Closed-cell spray polyurethane foam: Shrinkage and how to manage it

by tanya_martins | February 10, 2025 2:43 pm

By Rachel Lynde, P.E., Niklas Vigener, P.E., Spenser Simis

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Closed-cell spray polyurethane foam (ccSPF) insulation is used in exterior roof and wall assemblies to effectively combine air/vapor/thermal barrier and sometimes weather barrier (Figure 1). A critical aspect of ccSPF is it can undergo post-installation dimensional changes that are problematic when they result in breaches of the intended barriers. This article discusses conditions that lead to ccSPF shrinkage and separation from substrates and strategies to mitigate them.

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ccSPF is always combined with other functional components of the wall or roof assembly (e.g. water barrier, structural framing, ignition barrier, etc.) to create an effective, durable, and building code-compliant assembly. These components and their effect on building performance are outside the scope of this article. Similarly, open-cell spray polyurethane foam (ocSPF) is used in building envelope assemblies as an air and thermal barrier. It has different performance and technical characteristics than ccSPF and is also not addressed here.

ccSPF shrinkage and common installation defects

The chemical reaction that creates two-component ccSPF is the process of isocyanates (from one component) combining with polyols (from the second component) to create polyurethane that, with the help of a blowing agent, creates the ccSPF cell structure.¹ Water is part of one component, making moisture control critical during application. ccSPF components are pre-heated, but the chemical reaction is exothermic, resulting in the release of heat. The chemical composition of these components varies by manufacturer and includes admixtures that modify various properties, including reaction time, color, and ccSPF cell size.² During application, the ccSPF expands rapidly when the components are mixed during application, then shrinks as it cools, producing tensile and shear stresses on bonding surfaces. After hardening and curing, unrestrained ccSPF will shrink slightly (approximately 10 percent per manufacturer’s testing), inducing more shrinkage forces in cured ccSPF.

Like any material, ccSPF also undergoes thermal movement in service in response to in-service temperature variations. If the ccSPF is unrestrained, the shrinkage component of the cycle may result in separation from surrounding materials with different geometries and expansion coefficients, and the growth component of the cycle may result in compressive loads on surrounding materials (Figure 2). Shrinkage cracks sometimes do not appear until well after installation and after the foam has been covered. So repairs (e.g. respraying), while workable, are often impractical given the pace of construction.

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Designers and installers understand and compensate for these effects by sequencing installation to account for initial expansion (e.g. through installation in thin layers or alternating strips), compartmentalizing ccSPF installation between framing bays to limit large expanses that can cause splitting when shrinkage occurs, and by ensuring strong and uniform substrate bond that prevents delamination of the ccSPF from the substrate and evenly distributes strain.

Consequences of ccSPF shrinkage

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The cracks and separations caused by ccSPF shrinkage are especially problematic where the ccSPF is a component of the building’s air barrier (Figure 3). Air barriers require perfect continuity because even minor holidays allow rapid movement of conditioned air across the building enclosure, especially under differential pressure.³ This air movement dramatically reduces thermal energy efficiency. In any building subject to variable temperature and relative humidity (RH) conditions between inside and outside (particularly mechanically humidified buildings), moisture-laden air can travel through breaches in the air barrier to colder regions within the enclosure assembly, where condensation can occur. The resultant moisture accumulation and material deterioration are particularly destructive in wood-framed or light-gauge steel construction and without drying potential (Figure 4 and 5).

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Strategies to limit ccSPF shrinkage and separation from substrates

The following steps, which the designer of the ccSPF assembly should anticipate, can limit the impact of ccSPF shrinkage:

Substrate cleaning

Substrate materials with coatings or contaminants that interfere with adhesion (e.g. light-gauge steel framing coated with rust inhibitors, cast-in-place concrete covered with form release agents) require diligent cleaning, not just because these coatings may interfere with adhesion but because they can be chemically incompatible with the ccSPF. The effectiveness of the substrate cleaning must be evaluated using adhesion tests on ccSPF sample installations.

Mechanical restraint

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Substrate materials with very smooth surfaces (e.g. dimension lumber, laminated veneer lumber [LVL], architectural aluminum, stainless steel, and many air barrier membranes), even if they are free of contaminants, require roughening by mechanical means or the addition of a mechanically attached interference device that engages and restrains the ccSPF (Figure 6). Rigid lath used in stucco or tile construction is suited for this application and provides a physical bond substrate that holds the ccSPF in tight contact with the substrate to prevent shrinkage movement. The authors have had a satisfactory experience with metal (expanded or welded-mesh galvanized steel, welded-stainless steel) or fiberglass lath. All lath types must be furred to stand it off the substrate and allow the ccSPF to completely engulf the lath during installation and effect a tight bond to both lath and substrate (Figure 7). The lath should be installed on all substrates that do not provide sufficient bond and where lath shrinkage will cause breaches in the air barrier. Washered fasteners or staples are required to engage the lath, and fastener spacing must be sufficient to prevent bowing and lath displacement.

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Adding the lath is important at air barrier membrane tie-ins along the perimeter of doors, windows, storefronts, and curtain walls (Figure 8). The ccSPF can bond tenaciously but unpredictably to some air barrier membranes. When it shrinks, the resulting force can overcome the adhesion of the membrane and pull it away from the substrate, opening up gaps in the air barrier. This effect is observable and sufficient to delaminate membranes in even very small expanses of ccSPF.

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Installation technique

During ccSPF installation over the lath, the nozzle should be pointed perpendicular to the substrate, and the ccSPF should be applied in a side-to-side motion so it penetrates through and encapsulates the lath and thoroughly wets the surface below. This is especially important if the lath has small openings, such as galvanized expanded lath. When the nozzle is held at an angle, the ccSPF penetrating under the lath cannot create the expansion pressure needed to fill the space behind the lath. If the application angle is too shallow, the ccSPF may glance off the lath completely, causing a continuous void space under the lath.

Installation conditions

Temperature

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Ambient installation temperature and surface temperature affect the post-installation behavior of the ccSPF. Manufacturers list minimum ambient temperatures, which may require temporary heat if the building’s permanent heating system is not yet functional, or temporary enclosures along with temporary heat if the ccSPF will be installed from the exterior (Figure 9). In addition to ambient temperature, manufacturers stipulate minimum substrate temperatures (typically around -6 C [20 F]). To provide contractors with installation flexibility, manufacturers formulate different material grades for different ambient temperature ranges and for different substrate temperatures, offering several different formulations. These requirements require vigilant field review because, for example, ambient temperatures and surface temperatures of the interior side of a roof or wall can vary within the same building. Because using different ccSPF formulations on the same building is typically impractical, the installer must manage the temperatures of interior spaces and substrates using supplemental heating and installation timing. The surface temperature must be checked using a handheld infrared thermometer to confirm appropriate installation conditions. Note that the temperature of heat-sink substrate materials, such as concrete, usually cannot be elevated sufficiently with interior heat; instead, select a ccSPF grade matched to this condition.

Relative humidity

Ambient humidity awareness and control are also important, and ccSPF must not be installed if the ambient temperature is near (typically within five percent) the dewpoint. Similar to adhesive or sealant installation, this requirement is intended to limit the risk of poor substrate bond when a thin layer of condensation or dew is present on the substrate.
As for the substrate and ambient temperature, this requires either vigilant review or environmental controls, such as portable dryers or dehumidifiers, because the impact of a thin layer of dew on ccSPF bond is difficult to spot during application and defective bond is not apparent until test cuts and adhesion testing are performed.

Substrate moisture

High-moisture-content building materials, such as pressure-treated lumber or green concrete, can challenge ccSPF adhesion. Such adhesion problems may not manifest themselves until the installation is complete, and they could occur even if the substrate is surface-dry and has acceptable adhesion immediately following installation. Mitigating these adhesion problems without providing mechanical attachment is beyond the scope of this article. For wood framing, and as a starting point, the authors recommend confirming the moisture level of any wood framing is below 18 percent using a moisture meter. Framing lumber leaves the kiln at 19 percent but often sits exposed and re-absorbs moisture before the building is dried in. In this case, after the building is enclosed, portable dehumidifiers are set up and run for several days to several weeks before ccSPF installation is required to reduce the moisture level of the framing lumber.

Pre- and post-installation quality control measures

The following quality assurance and control measures are intended to guard against ccSPF adhesive failure and should be discussed and reviewed in a pre-installation meeting:

Detail review—Before installation, review the design to assess conditions that are vulnerable to ccSPF shrinkage or adhesion issues. These include fenestration tie-ins with sheet membrane, substrate materials with potentially high moisture content, smooth substrates to which the ccSPF may not bond, and areas that inadvertently invite very thick ccSPF applications. Identify materials with potentially high moisture content and anticipate mitigation strategies.

Applicator training/experience—ccSPF is a relatively mature technology, and reputable manufacturers provide product-specific applicator training. This training covers planning and quality assurance aspects such as product mixing and application, equipment checks and maintenance,
pre-installation review and testing, and appropriate product selection for anticipated environmental and substrate conditions. Appropriate training and job experience increase the applicator’s successful installation capability.

Use fresh component materials—Check manufacturing dates on component materials and verify how long open containers have been stored. More than two weeks of storage after opening can degrade components and installed ccSPF quality.

Sample installations and adhesion testing—Using the specified materials, perform sample installation and adhesion testing on the project’s actual substrate to assess substrate quality and required surface preparation.

Check ambient conditions and substrate moisture—At the beginning of each work phase, check the dewpoint, ambient RH, ambient temperature, and substrate temperature to confirm they are in the recommended range for the ccSPF product. Confirm the moisture content of porous substrate materials.

In-production checks—Keep in mind that substrate and ambient conditions vary, and the ccSPF mixing and dispensing equipment require set-up and adjustment at the beginning of each workday. Anticipate changes in these conditions and perform regular in-production checks to confirm acceptable adhesion. These checks should include test cuts that demonstrate the ccSPF bonds tenaciously. With strong adhesion, a ccSPF sample cannot be removed intact from the substrate but fails cohesively, with a portion of the ccSPF remaining bonded and requiring mechanical removal.

Conclusion

ccSPF insulation combines the functions of air, vapor, and thermal barriers and, when responsibly used, gives designers and builders design flexibility to conceive efficient and durable envelope designs. Its material properties and installation process can create challenges, including the potential for post-installation shrinkage, inadequate substrate bond, and vulnerability to adverse environmental conditions during installation. These challenges must be anticipated and managed during design, project planning, and construction.

Notes

¹ See the article “Troubleshooting Spray-Foam Insulation” The Journal of Light Construction at jlconline.com/how-to/insulation/troubleshooting-spray-foam-insulation_o

² Read the article “Spray-Foam Problems” Fine Home Building (October/November 2016), pages 46-49.

³ Learn more by reading “Sealed Attics Exposed to Two Years of Weathering in a Hot and Humid Climate” ASHRAE Transactions, (2016) vol. XXX, Part Z.

Authors

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Rachel Lynde, P.E., is a senior consulting engineer with Simpson Gumpertz & Heger’s (SGH) building technology group in Waltham, MA. She works on projects involving building enclosure investigation of existing buildings, remedial design and renovations, and design consultation of new building enclosure systems. She can be reached at rmlynde@sgh.com.

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Niklas Vigener, P.E., is a senior principal with SGH and leads their technical operations as chief technical officer. His experience includes designing, investigating, and rehabilitating building enclosure systems for new and existing buildings. He can be reached at nwvigener@sgh.com.

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Spenser Simis is the operations manager of Nate Holyoke Builders, a residential building contractor based in Maine. His work includes the planning and construction of new building and renovation projects, as well as maintenance and rehabilitation work to existing structures throughout the state. He can be reached at spenser@nateholyokebuilders.com.

Key Takeaways

Closed-cell spray polyurethane foam (ccSPF) insulation effectively combines air, vapor, and thermal barriers in building envelopes. However, post-installation dimensional changes, such as shrinkage and separation from substrates, can lead to breaches in air barriers, reducing energy efficiency and causing moisture issues. This article highlights conditions contributing to ccSPF shrinkage, including temperature variations, moisture content, and improper substrate preparation. Strategies to mitigate these challenges include meticulous substrate cleaning, mechanical restraint through lath installation, and adherence to precise application techniques under controlled environmental conditions. Quality control measures are essential, such as adhesion testing, monitoring ambient conditions, and ensuring applicator expertise.

Source URL: https://www.constructionspecifier.com/ccspf-shrinkage-prevention-installation-tips/

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