Sprayed-in-place polyurethane foam (SPF) systems have been on the United States commercial low-slope roofing market since the late 1960s. 

Sprayed-in-place polyurethane foam (SPF) systems have been on the United States commercial low-slope roofing market since the late 1960s. The application methods and material components have been continually upgraded over the years to the point where these systems can provide significant benefits on select roofing applications. One significant advantage of SPF systems is that they utilize a field manufactured, seamless foam insulation. With the addition of a weatherproofing coating, SPF systems can provide both waterproofing protection and thermal capacity in an economical and efficient manner.

SPF systems are manufactured from a chemically complex mixture of polymeric materials such as liquid isocyanates and liquid polyol resins, which are added with various cell stabilizers, blowing agents, combustion-retarding agents, catalysts and fillers. The polyurethane foam is developed by the heat reaction between the liquid isocyanates and liquid polyol resins coupled with the vaporization of the blowing agent. This reaction forms cells that change the material from a liquid to a cellular mass. The material is spray-applied over the existing substrate.

This photo shows a roof with spray polyurethane foam covered with a bright white acrylic coating to protect the SPF from ultraviolet degradation. (Photo courtesy of Aldo Products.)

SPF systems provide a high thermal resistance on a per-unit thickness basis and the finished system (including the coating) is very lightweight. The R-value of SPF is nearly R-6 per inch and a 3-inch-thick SPF foam application with coating weighs less than 1 pound per square foot. This allows for the widespread use of the material in recover applications without live or dead weight concerns that may be present for other conventional systems. In addition, the system provides significant compressive strength with a minimum of 40 psi. This is beneficial in reducing damage from foot traffic.

A primary advantage for roofing contractors is that system application only requires spray equipment, which is easily transportable, and material that is mixed and applied on-site. Application methods are easy to comprehend once the applicators are properly trained. Like many systems, there is an extreme dependence on the applicator’s skill level, particularly on proper operation of the spray equipment. Properly trained work crews can overcome potential system disadvantages by controlling the material thickness and applying material in an even and consistent fashion.

Once applied, the material becomes monolithic. The material is seamless and there is no transition between the field application and at vertical junctures or flashing points. This reduces the risk of potential openings at typically vulnerable junctures such as flashings, penetrations and terminations.

SPF thickness can be controlled during application to achieve proper slope for adequate roof drainage. This eliminates the cost - and trouble - of applying complex tapered insulation systems. Furthermore, field-applied crickets can be fabricated with additional SPF thickness at curbs and penetrations to divert water away from the penetrations and allow for proper flow to the drains.

If the substrate is properly prepared and the application is consistent with the manufacturer’s requirements, SPF systems provide extremely tenacious adhesion characteristics. Studies conducted in high-velocity wind zones after storm events such as hurricanes have found that SPF systems performed admirably. The material’s adhesion can also be considered a disadvantage when SPF removal is required. The material removal proves oftentimes prove to be slow, tedious and more expensive than the removal of other conventional systems.

Most problems associated with SPF systems have been from improper application procedures. Preparation of the surface is critical for success. This includes thorough removal of all loose particles. Polyurethane will not adhere to dirty, wet or oily surfaces. SPF application over dirty or wet surfaces could produce formation of blisters. Moisture also disrupts the chemical foaming reaction between the components, creating an imbalance that reduces the system’s moisture protection properties and reduces thermal efficiency.

The most common application error involves improper SPF thickness. There is a tendency on the applicator’s part to over-spray the material in some areas and not provide a constant thickness. Uneven application creates depressions in the surface - known as birdbaths - that regularly pond water.

Another potentially detrimental condition occurs when the application of the SPF lifts and the surface coating are not applied the same day. Delays in these applications can lead to delamination of the foam and peeling of the surface coating.

The proper application rate of SPF should be between a minimum of 1/2 inch and maximum of 1 inch per lift. Application thickness should be in compliance with project specifications and the manufacturer’s requirements. The total SPF system application (foam and coating) must be completed in the same day. If the roof area is too expansive to allow completion in one day, then only apply SPF in an area that can be completed that day. If precipitation occurs during the application process, inspect the system for ultraviolet degradation and remove all bad materials prior to completion.

The proper surface finish should be smooth with an orange peel or coarse orange peel texture finish. Surface finishes that resemble popcorn texture are vulnerable to coating defects such as cracking.