber. 

One (volume) part of isocyanate component is matched with one (volume) part of polyol component in the spray gun. 

The temperature of the environment and walls were 20-30°F and the relative humidity was kept between 40 and 60%. The 

spray was applied in a picture frame technique (see Figure 5). The picture frame technique is to first apply the liquid spray 

to the perimeter of the cavities created between the studs and the sheathing. The next step is to fill in the rest of the cavity to 

a nominal 2” of foam in one pass preferably from the top of the cavity to the bottom in a lateral motion. 

The chemical temperature setting at the proportioner was approximately 130°F for both the polyol formulation and the 

polyisocyanate and the hose temperature setting was approximately 123°F. The temperature settings chosen along with the 

environmental conditions gave actual chemical temperatures at the gun about 10°F lower than the primary heater settings 

for short application times but similar temperatures as the primary heater settings for longer application times. The dynamic 

pressures of the chemicals were between 650 and 850 psi. A typical formulation gave a cream time of 0.5 seconds and foam 

rise time of 4 seconds. 

The proportioner was in a trailer next to the building where the foam was being spray-applied to the mock walls such that 

the hose used to transfer the warmed chemicals to the spray gun experienced a variety of ambient temperature conditions. 

Any time the spraying stopped for more than 5 minutes, at least the material in the whip was purged prior to applying the 

foam to the walls. 

2 

3 

1 
16"

48"

96"

24

"

8" 16"

48"

96"

24

"

8"

1 

2 

3 

4 

5 

6 

7 8 9 

10 11 12 

Figure 4. Schematic of mock walls – THERMAXTM 

brand insulation sheathing or OSB Substrates with 

2X4 inch metal or wood studs, respectively. 

Figure 5. Applicator using picture frame technique - 

spray-apply to perimeter of cavity followed by filling 

in the middle from top to bottom in a side to side 

mode. 

RESULTS AND DISCUSSION

Cracks along the stud-mSPF interface of the exterior wall envelope can expose the wall to penetration by pollutants and 

incursion of air, water and moisture to the interior of the building. For example, infiltration from a crack as narrow as 10 

mils (0.25 mm) will allow electrically neutral particles near 100% penetration between the ranges of 0.1 and 10 microns in 

diameter at as low as 4 Pa air pressure [3]. To minimize air and moisture intrusion through the mSPF, suppliers of systems 

commonly evaluate mSPF as a monolithic system using the method described in ASTM E 2178 Standard Test Method of 

Building Materials. If the air leakage measured does not exceed 0.02 L/(s●m2

) at the static pressure differential of 75 Pa for 

a one inch thick sample of foam, the foam is considered an air barrier [5]. 

The goal of the project that led to the creation of a method to determine stud line cracking in metal stud walls was to 

develop a closed cell, nominal 2 LB/FT3

 density spray-applied polyurethane foam formulation (mSPF) that can be applied 

to wall cavities in winter season conditions. For most of the formulations evaluated the mSPF was applied to a nominal two 

inch thickness and to both metal and wood stud construction. The primary concern of the project was crack formation at the 

stud-mSPF interface in the cavities. 

Narrow crack widths and short narrow cracks are known to occur at almost any substrate temperature and are attributed 

to contaminants on the surface of the substrate. For this project, a crack depth of >1 inch was arbitrarily chosen to constitute 

a formulation failure. A depth of one inch represented about 1⁄2 the depth of the spray-applied foam. To simplify failure 

analysis, a relationship between crack width and crack depth was determined. Crack depths were measured as a function of 

crack widths on 23 walls. The correlation is shown in Figure 6. The maximum acceptable crack width was found to be 

0.067 inch (<1.7 mm). At 0.067 inch crack width, the crack depth was approximately 1 inch (25.4 mm). 
y = 8.5532x + 9.617

R2

 = 0.678

0

10

20

30

40

50

60

70

80

012345678

Aged Crack Width (mm)

Aged Crack Depth (mm)

1.59 mm depth limit

Figure 6 Observation of Crack Depth with Crack Width 

The measurement of crack widths, if present, for wood stud walls is straightforward. However, the metal studs used for 

commercial building are composed of c-channels. For the exposed flat side of the c-channel, crack widths analysis is strait 

forward. Unless the crack is very large, the observer cannot determine accurately whether there is a crack along the open 

side of the c-channel nor its width. 

Dimensional stability has been of concern for the spray-applied polyurethane foam industry as blowing agents phase out 

of use in the construction industry [6]. Stud line cracking can be considered an early dimensional stability issue. Once the 

foam has expanded and the reaction exotherm is nearly complete the foam begins to cool. The cooling foam begins to 

shrink and potentially pull away from the substrate. Shrinkage of polymer foams are known and can be attributed to cooling 

and pressure drop of the blowing agent [7], conversion of starting materials to polymeric, thermal contraction [8] and loss of 

blowing agents leading to slight vacuums[9]. When developing a new formulation, one of the important parameters needed 

by the formulator is the target lower substrate temperature limit for the system. The substrate temperature and the substrate 

thermal mass can affect post application shrinkage [10, 11]. The tensile strength of the newly formed foam will determine 

whether the shrinkage will lead to delamination. 

Attempts to measure dimensional stability under winter conditions in our lab included placing molds in freezers and once 

the equipment was ready, removing the molds from the freezer prior to spray-applying the SPF system. The difficulty 

encountered was condensation of moisture onto the surface of the mold, during seasons at high humidity. The lack of space 

was also problematic as the experimental designs were expanded into large scale mock walls. 

The decision was made to engage the NRC of Canada’s Center for Surface Transportation Technology which included a 

Climatic Lab in Ottawa. The Climatic Lab has the ability of controlling temperature and humidity of several large 

conditioned rooms. An extensive data collection system was also available. 

The Foam Deflection Force Gauge (FDFG) was developed to evaluate the compressive (foaming) and tensile (shrinking) 

forces of the foam forming system when spray-applied to the mock walls with metal studs composed of c-channels. The 

intent was to find a correlation between the forces and stud line cracking. The in-situ dimensional stability tool was also to 

be used as a pass/fail device for a formulation where delamination from the FDFG was considered a failure. 

The force measurements of the expanding foam against the studs required a customized measuring device in the absence 

of an off the shelf solution. Various FDFG designs were constructed and the final design used for the work of developing a 

winter season spray-applied polyurethane formulation is shown in Figure 4. 

The chosen load cell, LCFA-5, is sensitive to lateral forces which were present due to the initial design set. Two instances 

of lateral forces were noted during early design work from foam formation. The first was a simple jamming issue. The 

cylinder was attached to the bolt on the bottom of the LCFA-5 such that the height of the cylinder was adjustable. An 

adjustable height allowed the bottom of the cylinder to be lined up with the inner wall of the metal c-channel. A small gap 

was necessary between the cylinder and the c-channel to allow free movement of the cylinder. However, on several 

occasions, foam would expand into the small gap between the cylinder and the c-channel. As the foam expanded, it would 

push against the cylinder but upwards and laterally jamming it against the c-channel edge. The lateral forces were large 

enough to bend the bolt holding the cylinder to the LCFA-5. 

To resolve the intrusion of the foam into the narrow opening, an aluminum tape was placed over the cylinder and hole 

such that the gap was no longer exposed to the foam. The thin aluminum tape proved to be flexible enough to allow the 
Figure 7. Example of lateral force on FDFG (top 

curve) and no lateral force (lower curve) 

Figure 8. Uniform coverage of FDFG tape (first 

cure - 114A) vs non-uniform coverage (114B) 

observation of the forces acting upon the FDFG. Figure 7 graphically exhibits the affect of the lateral forces on the force 

analysis of developing foam. For the top curve (sprayed minutes after the first) the cylinder has been pushed against the cchannel wall preventing the cylinder from being pulled inward by the contracting foam at the same rate as the cylinder from 

the first wall. The FDFG indicated that for this sample both walls exhibited delamination from the studs (specifically the 

FDFG device at the 24 o.c. cavity) which for this study was an indication of failure. When the tensile or pull forces return to

zero, delamination has occurred and is quite often accompanied by load popping noises. Subsequently, all measurements 

were taken with aluminum tape applied to the inside of the header. 

A second source of lateral error was due to applicator error. If the applicator of the spray-applied spray polyurethane 

foam did not completely and evenly cover the FDFG tape, the expansion and contraction forces were uneven. The affect of 

under-covering the FDFG tape can be shown in Figure 8. For the under-covered sample, 114B, the amount of bulk foam is 

less, which reduces the total pull force against the FDFG. During the curing stage of the two walls, the expectation was that 

the wall, 114B, would fail and delaminate from the FDFG. The thought was that with less foam, the heat of exotherm would 

be reduced and therefore the green strength of the foam would be poor. However the wall did not fail. After reviewing the 

chemical temperatures between the two walls, it was found that the second wall was spray-applied about 5°F higher in 

temperature than the first wall which may have compensated for the potential exotherm heat loss. 

The FDFG and roll of aluminum tape were allowed to acclimate to the environmental conditions used for the evaluation 

prior to being attached to the mock walls. Hand gloves were found to be necessary when cutting and attaching the tape to 

prevent the fingers from warming the tape. On each installation of the FDFG, force measurements were recorded for about 

15 to 30 minutes before spray application of the system in order to determine any drift errors due to installation. The drift 

errors were usually measured to be negligible. See the first 1000 seconds of Figure 8 as an example of the flat baseline for 

the forces on the instrument during the first few minutes of environmental acclimation. 

Different measurement ranges of load cells were evaluated initially. The LCFA-5, with a 5 LB load capacity, was 

selected as it provided the best combination of sensitivity and protection against overloading. Several sensors were damaged 

that had lower than a 5 LB load capacity. 

The thermal mass can increase at the headers and footers of walls due to construction techniques. Also, the width of walls 

does not always fall at 16 inch on center but can be narrower near a wall penetration or up to 24 inch on center for some 

construction techniques. The mock walls (both wood stud and metal stud) were built to a nominal four feet wide by eight 

feet. Each wall contained three cavities and the headers for the walls were doubled in thickness by using two studs. The 

walls were constructed with three different cavity widths – 8, 16 and 24 inch on center. Each stud face within each cavity 

were given a number to correspond with its position. The numbers were from 1 to 12 as shown in Figure 4. The thermal 

mass of the headers was increased for all walls by adding an extra piece framing member to the top. The footers were 

always resting on the concrete floor. The walls were cooled to either 20 or 30°F (-6.7 or -1.1°C) prior to applying a variety 

of mSPF formulations – both experimental and commercial. According to the Quantile plot in Figure 11 for wood stud 

walls with OSB sheathing, the most likely locations for the crack to form was at positions 1, 2 and 3 – all studs associated 

with the 8 inch on center cavity. 
Figure 9 Failed Formulations - Distribution of 

Failure Time (min) on y-axis for Wood Stud walls 

Figure 10. Affect of wet metal studs on the pressure 

profile. 

The foam network structure was at its weakest early after application. To demonstrate this finding, the time-to-failure 

distribution of the set of wood stud walls evaluated is shown in Figure 9 for both the experimental and commercially 

available foam systems. 

75% of the walls that exhibited cracks, cohesively failed within 30 minutes. The only time adhesive failure between the 

foam and the substrate was observed was when water had condensed onto the substrate surface prior to spray-applying the 

system. The affect on the foam pressure profile for a formulation with and without the presence of the moisture in the form 

of ice on the metal studs is shown in Figure 10. 

The substrate application temperatures were only 10°F apart and yet both walls with the wet metal studs delaminated 

from the FDFG. Delamination is observed when the pressure returns to the pre-application pressure. Note that curve 106A 

of Figure 10 exhibited a sudden increase in tensile pressure on the FDFG. This phenomenon has been exhibited on a 

number of samples and is potentially due to the release of some lateral force around the sensor allowing a more true analysis 

of the tensile forces on the wall from the shrinking foam. Considering that the foam is spray-applied by hand, uneven 

application of the foam may shift the sensor slightly during the initial foam expansion. 

The walls that exhibited cracks at 60 minutes are those walls whose vertical studs were hit with a fist at the 60 minute 

mark. Contractors and local code officials in Canada often hit the studs in the winter to determine if there is a possibility of 

crack formation occurring during the evening hours when the temperature drops. About 25% of the wood stud walls that 

exhibited cracks failed due to the post striking technique at 60 minutes. 

Within the 60 minutes time frame (spray application to hitting the walls), the highest numbered crack position was noted 

for the wood stud walls and is shown in Figure 11. Generally, if a crack was observed at position five, then cracks were 

also observed at positions 1-4. The majority of the cracking occurred at the eight inch on center cavity. 

For the steel stud walls, the FDFG was solely relied on for pass or fail determination. Sometimes the sensors were 

allowed to stay on the walls longer than 60 minutes. The distribution of time to failure for the steel stud walls is shown in 

Figure 12. 

According to Figure 12, the failures occurred within about 30 minutes of the application in a normally distributed 

manner. The result is similar to that found with the wood stud walls in Figure 9. 

The crack position of the steel stud walls were noted only at positions 2 and 4. These positions corresponded to the side 

of the c-channels that were flat. Further cracking could have occurred in other positions but could not be verified. Of the 

steel stud walls where some cracking was noted, the width of the cracks were less than 25 mils and 5/6 times the cracking 

was observed only after hitting the foam at the 60 minute mark. The FDFG was always disconnected prior to hitting the 

walls. When cracking was observed prior to the 60 minute mark the FDFG indicated delamination 2/3 times. Of the 36 total 

steel stud walls that were evaluated in the 20-30°F substrate temperature range for one study, all visually passed the winter 

weather criteria for cracking. However, the FDFG indicated delamination in 11 instances or 31% failures. When cracking 

occurs at a stud-foam interface audible snapping emanates from the walls. For wood stud walls, the snapping sounds and 

the crack development can be tracked with ease. Our experience has been that the louder the snap the wider the crack width. 
0

1

2

3

4

5

6

7

8

9

10

11 -1.64-1.28 -0.67 0.0 0.67 1.28 1.64

0.1 0.2 0.5 0.8 0.90.95

Normal Quantile Plot

Figure 11. Quantile Plot for Crack Location. The 

highest stud position for crack formation is 

shown on the y-axis. 

Figure 12. Test of Normal Distribution of Time to 

Failure for Steel Stud Walls using the FDFG. 

For the steel stud walls the snapping may have been attributed to metal on metal slipping due to thermal expansion and 

contraction or simply minor crack formation. However, the FDFG confirms that the stud-foam interface is delaminating. 

Wood stud walls that did not indicate stud line cracking in the first hour were rechecked for cracks after about one month 

stored at ambient conditions. No new cracks were observed despite the fact that the walls were stacked and shipped prior to 

re-checking for cracks. Out of the 23 wood stud walls that had exhibited cracking in the first hour, only 12 out of 23 

continued to show cracks along the studs after aging. The ones that self re-sealed exhibited early cracking at <60 mils 

Figure 13 Starting Thicknesses for Samples that 

Re-Sealed (dark areas) versus those that 

Continued to Exhibit Cracks 

DOW CHEMICAL CLIMATE ROOM TESTS

May 25, 2009, Runs 14, 15 & 16

DOW 381, OAT = 30°F

10

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14:40 14:45 14:50 14:55 15:00 15:05 15:10 15:15 15:20 15:25 15:30

Time of Day

Temperature (F)

TC_1 TC_2 TC_3 TC_4 TC_5 TC_6 Gun_T_1 Gun_T_2

TC_1 - Run 14A - TWS

TC_2 - Run 14B - TWS

TC_3 - Run 15A - OSB

TC_4 - Run 15B - OSB

TC_5 - Run 16A - XPS

TC_6 - Run 16B - XPS

mock wall - THERMAXTM 

Sheathing Attached to Steel Studs 

mock wall - STYROFOAMTM XPS 

Attached to Wood Studs 

mock wall - OSB Attached to 

Wood Studs 

Chemical Temperatures Prior to Spray-gun

Figure 14 Spray Gun Chemical Temperature and 

Mock Wall Bulk Foam Temperature over time 
thickness (Figure 13). The self-healing can be attributed to the relaxation of the foam over time along with the replacement 

of the lost gases in the foam during the initial foaming processes with air. Therefore, post deconstruction to check for 

cracking in steel stud walls would have minimal value. 

Each time a unique foam formula was spray-applied, two mock metal stud wall cavities were instrumented with the 

FDFG and thermocouple at the widest cavity to measure the force and bulk temperature values over a period of 60 minutes. 

In a previous set of experiments, several mock walls exhibited failure on one of the two walls per delamination from the 

FDFG. The results were unexpected and a hypothesis was that the chemical temperature was not at the desired setting at 

least for the first wall that was spray-applied. For the current set of experiments, an adaptor was attached between the whip 

and the spray gun to measure the actual chemical temperature of both the polyisocyanate and the polyol system. 

The use of the adaptor and data acquisition system allowed the chemical temperatures to be monitored closely prior to 

spray-applied application of the formulations to the mock walls. A typical trace of chemical temperature and bulk foam 

temperature for a sample spray-applied to several mock walls within a short time is shown in Figure 14. 

While the applicator was waiting to spray the first wall, the spray gun and hose sat on the concrete floor of the room, 

which in this example was at 30°F. As soon as the applicator began to spray, the chemical temperature began to rise in the 

gun. For the first wall sprayed, 14A (THERMAXTM brand insulation sheathing over steel studs), the chemical temperatures 

were at their lowest values. The maximum exotherm temperature reached for the first wall was considerably less than for 

the second wall (14B) and was attributed to the fact that the chemical temperature had risen by about 10°F verses the 

application of the first wall. 

As the applicator continued to spray the rest of the walls, the chemical temperatures maintained a relatively constant 

temperature of about 110°F which was the targeted temperature for this application. Once the last monitored wall at 30°F 

was completed, the spray gun was set on the floor prior to spraying the non-monitored walls. While the gun was on the 

floor, the chemical temperatures dropped to as low as 100°F. 

The differences in the application temperatures on the two walls had a dramatic affect on the pressure profile obtained 

using the FDFG for those walls (Figure 15). 

The second wall (14B), spray-applied at 8-10°F higher chemical temperature had very minimal tensile pull indicating a 

higher extent of reaction and therefore improved dimensional stability as the foam cools. Neither wall exhibited 

delamination from the FDFG and therefore both passed the evaluation but the affect of chemical temperature was probably 

the cause of inexplicable failures in previous studies. 

Pressure Profile from the FDFG for Mock Walls 

THERMAX(tm) Over Steel Stud Walls at 30F Substrate 

Temperature 

-0.40

-0.30

-0.20

-0.10

0.00

0.10

0.20

0.30

3000 4000 5000 6000 7000 8000

14A-lbs/inch 14B-lbs/inch

Figure 15. Affect of Chemical Temperature on 

Pressure Profile from FDFG. 0

500

1000

1500

2000

2500

3000

3500

4000

4500

Mean

Std Dev

Std Err Mean

Upper 95% Mean

Lower 95% Mean

N

1079.2121

718.70138

125.10985

1334.0525

824.37169

33

Moments

Time at which Tensile

Force becomes >0.1 LBS/IN

F

P

F

P

Total

Level 

8

25

33

Count

0.24242

0.75758

1.00000

Prob

 N Missing 0

2 Levels

Frequencies

FDFG Delamination

Distributions

Figure 16. Distribution of Time to Reach a Drop of 

0.1 LBS/IN and the frequencies of passes vs 

failures. 
From Figure 15, one can see that the time spent above 0.1 LBS/IN, an arbitrarily chosen change in pressure from the 

beginning value, was twice as long for wall 14B versus wall 14A. Of the mock steel stud walls evaluated the distribution of 

times to reach a drop of 0.1 LBS/IN was plotted and shown in Figure 16. The shaded areas indicate the walls that failed or 

exhibited delamination from the FDFG and how much time the foam spent at a relatively high force (a drop from the start of 

spraying of less than 0.1 LBS/IN). Clearly if the tensile pull occurs too soon, the foam will fail. 

CONCLUSIONS

The Foam Deflection Force Gauge (FDFG) has been developed to investigate in-situ the dimensional stability of freshly 

spray-applied mock walls built with steel studs. With the FDFG, the intermediate scale test using 4X8 foot mock walls gave 

real time pressure profiles of the expanding and gelling polyurethane foam. The FDFG, which employs a modified 

miniature compression load cell to evaluate the forces applied to the studs, was able to significantly reduce the number of 

false positives for steel stud walls where stud line cracking was difficult to observe. 

The FDFG can be constructed using commercially available components and can be used to evaluate foam forming 

characteristics. The FDFG was especially useful in this study when intermediate-scale sized mock walls with steel studs 

were the substrate and commercial-scale spray application equipment was employed. The device gave information about 

dimensional stability of the green foam – or foam early in the curing process. Coupled with bulk foam temperature analysis 

and chemical temperature monitoring at the spray gun, the modified dimensional stability test was successfully utilized to 

develop a spray-applied polyurethane foam insulation that can be applied in winter like conditions. 

REFERENCES

1. Definitions, 2009 International Building Code, Section 1402.1, 2009 (second printing). 

2. “Heat-Air-Moisture Transport, Measurement on Building Materials,” in ASTM Stock Number STP1495, 

Mukhopadhyaya, and Kumaran, eds., ASTM, 2007 

3. Liu, D-L., Nazaroff, W. 2003. “Particle Penetration through Building Cracks,” Aerosol Science and Technology, 37, 

565-573. 

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BIOGRAPHIES 

Steven Crain 

Mr. Crain is a Technology Development Leader in the Dow Building Solutions of The Dow Chemical 

Company. He received his Bachelors of Science degree from Michigan State University in 1983. He 

has conducted product and process research in styrenics and urethanes for over twenty seven years. 

Steve has worked on a number of urethane related projects in the thermoplastic urethane, 

isocyanurate board stock and spray polyurethane insulation fields for the last 10 years. He has coauthored six technical papers and holds four U.S. patents. 

Dalwinder Gill 

Mr. Gill is a test and measurement technologist with National Research Council of Canada. He 

holds an Electronics Engineering Technologist Diploma from Southern Alberta Institute of 

Technology. He has worked with a calibration lab and global high tech firm. He has over 18 years 

experience in the field of electrical, mechanical and optical metrology. He has developed many test 

and calibration methods for unique setups and written corresponding procedures. 

Medhat Hanna

Mr. Hanna is the project manager for the Climatic Engineering Division of the National Research 

Council of Canada. Mr. Hanna has over 15 years of experience in consulting, manufacturing, 

research, and project management in the rail, aerospace, and high-tech sectors. He holds a 

Bachelor’s Degree in Mechanical Engineering from Concordia University in Montreal, Quebec, 

and a Professional Engineering license from the Order of Engineers of Quebec. He has coauthored over 15 technical research papers published by the Federal Aviation Administration 

(FAA) and Transport Canada (TC) in the field of Aircraft Ground Deicing. He also prepared 

dozens of technical reports related to environmental testing projects. 

Donald Leblanc 

 As General Manager of the Climatic and Environmental Engineering Division, Mr. LeBlanc is 

responsible for the business development and operation of CSTT’s Climatic and Environmental 

Engineering Division in Ottawa. In addition to his twelve years at CSTT as manager of the 

environmental facilities, he has over twelve years of experience in private sector test engineering, 

including testing in material research, packaging and product design. Mr. Leblanc is accredited 

with a P.Eng from the Association of Professional Engineers of Ontario. He is also a member of 

The Engineering Society for Advanced Mobility Land Sea Air and Space (SAE), the Institute of 

Environmental Sciences and Technology (IEST) and of the American Society of Heating, 

Refrigerating & Air-Conditioning Engineers, Inc (ASHRAE).