PUF The Magic Roofing

The above are just a few of the advantages typically associated with sprayed-in-place foam systems. The question then becomes why, with all their potential advantages, have polyurethane foam (PUF) systems not captured more of the roofing market? This article will explore some of the reasons that have accounted for their limited growth.

Market Share

It is always difficult to obtain exact numbers on the share of market held by a particular product or system. In the case of sprayed-in-place polyurethane foam, it is safe to say that its portion of the market certainly is less than 10 percent.

Industry sources consistently place PUF in a range from 3 percent to 8 percent for both new roofing and reroofing. The annual surveys of subscribers conducted by Contractors Guide shows results that agree with these figures. (See table 1.)

                                                       Table 1

       Question: When doing commercial/Industrial work, approximately what

                             percent of your business is spray foam?

               1988                         1987                      1986                    1985

               3.7%                           6%                       3%                       3%

Source: 1988 Roofing Contractors survey, Contractors Guide

The expected growth of this product system also is difficult to project. At a recent meeting of the Polyurethane Foam Contractors Division of the Society of the Plastics Industry (SPI), Dr. P.J. Manno of Dow Chemical Co., Midland, Mich., indicated the total demand for rigid foam in the United States would grow at an average rate of 5 percent per year to the year 2,000.

Building and construction applications make up more than half of rigid foam consumption. Therefore, such applications should be at or near this growth rate. Manno suggested that demand would be influenced by a variety of factors, such as chlorofluorocarbon regulations, R-value drift, supply of raw materials, energy concerns and recycling and waste disposal issues.

  Sprayed-in-place foam has been available for about 20 years. It has not achieved the market share and growth that was originally predicted, even though it was available during the energy crisis of the 1970s. Those times should have spurred its acceptance and given it broad exposure in the marketplace.

Slow Growth

A number of explanations have been suggested to account for the slow market penetration of PUF roofing systems. Some of the ones most often cited are:

  • Overpromotion when they were first introduced as the answer to all roofing problems;
  • Lack of acceptance by the larger contracting organizations;
  • Sensitivity of the technology and its application to certain weather and climatic conditions;
  • Complexity of the system and the need for highly trained applicators and expensive equipment;
  • Total reliance on the coating to protect the foam from water, ultraviolet rays and physical abuse;
  • Concern about the fire resistance properties of the system.

This article began by enumerating some of the desirable features of PUF systems. When scanning this list of advantages, it would be quite easy to get caught up in the positives and lose sight of the limitations of PUF.

It is typical for technology developed outside of roofing to suffer through a learning curce when the realities and complexities of this industry are discovered. The combination of the potential benefits which the system offers along with its promoters’ inexperience in the roofing industry very likely could have led to some of PUF’s early difficulties.

Some of the problems associated with PUF systems will be discussed later in this article. But proper specification and application are much more critical to urethane foam than any other roofing system.

The fact that the larger roofing contractors have not fully adopted PUF is due to a number of factors. The early applicators typically did not come from the ranks of roofing contractors.

The high level of applicator training and the expense of new equipment discouraged contractors with established businesses from investing in an unfamiliar system, which even today is considered foreign to their typical product options.

The complexity and weather sensitivity of PUF’s application also has generated some concern about the contractor’s liability for improper installation.

A very important factor which has slowed PUF’s acceptance by roofing contractors is its limited specification by architects. If these systems were part of roofing bid invitations more frequently, contractors would show a greater willingness to commit their time and resources to urethane foam.

Complex Application

There is no doubt that weather limitations have restricted the growth of PUF to predominately Sun Belt states. There may be good reasons for this perception.

Polyurethane foam is based on complex chemistry. In essence, the same chemical reactions which are used to manufacture urethane and isocyanurate board stock under plant conditions are used to foam in the field.

Just a few of the variables that affect foam quality in field applications include spray pattern and technique, mix ratios, component temperatures and the age of the chemicals being used.

Polyurethane foam must be coated within 72 hours of application. Foam, if left exposed, will begin degrading to a minor degree the same day it is installed.

It is recommended generally that multiple passes of foam and their coating be applied the same day. The coating must protect the foam from water, ultraviolet light and physical abuse, such as foot traffic, hail impact and birds pecking through it to gain access to the insulation.

Coatings must possess sufficient fire resistance to achieve suitable ratings for code approvals. They also must adhere well, both to protect the foam and to prevent their own blow-off from wind uplift forces.

The performance of the coating is related substantially to the surface texture of the foam. An uneven foam application with significant surface roughness creates a number of potential difficulties.

When high and low points are present in a foam surface, the coating will tend to flow off the high points and accumulate in the low areas. The result is much less coating on the raised points than desired.

Because the high points receive the greatest abuse from foot traffic and other sources of erosion, this reduced thickness of coating can lead to problems in extended protection of the foam.

If the foam surface is uneven, inexperienced applicators sometimes miss the backside of raised points on the foam surface due to improper angling of the spray gun. The use of a paint roller also is a problem on an uneven surface because the low points will not come in contact with the applicator surface.

A foam with a great deal of surface texture will have significantly more area to coat. If material usage is not adjusted to compensate for the increased surface area, an application of coating below the recommended thickness will result.

Over the years, sprayed-in-place foam has had a reputation for being deficient in fire resistance. This perception undoubtedly has discouraged some from pursuing the application of foamed-in-place materials.

Urethane board stock also has suffered to some degree from the same concern. But a quick scan of the Underwriter’s Laboratories (UL) Building Materials Directory indicates that there are a great number of sprayed-in-place systems which carry a Class A rating on slopes as high as 4 ½ inches per foot.

The Factory Mutual (FM) Approval Guide lists several coatings carrying a Class I approval over urethane that is spray-applied to an existing roof. The rating is contingent on the foam having a flame spread rating of 75 or less when tested by ASTM E84. It appears that reasonable fire ratings can be achieved with PUF systems providing proper material combinations are selected.

Questions of Geography

The proponents of PUF systems point to highly successful applications in all parts of the United States, or more generally, in all parts of the world. This undoubtedly is correct. On the other hand, there are very definite climatic conditions which increase the difficulty of installing PUF and the chance of problems.

A recent Naval Civil Engineering Laboratory publication titled Users’ Guide for Polyurethane Foam Roofing specifies the weather conditions that must be avoided for a quality foam installation. Authored by K. Coultrap, R.L. Alumbaugh and E.F. Humm, this report indicated that to avoid problems, the following criteria should be used.

No spraying should be done if there is visible moisture on the surface to be foamed or if conditions of rain, snow, fog, or mist are present.

The dew point must be more than 5 degrees F below the temperature of the surface being foamed.

Wind speeds cannot exceed 12 miles per hour unless some form of windscreen or shield is used.

Surfaces cannot be below 50 degrees F to 60 degrees F – otherwise the proper foam rise will not occur.

The temperature of the surface to be foamed must be lower than 120 degrees F.

Additional conditions are outlined in the report, but the above ones will be sufficient to illustrate the following example.

Monthly summaries of Local Climatological Data (LCDs) were obtained from the National Climatic Data Center in Asheville, N.C. LCDs were obtained for all 12 months of 1987 and for six major cities in various parts of the United States. The six cities chosen were Cleveland, Atlanta, Washington, D.C., Los Angeles, Phoenix, Ariz., and Miami.

Using the criteria listed above for proper foaming, the data in the LCDs was analyzed to determine the number of days in the year that met all the recommended conditions. The results are summarized in Table 2.

                                                             Table 2


# of days n 1987 meeting all conditions


Cleveland, OH


Short summer season. Windy and humid.

Atlanta, GA


Long summer season.

Washington, D.C.


Summer season is very humid and windy.

Los Angeles, CA


Wind and humidity often are a problem. Temperature generally is acceptable.

Phoenix, AZ


Should expect good foam jobs; conditions are very favorable.

Miami, FL


Wind and humidity creates problems. Temperature generally is acceptable.

Source: National Climate Data Center, Asheville, N.C.

In the analysis, surface temperatures were considered to be the same as ambient. No temperature gain or loss was assumed due to factors such as heating from the sun’s rays, radiative losses on clear nights, interior building conditions or other factors.

The highest sustained minute of wind speed was used when given in the LCD. When this data was not available, the average wind speed was used for the analysis.

Putting the information into perspective, it is not meant to indicate that good sprayed-in-place installations cannot be made in cities such as Cleveland or Washington. But the data does indicate that great diligence and experience is necessary when working in these cities and others with similar weather patterns.

It can be argues that a full day is being considered unfavorable for application because of conditions which exist only during part of the day. This is so, but real world conditions do not typically allow an applicator to stop and start a job when conditions drift from favorable to unfavorable and back again.

The data makes a clear point – the weather less favorable for obtaining high quality foam jobs in certain parts of the United States than in others.


As discussed earlier, coatings are critical to the performance of polyurethane sprayed-in-place systems. It should be realized that, in effect, the application of the surface coating is the manufacture of a single-ply membrane on the roof.

The industry is now recommending the use of foam with a density of 2 ½ to 3 pounds per cubic foot (pcf) rather than the previous recommendation of 2 pcf. This higher density produces a foam which has improved compressive strength, resistance to mechanical abuse and dimensional stability.

An insulation with a density of 2 ½ to 3 pcf also has lower vapor transmission than one with a density of 2 pcf. The higher density recommendation has taken some of the performance load off the coating, especially in the area of hail resistance.

A coting over a soft substrate is easier to puncture than one over a hard, resistive material. But the quality of the coating and its application still are paramount to the extended performance of the total system. If the coating is misapplied of improperly selected, the foam will degrade quickly from the action of water and ultraviolet light upon it.

A number of different coating types have been employed in PUF systems throughout the years. As with the foam, their quality has improved as knowledge of the required performance attributes has grown.

They are classified somewhat arbitrarily as breathable (having a permeance of 1.0 perms or more) and non-breathable (having a permeance of less than 1.0 perms).


Acrylics usually are supplied in one-part, water-based form. They can be cleaned up easily, have no solvent odor and are available in light, reflective colors. Acrylic latex coatings do not perform well in water-ponded areas and should be used in applications having good slopes to drains.

One-part silicones cure by the absorption of moisture from the air. They have very good weathering characteristics but can have problems with recoat adhesion after outdoor exposure. Silicone coatings have high permeability.

Two-part silicones require thorough blending in either a batch process or a mixing device of a spray system. More expensive than one-part silicones, the two-part products give dependence and predictable cure times if they are mixed properly. Because of the high permeability of these coatings, all silicones should be used in applications having good water drainage.

For one-part urethanes, as with single-component silicones, moisture in the air acts as the catalyst which causes the cure of these materials. Since the aromatic chemicals are used to form the reactive polymers, weather resistance is only moderate. Urethane coatings have lower permeability than the silicones.

Two-part urethanes must be mixed well. They also are more costly than one-part urethane coatings. Their cure can be controlled to predetermined requirements by adjusting the amount of catalyst used. They can be formulated for excellent weathering characteristics if aliphatic chemicals are used in their production, but this increases their cost.


Butyl coatings have the lowest permeability of those used for PUF roofs. A good weathering top coat of some type is recommended because the butyls have marginal exterior exposure resistance.

Butyl coatings are high in cost and are supplied as two components which require mixing. They are solvent-based and are specified when their very low permeability is necessary.

Hypalon, a trademark polymer supplied by Du Pont Co., Wilmington, Del., can be compounded into coatings as well as used in single-ply sheet goods. Coatings made with Hypalon are the most expensive used in PUF systems. Single-component and solvent-based, they have excellent weathering properties and are available in a variety of colors.

Urethane coatings can be modified with tar, asphalt or other ingredients to reduce their permeability. One-part asphalt and two-part tar modified urethanes are available for application over foam.

They are black in color and require top coating for aesthetics and reflectivity. The use of tar or asphalt lowers the cost of these coatings and makes them competitive with acrylics if a top coat is not desired.

One guideline for selecting a coating type is the direction of moisture drive. If moisture is being driven into the building, such as in a cold storage operation or air conditioned buildings in hot climates, an impermeable coating should be used to protect the foam from excessive absorption of water.

On the other hand, if high humidity is present in a building (if it contains, for example, a swimming pool, food preparation area or gymnasium), moisture should be released through the use of a permeable surface coating. In this case, a vapor retarder of some form also must be used beneath the foam to reduce the amount of moisture reaching the insulation from the interior.

The use of permeable or impermeable coatings often is a subjective decision. The choice of 1.0 perm as the cutoff point between the two types obviously is a matter of convenience.

In situations where good roof drainage is present, a quality coating in either of the permeability categories will serve very well. But when job parameters dictate, it is important that a coating with a suitable permeance value be selected.

Chemistry of Foaming

An understanding of the chemistry involved in the application of foam will help to explain the sensitivity of the operation. The basic chemical reaction that takes place is between materials called isocyanates and other compounds referred to as polyols.

The isocyanates react with two polyols, whereas the polyols are multifunctional and react with several isocyanates. Because of the proportions in which these two chemical combine or react, a three-dimensional network is formed.

The foam rises because a material called a blowing agent is included in the formulation. This blowing agent is one of the chlorofluorocarbon chemicals (CFCs) which we are now hearing so much about. The boiling point of this blowing agent is about 75 degrees F.

When the isocyanates react with the polyols, heat is generated because in chemical terms this is an exothermic reaction. The heat from the reaction plus the ambient temperature cause the blowing agent to vaporize or boil. This, in turn, causes the chemical mixture to grow or rise.

The materials will expand to 20 or 30 times their original volume. Thus, one gallon of chemicals will produce 20 to 30 gallons of foam. As the foam rises and sets, the blowing agent is trapped within the cells created during this chemical process.

The trapping of the chlorofluorocarbon accounts for the foam’s good R-value because CFC’s do not transfer heat efficiently. Thus, heat will not pass through the cells of the foam as easily as if they were filled with air.

Other ingredients contained in the urethane foam are surfactants to control the size and shape of the cells, catalysts to control the reaction speed between the isocyanates and polyols, and fillers to lower the cost of the foam and adjust its strength, color, spray, and other characteristics.

This brief explanation of the chemistry of the foaming process will be used to consider the moisture, temperature and wind conditions that are not favorable to achieving a quality foam application on a roof.


The chemical group in the polyols that causes them to react with isocyanates also is contained in the water molecule. This means that the isocyanates will react with water, if it is present in sufficient amounts, instead of the polyols.

Depending on whether the form of the moisture is gas or liquid and whether it is present as humidity or on the roof surface, moisture can cause problems with adhesion, foam strength, R-value and water resistance, to name a few potential concerns.

It is easy to understand why so much stress is placed on moisture conditions during foam application. All surfaces to be foamed must be dry and humidity must be at an acceptable level. Of course, rain, snow, fog, or mist also must not be present.


It is important that the CFC blowing agent be vaporized properly during application. Too low an ambient or surface temperature (50 degrees or lower) will result in improper foam rise due to insufficient vaporization of the blowing agent.

On the other hand, too high a temperature (120 degrees F or greater) will result in excessive blowing agent pressures. The CFC material literally will blow holes in the polymer cells being formed, creating pinholing and increasing density of the urethane foam due to collapse of the cells.


Excessive wind can be a problem for several reasons. The first involves potential overspray of the foam chemicals onto areas adjacent to the roof surface. These chemicals have very aggressive adhesion to almost anything they contact.

Overspray of the foam chemicals usually must be corrected by mechanical or chemical removal followed by refinishing of the affected surface. Additionally, overspray should be minimized because there is a health hazard associated with some of the foam ingredients, especially the isocyanates.

Another problem has to do with the uneven foam surface produced when wind blows across an uncured application. When this happens, the liquid chemicals will be displaced and produce a surface that is known as rippling.

The above conditions can be alleviated to some degree by the use of widescreens or barriers. Some contractors even will go to the extent of putting up tents or air structures to enclose their working area. Masking of all surfaces on or near the roof where overspray would be a problem is highly recommended.

Issues Facing Foam

Although the predictions for foam continue to be positive, the PUF roofing industry is facing a number of uncertainties, both near and long-term.

There has been substantial discussion in the industry regarding the retention of R-value by plastic insulations. The debate has centered on board stock because of its greater use. But the same concerns apply to foamed-in-place urethane.

The basic issue involves the potential loss of the chlorofluorocarbon materials contained within the cells of the foam. From a chemical viewpoint, CFCs are large, cumbersome and slow-moving molecules. Since energy is transferred within the cells of the foam by collisions of the gas molecules, the insulation value is improved by the presence of the CFC’s.

But if the CFC materials are lost through the cell walls, diluted or replaced by light, fast-moving molecules of air, the insulating capabilities of the plastic foam are decreased.

A number of studies have been published on the loss of R-value by foam insulation. In a paper given at the Eighth Conference on Roofing Technology in 1987, researchers at Dynatech Scientific Inc., Cambridge, Mass., measured R-value losses for polyisocyanurate foam as high as 34 percent after 90 days at 140 degrees F.

These findings certainly are of concern. The building owner is not receiving the amount of insulation expected when the job was contracted. In addition, the results also may indicate that CFCs are being lost to the environment. The consequences of this latter occurrence have been well-publicized over the last few years.

Because of environmental impact, the nations of the world have agreed to limit the production of chlorofluorocarbon materials. All the details of this curtailment have not been released yet.

Expectations are that the limitations will go into effect in July. The cap on production at that time will be the 1986 level, which is 20 to 30 perfect lower than is being produced today. By the year 2,000, the production limit is anticipated to be at 50 perfect of today’s levels.

Much study and research has been done to find suitable substitutes for CFCs. It appears that chemicals called hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) will be the best alternatives.

But these materials do not have the same level of insulation efficiency as CFCs. Additionally, it is likely there will be some adjustment of formulas necessary as these materials are substituted into both board stock and sprayed-in-place formulations.

Other questions that will need to be answered are where the available CFC chemicals will be used, how the cost of the substitutes will affect foam prices and whether or not other performance properties of the foam will be influenced in any way.

In 1988, urethane and isocyanurate insulations were in short supply and prices soared. This situation was created because the availability of isocyanates was limited because of a shortage of ethylene, a starting ingredient of isocyanates.

If pries and supply continue to be a problem in the future, alternate types of insulation will continue to gain greater market share, as phenolic did in 1988. Although the shortages were felt primarily in board stock, the sprayed-in-place materials use the same raw materials, and their prices and availability were affected as well.

Increasing prices and loss of R-value will make it more difficult to promote PUF systems I the basis of energy payback. On the other hand, a return to energy shortages similar to the 1970s certainly will lead to increased usage of these systems regardless of cost.


Sprayed-in-place urethane systems have been in the marketplace for a considerable period of time. For a variety of reasons, they still are placed in the category of a specialty product.

Their growth undoubtedly has been hampered by a number of job failures, most of which have been the result of applicator inexperience.

It is very difficult to take a product system with the technical complexity and weather sensitivity of PUF and attempt to make an impact on an industry which is seeking the opposite installation parameters.

There is and always will be a place for PUF in the roofing industry. It offers solutions to certain roofing problems which cannot be handled easily in any other way. Trained and experienced installers can produce an excellent job if proper precautions and weather factors are kept in consideration.

The choice of coating materials is a key one. Many job problems have resulted from the improper selection of a coating because its critical role was not understood completely.

It is believed that barring technical breakthroughs, current expectations are that sprayed-in-place foam probably will remain at about 10 percent or less of the roofing market. Its popularity will continue to be primarily in geographical areas favoring good application conditions.

Many roofing contractors will continue to be reluctant to take on the day-to-day problems that go along with foam application.

Originally published in CONTRACTORS GUIDE, January 1989