Spray polyurethane Foam (SPF) has played a key role in restoration programs for the dome and exterior of New Orleans’ Superdome.
A combination of economic drivers and technical advances has driven the development of an expanded array of solutions for roofing systems and energy-efficiency improvements in commercial buildings.
Depending on the situation, project-specific conditions and ultimate objectives, these roofing solutions may call for the specification and installation of any of several roofing types, including polyvinyl chloride (PVC), thermoplastic polyolefin (TPO), ethylene propylene diene monomer (EPDM) or synthetic rubber, built up roofs (BUR), metal, extruded polystyrene foam, sprayed-in-place polyurethane foam (SPF), and others.
This review focuses on one of these choices, SPF, and the quality-control measures necessary to achieve a successful installation.
The SPF file
Another high-profile job for SPF: Virginia Tech’s “Lumenhaus,” a star of the 2010 Europe Solar Decathlon, on exhibition in New York City. See A Shining Moment for SPF: Supporting Role in ‘LUMENHAUS.’
SPF is a unique roofing material that is quite different from other conventional roofing and insulation systems, and it can be used to solve some of the most challenging reroofing applications. Since SPF is seamless, it is suitable for roofs with unique configurations such as low-slope roofs with multiple penetrations for mechanical equipment or electrical service systems.
SPF has also been used on roofs with unique shapes, such as the Superdome in New Orleans and Planet Hollywood in Orlando. In the case of the Superdome, SPF was used in restoration programs for both the stadium’s roof and exterior; see Polyurethane Products Restore Superdome Exterior and New Dome for Saints Home.
SPF also can provide a viable solution for building owners interested in energy savings, a valuable asset for designers, specifiers and owners looking to formulate building systems that can deliver energy-use reductions that go well beyond code requirements prescribed in ANSI/ASHRAE/IESNA Standard 90.1-2004, Energy Standard for Buildings except Low Rise Residential Buildings. (See ASHRAE Publishes Guide on Implementing Energy-Efficiency Plans in Commercial Buildings and DOE Issues 3rd in Series of 50% Savings Energy Design Guides, on Retail Buildings).
Improvements to the R-value in walls, roofs and HVAC ducts can be realized using SPF, and it also is a good option to consider in retrofit applications to improve energy performance.
Since SPF can also be used to eliminate air infiltration, it can be used to address air leakage in unique configurations that may have been overlooked during the original design and construction.
Unfortunately, SPF has received some “bad press” over the years, even though most of the problems have been attributed to poor application practices such as applying SPF over dirty or wet substrates, or during weather that was unsuitable for foam application.
Organizations such as the Spray Polyurethane Foam Alliance (SPFA; www.sprayfoam.org) and the National Roofing Contractors Association (NRCA; www.nrca.net) have developed greater awareness and broader knowledge of SPF in the industry. These organizations have developed guidelines for application, specifications, detailing, and quality control that have resulted in improvements to the quality of SPF application. NRCA, in fact, recently introduced an updated manual on SPF roof systems, Metal Panel and SPF Roof Systems—2012.
Accreditation courses for applicators have been pivotal in improving the application quality of SPF. Most SPF applicators currently do a good job thanks to accreditation courses offered through SPFA and the Canadian Urethane Foam Contractors Association (CUFCA; www.cufca.ca), and by SPF manufacturers such as NCFI Polyurethanes.
The critical role of quality assurance
Figure 1: SPF application under way.
Photo courtesy of NCFI Polyurethanes
Like all field-applied foams and coatings, quality control and quality assurance is critical to the successful performance of SPF roof systems. But unlike many other roofing materials, an SPF roof is assembled in the field. Materials such as extruded polystyrene foam, single-ply membranes of EPDM and TPO, and form flashings are manufactured in controlled production settings with rigorous quality processes in place. Manufacturing plants are equipped with automated systems to control temperature and humidity or to catch pumps that go off ratio so that corrections can be made before multiple runs of material are manufactured improperly.
Since SPF serves as the thermal boundary, moisture barrier and flashing, quality control is extremely important during application to ensure the system is properly “site-manufactured.” A successful application of SPF depends heavily on the applicator’s skill and the employment of a quality-control/quality-assurance plan to establish that the substrate is properly prepared, that the foam mix ratio is correct, and that proper ambient conditions are maintained.
Continuous field quality control/quality assurance is necessary throughout the application process in order to achieve a successful SPF application.
Key materials used in SPF systems include spray polyurethane foam and protective surfacing. Primers can be used to facilitate adhesion, but are not a substitute for proper surface preparation.
A number of quality-control and quality-assurance measures that can be utilized on SPF projects are described in the following discussion. Some of the methods discussed are traditional quality measures that have been utilized in the SPF industry for decades, and some describe new technology that can help ensure the success of future SPF applications.
Proper surface preparation is critical to the performance of SPF systems. A good deal of the SPF is applied in re-roofing applications, so an initial, thorough examination of the substrate is critical. Common substrates in re-roofing applications include built-up roofs (BUR) and adhered single-ply membranes.
Listed below are general surface/deck preparation recommendations from SPFA. The association also provides recommendations for specific substrates such as built-up roofs and metal decks (source: SPFA AY 104, Spray Polyurethane Foam Systems for New and Remedial Roofing).
- The roof deck shall be securely fastened to the building structure and conform to proper load limits defined by the applicable building code. Special attention should be focused on the deflection rate under all type roof conditions, including but not limited to, foot traffic, mechanical equipment utilization, as well as live and dead loads.
- When a primer and/or a vapor retarder is specified, there must be adequate adhesion of all components of the system to secure the entire system against wind uplift and movement.
- Prior to application of primer, vapor retarder or polyurethane foam, the deck shall be properly cured, dry, and free of loose dirt or any contaminants that may interfere with proper adhesion of any of these respective components.
- Deck contaminants, depending on their severity and quantity, may be removed by use of air pressure, vacuum equipment, hand power broom, chemical solvents, sandblasting, manual scraping, etc.
Moisture management. Assessing the substrate for moisture is also important with SPF. Excessive moisture impairs SPF curing by interfering with the reaction of Part A, polymeric isocyanate, with Part B, resin-containing polyols. Such an imbalance can create soft, spongy foam with defective cell structure that is susceptible to moisture absorption, and is weaker and less thermally efficient than correctly mixed foam (Griffin & Fricklas).
Moisture surveys can be done to detect existing saturated insulation or coverboard materials. Survey methods include infrared thermography to scan large areas identifying areas with moisture.
Smaller, more localized areas can be identified through the use of moisture meters. One such meter is based on conductivity to check the relative moisture content in materials; two contact pins on the end of the instrument are used to measure the conductivity of the material to establish the moisture content. Figure 2 shows a conductivity meter used on fibrous coverboard under a BUR.
Figure 2: Delmhorst meter used to check coverboard material under BUR.
Non-destructive moisture meters can also be utilized to identify moisture. A GE Aquant Protimeter utilizes radio frequency to assess and monitor the relative moisture level in porous materials. The instrument uses a radio-frequency transceiver principle to determine relative moisture levels in the substrate at up to ¾ inch depth. This author has successfully utilized this instrument to identify wet areas in roofing materials. The instrument provides a method to quickly assess the moisture content of various substrates. Figure 3 shows the instrument being used on a newly installed area of BUR.
Figure 3: Aquant instrument used to detect moisture under BUR.
The Tramex RWS Roof & Wall Moisture Scanner (Figure 4) is another type of moisture meter that can be used for non-destructive moisture evaluations on roofs. The Tramex unit uses electrical impedance, which transmits signals through electrodes at the base of the unit. The instrument has four ranges of sensitivity and can measure up to a maximum depth of 4 inches (10 cm). The signals measure the change in electrical impedance, which is translated into a relative moisture content reading that is displayed on an analog dial. The instrument works on SPF roofs and other non-conductive roofing materials.
Figure 4: Tramex RWS, used to detect moisture in SPF and other building materials.
Environmental conditions. Needless to say, entrapped moisture in SPF can turn out to be disastrous and must be avoided. In order to achieve a quality SPF application, the installation must follow the specifier’s and manufacturer’s written instructions with regard to environmental conditions. Zero tolerance is the approach that must be followed. Unlike other roofing systems SPF cannot be applied when weather conditions present unsuitable temperatures, humidity and wind.
NRCA recommends that SPF should not be applied when the surface temperature is less than 5 F above the dew point or when the surface temperature is below 50 F or above 180 F, or when wind speeds exceed 12 mph at roof height unless wind screens are installed. The manufacturer’s recommendations for ambient conditions during application must be checked and may be more stringent.
The ambient conditions should be frequently monitored to determine if conditions are within allowable tolerances. Unsatisfactory conditions can cause unacceptable physical properties in SPF and can ultimately lead to damaged foam, blisters or delamination.
Portable weather devices such as an anemometer, portable wind meter, psychrometer, and a surface temperature gage will aid in monitoring the weather conditions on a roof. A portable wind meter is an easy-to-use and inexpensive device used to indicate wind speed measurements. Figure 5 shows a Dwyer® handheld portable wind meter.
Figure 5: Dwyer® Wind Meter for checking wind speed.
All-in-one digital psychrometers such as the Positector® unit pictured in Figure 6 give precision ambient conditions that include air and surface temperatures, wet-bulb temperature, relative humidity, and the dew-point temperature instantly at the press of a button.
Figure 6: PosiTector psychrometer for measuring ambient weather conditions.
SPF material considerations
The applicator must also understand the physical properties of the SPF material that is specified. Two of the most important properties of the applied foam are the density and compressive strength. The SPFA and NRCA provide minimum physical properties for foam applied on roofs. The properties very slightly between the two organizations and are shown in Table 1.
While testing for compressive strength and density are designed to be laboratory tests, field testing is possible and recommended. Portable foam testers are available to evaluate the foam compressive strength of a sample in the field. Comparisons between laboratory tests and field test equipment for compressive strength show that results vary less than 10%.
Open-cell content can also be quickly checked in the field by simply placing a sample in water to see how much weight the sample gains. Quick field tests of compressive strength and density on mock-up samples are beneficial before production spraying begins, to reduce the potential for spraying defective foam.
Foam appearance and foam thickness must also be monitored during application. Certain visual characteristics during application provide clues that the material being applied is faulty. The NRCA provides descriptive visual characteristics to help identify when certain deficiencies may be occurring. Visual characteristics associated with a lack of isocyanate (component A), a lack of resin (component B), excessive shelf life, improper storage, and contamination by moisture are described in the NRCAMetal Panel and SPF Roof Systems manual.
The foam should be continually examined for these deficiencies during installation to avoid the application of defective material.
Foam thickness must also be monitored throughout the SPF application process. SPF must be applied in a minimal pass thickness of ½ inch, and the total SPF thickness should be a minimum of 1 inch. The designer and/or the manufacturer will provide specific thickness requirements for the application.
SPF should be applied uniformly with a tolerance of plus ¼ inch per inch of thickness minus 0 inch (SPFA AY-104). Thickness can be easily determined by using a small-diameter probe such as a needle or wire and measuring the depth of penetration.
SPF roofs should be designed with positive drainage, and standing water should be avoided. It is normally recommended as part of the design that a minimum of ¼ in./ft. (2%) slope be achieved. In some cases it is necessary to build up areas during installation to promote positive drainage and eliminate low areas.
The surface texture of SPF must also be monitored closely. SPFA and NRCA provide visual surface texture photographs that serve as as industry reference standards for SPF texture. The visual standards are provided in a number of SPFA documents; AY 145 Surface Texture of Spray Polyurethane Foam is one source.
Appendix 1 of the NRCA Roofing Manual: Metal Panel and SPF Roof Systems offers a similar visual reference standard. The visual guide provides photographs and descriptions of six surface textures. These textures from smoothest to roughest include smooth, orange peel, coarse orange, verge of popcorn, popcorn, and tree bark. It is recommended that protective surface coverings not to be applied over a surface texture rougher than “verge of popcorn.”
Proper detailing of SPF systems is also extremely important. Although SPF is considered the primary flashing in many cases, additional construction details are sometimes needed, and involve the use of other components such as metal counter flashings, drain heads and pitch pans. The applicator must be aware of these other components and allow for proper tie-ins during SPF installation.
Chapter 6 of the NRCA Metal Panel and SPF Roof Systems manual provides an excellent resource for construction details. More than 60 construction details are provided that illustrate necessary details for tie-ins at parapets, edge flashings, equipment support curbs, pipe enclosures, etc.
Since SPF is sensitive to ultraviolet light, it is important that additional lifts of foam and protective surfacing are applied in a single day. If 24 hours elapse between SPF and protective-surfacing application, the foam must be inspected for UV degradation, oxidation or contamination. As closed-cell SPF is exposed to UV, it gradually changes color from a cream to burnt orange, similar to spray-can foam (Great Stuff®) when exposed to UV. Figure 7 illustrates SPF over-exposure to UV where the coating had been removed.
Figure 7: Protective coating damaged by UV (burnt orange color indicates damage).
If foam damage is observed, the SPF manufacturer should be contacted regarding a corrective action plan to repair the foam damaged by UV overexposure.
Protective surfacing and coating
Protective surfacing can take the form of a coating or loose aggregate, and either will work. SPFA and NRCA both recommend that aggregate surfacing be embedded in a topcoat; however, it should be noted that some SPF roofs have been installed using aggregate surfacing without the use of coatings.
Elastomeric coating materials are often used over SPF, and typical chemistries include acrylic, butyl rubber, Hypalon®, Neoprene®, silicone, polyurethane elastomer, PVDF (fluoropolymer), and modified asphalt (SPFA AY 102, A Guide for Selection of Elastomeric Protective Coatings over Sprayed Polyurethane Foam). The type of coating used on SPF is selected based on environmental conditions, code requirements, anticipated foot traffic, and moisture-vapor transmission.
In this author’s experience, vapor transmission is an extremely important factor that is often overlooked. The vapor retarder must be placed on the side that has the highest prevailing humidity. Improperly placed vapor retarders can lead to moisture buildup in building materials.
A number of factors must be considered in the placement of vapor barriers, and in some cases they are required below the roofing system. In addition, a “breathable” (vapor-permeable) coating is sometimes needed as the protective covering, making acrylics or silicones the recommended choice due to their permeability property.
Tight quality-control/quality-assurance measures are required in order to produce the optimal dry film thickness (DFT) of the coating. The objective is sufficient protection of the SPF while allowing interior vapor to diffuse through the coating film, reducing the potential for blistering or wetting of the foam material.
Quality procedures for coating application
As with any coating application, various quality control/quality assurance measures are required to produce a quality product. Some manufacturers require specific inspections for warranted projects, and inspections are needed to determine if the application is compliant with written specifications and manufacturer instructions.
Quality-control/quality-assurance inspections for coating over foam include verification of the following:
- Protective coverings in place around HVAC equipment, parapet walls or other items where coating is not to be applied
- Proper surface texture of the foam, surface cleanliness, and removal of contaminants
- Moisture content of the foam
- Ambient conditions and surface temperature for coating application
- Specified materials use
- Material shelf life
- Proper material mixing
- Application of multi-component materials within the stated pot life
- Application equipment
- Pot agitation and temperature
- Intercoat cleanliness
- Recoat-time compliance
- Visual appearance for runs, sags, pinholes, missed areas, and uniformity of coating coverage
- Wet film thickness
- Dry film thickness (DFT)
Typically, two coats of elastomerics are required over SPF. Uniform coverage is important not only for external protection but also for proper “breathability” when applicable.
Coverage rates have traditionally been monitored by calculating theoretical coverage rates and using destructive testing with an optical comparator to measure the dry film thickness.
When calculating coverage rates, it is important to note that theoretical dry film thickness (DFT) coverage rates of a coating are based on the fact that 1 gallon of a 100% solids paint applied at a 1-mil thickness will cover 1,604 square feet of surface area. This theoretical rate assumes that all of the coating (100%) in that gallon is solid film-forming material (not thinned), and will always be spread out at a 1-mil wet film thickness (WFT), producing a 1-mil DFT over a 1,604-square-foot area.
Many elastomeric coatings are not 100% solids by volume, however, so the dry film thickness will be less than the wet film thickness.
For example, if an elastomeric coating is 70% solids and the manufacturer recommends applying the elastomeric at a 3.5-mil dry film thickness, some simple calculations are needed to determine actual square-foot coverage rate. An unthinned 70% solids-by-volume coating will cover about 1,123 square feet at 1 mil DFT (1604 x .70 = 1,122.8 sf), and when applied at a 3.5-mil DFT, the coverage rate will be approximately 321 square feet per gallon (1,123 sf / 3.5 mils = 320.8).
Once the theoretical coverage rate is calculated, additional material percentages must be added to allow for texture of the SPF and wind loss. For example, 10% should be added for SPF with an orange-peel texture and 1% for wind loss. Determining uniform coverage using spread rates can be used to provide approximate coverage, but should be supplemented by other methods to determine actual DFTs due to uncertainties in material loss.
Optical comparators have been used widely in the SPF industry to check DFTs. SPFA recommends slit samples be taken at the following rate: 10 samples for the first 100 squares (square=100 square feet), plus one for each additional 25 squares. The recommendation is to examine each sample using the optical comparator to determine the average and the minimum thickness of the base coat and topcoat.
Non-destructive coating thickness gages also have been found to provide accurate and reliable results in determining DFTs in the coating industry. Ultrasonic coating thickness gages can be used over non-metallic surfaces such as wood, concrete and plastic, and can measure up to 150 mils. This type of instrument can provide the total coating thickness and coating thickness of each layer.
Figure 8: PosiTector 200® ultrasonic coating thickness gage.
Photo courtesy of DeFelsko Corp.
Figure 8 shows a photo of an ultrasonic gage. The instrument gives DFT measurement for each layer and the total thickness on the left side of the screen and averages and standard deviations on the right side of the screen. The unit displayed can store up to 10,000 readings in up to 1,000 batches. This type of gage provides quick and accurate DFT measurements non-destructively, conforming to ASTM D6132, Standard Test Method for Nondestructive Measurement of Dry Film Thickness of Applied Organic Coatings Using an Ultrasonic Gage.
Quality control’s central role
in SPF applications
Since SPF roof systems are fabricated in the field and are not manufactured in a controlled production and quality-assurance setting as is possible with manufactured roofing products, thorough quality control and quality assurance is necessary in the field to produce quality SPF systems.
The Spray Polyurethane Foam Alliance (SPFA) provides recommendations for quality control and physical testing in section VII of the AY 102 manual on guidance for the application of elastomeric coatings over SPF. Additional information regarding quality control for SPF is provided in the NRCA/SPFA Quality Control Guidelines for the Application of Sprayed Polyurethane Foam Roofing.
This discussion has sought to summarize a portion of the quality-control/quality-assurance requirements for the installation of SPF, together with additional control measures that are commonly used in the coating industry and that can prove beneficial when applying the protective surfacing coating over SPF.
Griffin, C.W., and Fricklas, R.L., 2006. “Manual of Low-Slope Roof Systems” 4th edition, McGraw-Hill Companies, New York, N.Y.
Spray Polyurethane Foam Alliance, AY 102: A Guide for Selection of Elastomeric Protective Coatings Over Sprayed Polyurethane Foam.
Spray Polyurethane Foam Alliance, AY 104: Spray Polyurethane Foam Systems for New and Remodel Roofing.
NRCA Roofing Manual: Metal Panel and SPF Roof Systems—2008.
Kevin Brown is the Manager of the Commercial Services Group for KTA. In this position, Kevin develops and implements maintenance programs for commercial clients nationwide who are experiencing architectural/commercial problems related to paint failures. He has over 12 years of experience in the field of retail facility management overseeing building maintenance and preventative maintenance programs for over 1,700 stores including store repaints, floor coating replacements, and long-range budget planning.