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Home | Spray Foam Industry | Shrinking Foam & Dimensional Stability

Shrinking Foam & Dimensional Stability

How Incorrect Dimensional Stability Can Prevent SPF From Performing Correctly

By Mason Knowles 


One of the more common issues I run across in my consulting business is shrinking or contracting closed-cell spray foam (ccSPF).  This is where the foam shrinks from a stud creating a gap between the stud and the foam.  This gap can prevent the spray foam from performing effectively as an air barrier, reduces its insulation effectiveness, and can create conditions that lead to condensation.

So what is the connection between shrinking foam and dimensional stability and why is it important?

All foam plastics expand or contract to some degree.  This expansion and contraction is referred to as dimensional stability.  It is important to have the correct dimensional stability for the application.  For example a ccSPF that works well in a cold storage application may not perform as a roofing product.

SPFA in its technical guidelines and educational courses refers to ASTM C 1029; Standard Specification for Spray-Applied Rigid Cellular Polyurethane Thermal Insulation based primarily on compressive strength and dimensional stability for the different types of foam used for various applications, explained in the table below.

Type I ccSPF typically can be used for interior applications where the temperatures and humidity are more moderate.  Type II ccSPF can be used in more extreme temperatures and humidity such as cold storage facilities, mushroom farms, and swimming pools.  Type III ccSPF is used in exterior applications such as roofing and exterior tank insulation. Type IV ccSPF is fairly rare but would be used where the foam requires exceptional impact resistance such as heavy foot traffic or severe hail regions.

ASTM D 2126; Standard Test Method for Response of Rigid Cellular Plastics to Thermal and Humid Aging is used determine the dimensional stability of foam plastics. It measures the volume change of a sample when exposed to high heat and humidity.  In most cases the foam sample expands, although it can shrink.  The resulting change in volume is reported as percent volume change in a linear direction.

When checking out a ccSPF’s dimensional stability it is important to verify the supplier is using similar temperatures and humidity for tests. For example, if the supplier tests their foam at 90°F and 50% humidity, their dimensional stability measurements are more likely to be lower (i.e. better) than at the higher temperature and humidity. If in doubt about the dimensional stability of a ccSPF use the one with the higher compressive strength.

Poor dimensional stability can cause ccSPF to contract from the studs. In some cases, the contraction can be severe.

Poor dimensional stability can cause ccSPF to contract from the studs. In some cases, the contraction can be severe.

The information in the table above was obtained from the technical data sheets of five different foam manufacturers nominal two pcf (pounds per cubic foot) ccSPF.  As you see, some used the current C-1029 humid aging procedure and some used different procedures.

Foam A’s value would be hard to compare to the others since the foam was exposed to 75°F temperature instead of 158°F. ccSPF is typically more stable at moderate temperatures than at higher temperatures.

Foam B reported a total volume change. It is generally higher than when reporting a linear change.  So a 3.06 percent total volume change under the test conditions would be considered good for a two-pcf-density ccSPF.

Shrinking ccSPF can warp and crack drywall

Shrinking ccSPF can
warp and crack drywall

Foam C just reported pass.  It was unclear from the data sheet what value it was using.

Foam D & E used the current C 1029 procedure and reported value.  The foams can be categorized properly and specified for various applications using SPF industry guidelines.

Using a type I ccSPF with a lower dimensional stability measurement instead of a Type II ccSPF in a cold storage or hot/humid environment can result in shrinking or contracting foam.  Gaps and voids created could compromise the air tightness, insulation efficiency of the assembly and could cause condensation issues.

In roofing applications ccSPF with poor dimensional stability can cause blisters

In roofing applications ccSPF with poor dimensional stability can cause blisters

In a roofing application using a Type II instead of a Type III ccSPF usually results in blisters within the foam and/or expansion of the foam at the roof edges.

Most shrinking/contracting foam applications that I have been hired to investigate involve a ccSPF that was correctly specified but not installed properly.  Some factors that can affect the dimensional stability are as follows:

Off Ratio Foam

ccSPF should be installed with no more than two percent variation of the 1 to 1 ratio of A side to B side.  Off ratio foam will result in a material that does not exhibit optimum physical properties.  B-rich foam will have weak cell structure with poor dimensional stability and potentially strong odors. In extreme cases it will be soft and gummy and in less extreme cases, softer and exhibiting lower compressive strength.  A rich foam conversely will be much harder and potentially friable and brittle with very high compressive strength.  Off ratio foam can contract and shrink from the studs (illustrated on the previous page).

Spraying ccSPF too thick in one pass can cause poor cell structure, lower compressive strength, and poor dimensional stability

Spraying ccSPF too thick in one pass can cause poor cell structure,
lower compressive strength, and poor dimensional stability

Sprayed Too Thick

SPFA in its technical guidelines and educatonal materials recommend that foam be sprayed in minimum lift thickness of 0.5 inch to a maximum of 1.5 inches or as recommended by your material supplier.  Spraying the foam in 0.5 inch to 1.5 inch lifts allows the applicator to reach the desired thickness and physical properties to provide an effective and stable insulation.  In addition sufficient time must be allowed between lifts to allow exothermic heat from the chemical reaction of the rising foam to dissipate.  Most ccSPF systems require 10 to 15 minutes between lifts for the heat to dissipate.  When foam has been installed too quickly over freshly sprayed foam, thermal degradation of the foam can occur which causes the foam’s cell structure to lose strength and become soft in the center.  The soft foam is less dimensionally stable and more prone to contraction and shrinking.

Wrong Temperature, Pressure, and Mechanical Mixing

The right combination of heat, pressure, and mechanical mixing of the materials is required to produce the desired physical properties of each foam system.  Varying one of these factors will affect the others.  For example, raising the heat of the material allows a person to reduce pressure and impingement mixing.  Raising the pressure allows a reduction of heat and increasing the impingement mixing of the foam allows a reduction of heat and pressure.  But if any of the factors are varied without changing the others to accommodate it, the quality of the foam suffers.  Foam that is lacking sufficient temperature or pressure will not mix properly causing poor cell structure, strength, and dimensional stability.  Foam that is too hot will form high exothermic temperatures that can cause elongated foam cells, cracks, and fissures in the foam, poor dimensional stability and strong odors within the foam.

How do you identify spray foam that is likely to contract?

Obtain quality control samples before and during application.  Check out the middle of the foam core for discolored foam, elongated cells, cracks, fissuress.  Use the thumb test or a field compression tester to check out the compressive strength of the foam both perpendicular and parallel to rise.  If it is softer in the middle than at the top and bottom, there is a good chance that foam could shrink.

A portable compression tester can be used in the field to obtain more precise compressive strength measurements

A portable compression tester can be used in the field to obtain more precise compressive strength

An experienced foam applicator or inspector can typically tell within 5 psi the compressive strength of the foam by using the thumb test.  Obtain a core sample of foam from the substrate.  Cut off the top and bottom 1/3 of the core.  Apply firm pressure with the thumb to the top of the sample.  If the foam depresses ¼ inch then it is approximately  25 psi, if it depresses 1/8 or less then it is greater than 40 psi.  Perform the same test to the sides of the sample as well.  It should have similar compressive strength.  If it is much softer, then the foam is more likely to shrink or contract.

If the foam is suspected of inadequate compressive strength, a sample can be obtained and measured with a field compressive strength tester such as a Com-Ten tool.  Typically a 2-pcf (pounds per cubic foot) density ccSPF would exhibit between 20 to 30 psi.  If the foam is softer or harder than this, it is an indication of an off-ratio or poor mix.

Repair all foam that measures less than 20 psi but greater than 15 psi as follows:

  • Identify areas of suspect adhesion and poor quality foam by performing test cuts adjacent to observable cracks
  • Remove cracking or shrinking foam by cutting back foam in a 45 degree angle until it reaches foam that exhibits good adhesion and physical properties.  (Note V groove cuts should be used when the crack is in the center of the cavity).
  • Prepare substrate as required to accept new SPF.
  • Install new SPF to specified thickness in a picture frame pattern in lift thickness of ½ to 1 for the first lift and no more than 1.5 inches for subsequent lifts.  Wait 10-15 minutes between lifts or until the exothermic heat reaction has dissipated (or in accordance with manufacturer’s written instructions). •


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