Fires in the wildland-urban interface are becoming an increasingly significant problem as both their numbers and size increase. Between Dec. 1, 2005, and March 1, 2006, there were 8,703 wildfires in Texas, 85% of which burned within two miles of a community. The use of Class A foam in many of these incidents demonstrated that it can provide both effective extinguishing capability and a means of property protection in the interface.

Water has many characteristics that make it a good extinguishing agent. It's plentiful, readily available, easy to transport, non-toxic and able to absorb large amounts of heat. It does, however, have two downsides as an extinguishing agent. Its high surface tension makes it form droplets, which limit its surface area so that only the water on the surface of the droplet is able to absorb heat. In addition, all Class A fuels are carbon-based materials, and plain water makes no distinction between carbon and any other material.

Adding a “surfactant” or surface-active agent to water to create a Class A foam reduces the water's surface tension, allowing it to spread into a thin sheet, thereby increasing the surface area and heat-absorbing ability. More importantly, the reduced surface tension allows the water to penetrate more easily into Class A fuels, thereby raising their moisture levels.

Surfactants also are chemically attracted to carbon. The addition of a surfactant to water helps the water remain in contact with Class A fuels longer, which helps with cooling and protection.

FINISHED CHARACTERISTICS

Foam can be made in different forms depending on the intended tactical use. Wet foam can best be described as melted ice cream. It's wet and sloppy and has a short drain time. The water drains from the bubble structure very quickly. Tactically it's used for fire extinguishment, mop-up and overhaul, and to raise fuel moisture levels.

Another form is fluid foam, which has the look and feel of shaving cream. It holds peaks and will hang on vertical surfaces. This type of foam holds water for a longer period of time. Tactically its primary use is to hold water in place to inhibit fuel ignition.

The length of time a foam will hold moisture and remain as a foam blanket depends on many factors, including how the foam was made. The nozzle; foam system; and proportions of water, air and foam all have an effect. Weather conditions, too, can be a factor. Temperature, wind and humidity can all have an effect on the bubble structure. The surface to which the foam is applied also has a bearing on its longevity. Dry barn boards will absorb moisture more quickly than newly painted surfaces. Sheet metal and vinyl siding will affect the drain time of the foam blanket.

It's important to know how long the foam will last so it can be applied at the proper time during an incident. The way to learn that timing is through training and experimentation prior to an incident. Apply foam to various surfaces under various conditions and note how long the foam blanket stays in place. Keep in mind that even when a foam blanket breaks down, its ability to inhibit ignition isn't completely lost. As the water drops out of the foam, it's absorbed into the fuel, raising the fuel moisture.

Class A foam has a number of characteristics that make it useful in fighting interface fires. Water in the form of foam provides the maximum surface area. This high surface-to-mass ratio covers a large area and spreads well over interface fuels needing protection. A foam blanket also provides a white opaque surface that reflects heat away from objects that it covers.

Styrofoam insulation prevents heat transfer because it contains small air pockets. The same principle applies with Class A foam. A layer of Class A foam bubbles contains thousands upon thousands of tiny air pockets that help insulate fuels from heat.

Foam has one other advantage in the interface: Because it's easy to see, it's easy to tell where it's been applied. Personnel can quickly see which structures have been protected and which have not.

NAFS AND CAFS

To create Class A foam, a foam concentrate is added to water to create a foam solution. When air is agitated into the foam solution, foam bubbles are created. This is done through either normally aspirated foam systems or compressed-air foam systems.

With a NAFS, the foam solution is pumped through a hose line. Air is drawn into an aspirating nozzle through a Venturi effect and agitates through a screen to create foam bubbles. The bubbles can be made in a variety of volumes or expansion ratios depending on the size of the nozzle used. A NAFS combines two pumping systems, a water pump and a foam pump called a proportioner.

A CAFS represents the most efficient method of producing foam bubbles. With a CAFS, air is injected into the foam solution under pressure. The agitation occurs as the air and foam solution travel through the hose line, scrubbing and creating finished foam bubbles. Because the air has been injected under pressure, it adds energy and creates a high-energy fire stream.

CAFS-created foam has a small consistent bubble structure. This structure supports itself, making the bubbles last much longer than those from a NAFS. The high energy of the stream also provides a longer discharge distance. A CAFS combines three pumping systems: a water pump and foam proportioner as in a NAFS, plus an air pump or compressor.

Foam-capable apparatus can be specifically configured to be more effective in interface operations. Types 1, 3 and 6 apparatus all have their place in the interface. The type best suited for an operation will depend on fuel types, terrain, access and the tactics to be used. The most useful NAFS and CAFS pumping systems for the interface will be capable of pump-and-roll.

Some specific equipment is required for foam operations in the interface. When using NAFS, a variety of nozzles that will allow a range of foam expansion ratios should be available. The expansion ratio is the ratio of the volume of finished foam compared to the volume of water used to create the foam. Generally, the most appropriate nozzles will those allowing a medium expansion ratio (20:1 through 200:1). This foam is still wet but also provides sufficient volume to cover fuels.

When using CAFS, smoothbore nozzles are the most effective tool. These nozzles consist of a ball valve coupled to one or more different sized tips. The ball valve used should have the largest diameter waterway available. For example a 1H-inch ball valve would have a 1⅜-inch waterway. The large waterway delivers the driest, most fluid foam. Simply adding a smaller diameter tip will break bubbles, stripping air out of the foam and making it wetter. Use of a fog nozzle with CAFS isn't appropriate because it destroys the bubble structure, making the foam much less effective in interface operations.

Since CAFS bubbles form as they scrub in the hose line, a sufficient amount of hose is necessary to create good foam. With 1-inch hose 50 feet is adequate while 100 feet of 1H-inch is necessary to produce a quality foam. For obvious reasons, these hose lengths will not work for pump-and-roll. There are two ways to deal with this dilemma. A booster reel can be used to gain sufficient hose length, but the use of CAFS in booster reels can be problematic. A better option is using a mixing chamber, which is a baffled piece of pipe that causes agitation and creates CAF in as little as 12 to 18 inches of space. Using a mixing chamber, quality foam can be made with 10 to 15 feet of hose.

Bumper turrets are another effective tool for interface use. They work well in both stationary and pump-and-roll operations. They're an effective tool to apply CAF to large surfaces.

INTERFACE PROTECTION TACTICS

When using foam in the interface, the first thought of many firefighters is just to paint the houses white. In reality there are many more interface applications, but several factors need to be considered first.

The amount of water required to raise the fuel moisture is a primary consideration. Where fuels are light, less water is needed, and therefore a drier, higher-expansion foam may work well and cover more area. With heavier fuels, a wetter foam will be needed to provide sufficient moisture to do the job.

Fuel type also will determine how wet the foam needs to be and the volume needed for coverage. Tall fuels or those that are farther from your access may dictate the use of a CAF stream.

Current fuel moisture and humidity levels should be considered. These will determine if foam application is needed, and if so, when it should be applied. Wind is another weather factor because it will carry the foam both during and after application. The higher the wind, the wetter the foam will have to be to reach the target and remain effective.

Once fuel and weather considerations have been assessed, objectives for foam use in the interface can be set. Such objectives can include:

• Extend capability of both water and personnel.
• Create foam lines and barriers.
• Raise fuel moistures of wildland fuels.
• Raise fuel moistures of structural fuels.
• Coat structural fuels with a protective layer of foam.

At interface incidents, particularly early on, water, apparatus and personnel are often in short supply and stretched thin. Because foam makes water more effective, it extends the capability of a given volume of water. It also allows a crew to accomplish more in less time.

Class A foam can be used to create lines or barriers. These may be designed to hold a fire bumping against the line, to burn from, or to burn in between. Burning off of a foam line is typically better then having the fire run up against the line because of the amount of heat the line has to hold and the proper timing to apply the foam. When laying down a foam line, the foam must release its moisture quickly enough to wet the fuels. It's often better to work the moisture into the fuels as opposed to simply letting it drain out. One way of doing this is to apply the foam in front of the apparatus tire track and driving over it. Another is to use the energy of a CAF stream to force the moisture into the fuels. When igniting off of these lines, the fuels must be wet. This makes timing the ignition is important.

Another tactic is to use the foam to create a black line by first laying down two foam lines in a pump-and-roll operation. The distance between the lines is determined by the fuels and the flame lengths. The truck is then followed with a firefighter using a drip torch to burn out the fuels between the foam lines, thus creating a black line. The advantage here is that the foam line needs to remain effective only for the length of the burnout.

Foam can be used to raise fuel moistures of wildland fuels in the interface. The technique is to use wet foam to coat as much of the fuels as possible. As the wet foam penetrates the fuels, it raises fuel moisture. This tactic also can raise the humidity levels within a stand of trees. After the wet foam is applied the moisture can be held in place by coating the fuels with fluid foam to provide an insulating blanket. Use of this tactic will be time- and resource-dependent. If time and/or resources are short, wetting the side of the fuels facing the flame front will be most advantageous.

Protecting structures with foam is best done in a two-step process that again is time- and resource-dependent. In some cases a pump-and-roll operation applying foam from the street may be the only option. The most effective tactic, if time and resources allow, however, is to first apply wet foam to wet the structure and then apply fluid foam as a cap to hold the moisture in, insulate and reflect away heat. This can be done quickly by backing into the driveway and pulling a 1H-inch line to the structure. This line will have a wye and two 1-inch lines attached with smoothbore nozzles.

Two firefighters, one on each 1-inch line, can begin applying wet foam to the structure, starting in the front and working around both sides to the rear. They then signal the pump operator to change the wet foam to fluid before working their way back around to the front. Care should be taken to cover not only the large surfaces but also any ember traps, such as eaves or decks. Applying foam to the ground fuels around the structure also will stop a ground fire.

Triage and timing are key to a successful operation. Some structures cannot be protected and should be triaged out of the operation. Applying the foam at the right time will provide the best protection as the fire passes. Learning the proper application time comes from training prior to the incident.

When fighting interface fires, the primary objective usually isn't to save involved structures. Wildland firefighters aren't typically equipped with structure gear. One rule of thumb says that if 25% of the roof is involved, the structure is lost. However, by using CAF on the exterior, successful structure attacks can be made on heavily involved buildings. The advantages of being able to extinguish these fires are first to save the property and second to prevent large amounts of firebrands from starting numerous spot fires.

The key to a successful foam operation is training before the incident. Engine companies must know their apparatus and understand both how foam works and foam tactics. Strike team leaders and division supervisors must determine how best to use their resources and what tactics to use. Oversight personnel such as operations chiefs and incident commanders must understand what resources to order and where to place them.

Class A foam systems, while not “silver bullets,” can be very effective tools. Understanding how foams are generated and how to use them will enhance your interface operations.

Keith Klassen is a captain with the Summit Fire District, Flagstaff, Ariz.