This article was originally published in the November/December 1998 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.


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Home Energy Magazine Online November/December 1998

Insulation in the heating system--A Source of Black Stains

by Lila C. Albin


Home Energy readers are collecting clues to the mysterious stains found more and more often in homes. It turns out that many black, sooty stains are the result of deteriorating insulation in the HVAC system.
Figure 1. Key pieces of the furnace are the heat exchanger, the fan, the return air duct, and the supply air duct. The heat exchanger is always insulated. The area where the fan sits--the fan housing area--is insulated in most systems. Ductwork can also have internal insulation, especially in industrial air handlers. The black coating on the insulation off-gasses into the airstream and is spread throughout the house.

Insulation Construction and Deterioration

Fibrous glass insulation is made of glass wool fibers woven into a blanket with the glass fibers held together by a resin glue. The airstream side of the insulation blanket is treated with a surface-hardening polymer to toughen it against the rigors of air movement.

The resin glues used in insulation are, chemically speaking, in a chemical class known as plasticizers. Plasticizers are used to make a polymer product flexible, and they can be added for varying degrees of hardness in the final product. The one used in black insulation is usually either urea-extended phenol-formaldehyde resin or urea-extended phenol-melamine-formaldehyde resin. The coating will probably contain a fire-retardant such as antimony trioxide.

These substances are volatile organic compounds, or VOCs. They stay airborne to the extent that they reach equilibrium. This means that the amount of VOC vapor over the surface of the insulation will be the same as that inside the insulation as long as the temperature remains the same.

Due to the nature of air handlers, the air temperature inside them changes frequently. When there is heat supplied by the system, the amount of off-gassing VOCs increases. The addition of heat to a molecule increases the kinetic energy of that molecule, increasing the distance the molecule will travel, and decreasing the concentration of molecules over the insulation surface. 

This concept is expressed by the Ideal Gas Law: PV = nRT (where P = the air pressure, V = the volume, n = the number of molecules of gas, R = the gas law constant, and T = the air temperature.

Also, Charles' Law states that at constant P and n, the volume is directly proportional to the temperature. Therefore, if you increase the temperature, the volume occupied by the gas also increases. As the volume increases, the concentration decreases, disturbing the equilibrium and resulting in more VOC molecules being released from the insulation surface.

These 25-year-old insulation particles are shown under 20,000X magnification. Note the relatively large size of the particles compared to candle soot particles (at right).
These particles of candle soot are shown under 20,000X magnification. They are much finer than particles from insulation.
Sooty stains on a carpet outline the spot where the armchair normally sits. The staining in this two-year-old home first appeared about three months after the owners moved in. The furnace had internal insulation directly inside the access door of the fan housing area, with a black coating that came off easily when touched. 
The stains on these walls and ceiling show a characteristic deposition pattern. This is an older house with a new furnace but old ductwork. The stains appeared where the wall is cooler than the airstream.

Commercial Systems

Commercial air handling devices or HVAC units are not immune to the problem of deteriorating insulation. However, the pieces of insulation that are deposited from commercial HVAC units tend to be larger, resembling small chunks of charcoal. They are generally noticed in the morning, when an employee comes in to work, as though they had magically appeared overnight. Frequently, the HVAC system associated with the particles will be shut off for part of the evening. The expansion of the ductwork when the unit is brought back on-line is enough to jar pieces of polymer-coated insulation loose and deposit them into the occupied spaces.

The problem also may be set off by repair work on the system, accidental jarring of the ductwork, or adhesive failure that allows a section of insulation to flap in the airstream. It may happen during a long cold spell in the middle of winter when the furnace has been working overtime to keep up with unusual heating demand. The problem doesn't usually occur in commercial air handlers until they are at least four or five years old.

Stains in homes have many different sources. In my job as an industrial hygienist, I have seen a wide variety of substances that can create stains, including mold growth, vehicle exhaust, combustion appliances, tobacco smoke, cooking fumes, and even candles (see Black Stains in Houses: Soot, Dust, or Ghosts? HE Jan/Feb '98, p. 15). Deteriorating construction materials or household items can also cause odd-looking stains and deposits.

In my experience, the most common source of stains is deteriorating insulation within the air handler--the forced-air heating and cooling system that, together with the ductwork, supplies the heated or cooled air to the house. This insulation gives off a characteristic soot that is spread by the ductwork throughout the house. With a little investigation, it is easy to identify whether or not it is the air handling system that is the source of a staining problem.

Search for the Source Internal insulation has been used in air handlers for more than 40 years to provide thermal regulation of the metal ductwork, to prevent moisture from condensing in the ducts, and to muffle the fan and motor noise.

This insulation can have many appearances. I have seen yellow and pink batting lining the most accessible parts of air handlers. But, the most common insulation that I see in both industrial and residential air handling devices has a black outer coating. This kind of insulation has an underlayer of fibrous glass batting, a black woven mesh over the fibrous glass, and a carbon-black acrylic, vinyl, or latex polymer coating on the side of the insulation that comes into contact with the moving airstream. The fibrous glass batting is held together with a binder, and it may be blended with cellulose fibers. The black outer coating also contains binders and will generally contain a fire retardant.

When first installed, the airstream surface of the insulation is hard to the touch, and it is difficult to puncture the black woven mesh or tear it from the fibrous glass batting underneath it. Over time, a certain amount of chemical off-gassing from the materials in the insulation occurs. Eventually, this will make the coating brittle, much like the classic cracking and splitting of automobile dashboards when the weather is cold. It can no longer expand in the heat and then contract in the cold as originally intended, and therefore it cracks.

When the black outer coating has off-gassed enough of the plasticizer to become brittle, the surface will be like carbon-black, charcoal, or graphite powder, and it will be easy to puncture the mesh or tear it from the rest of the insulation. This loose surface coating is easily picked up on the airstream and distributed throughout the duct system (see Figure 1).

Check the furnace and ductwork with a flashlight and mirror. In some air handling units, the insulation will be readily visible from the access door. If the black coating has deteriorated enough that it easily transfers to a hand lightly touching it, then it can also easily be picked up by the airstream and carried around the house.

Ruling Out Any Doubts In staining cases that are not clear-cut, mold may be suggested as the culprit in sooting stains. But, black sooting stains are easily distinguished from stains caused by mold growth. Although the area in which a stain appears could be sampled and cultured (see Sampling Stains for Fun and Profit, HE Sept/Oct '98, p. 12), and the sample would probably grow a few mold colonies, mold can quickly be ruled out. First, mold growth can be many colors, including white, black, various shades of green, or pinkish brown. Second, the outer edges of mold stains will look fingerlike or feathery. In contrast, sooting stains will be grayish black and will have a more uniform outer edge.

It is more difficult to rule out candle soot or car exhaust as potential sources. However, my experience is that candles are rarely the cause of sooting stains. Heavy candle use can contribute to staining near the candles, but it does not typically cause soot stains throughout the house. Furthermore, in many of the cases of soot staining I have seen, the owners did not use candles except for special occasions such as Thanksgiving and birthdays, and the stains clearly came from a different source.

If you are still unsure of the source of a sooting stain, one place to go for help is a laboratory. A lab can use scanning electron microscopy to determine, for example, whether a stain is caused by deteriorating insulation or by candle soot. Particles in stains from insulation material are significantly larger than particles from candle soot. Another way a lab can spot the difference is by running a chemical analysis of the material, checking for the presence of chemicals released by insulation off-gassing and degradation.

But, the clearest indication that deteriorating insulation in the air handler is the source of sooting stains is the way the stains appear in the house.

Soot Deposition Patterns Often the stains first appear on the walls and ceiling, particularly on exterior walls. They concentrate on inside surfaces along the ceiling joists and wall stud boards, and especially around nail heads, where the surface temperature is lower than the temperature in the surrounding area.

This pattern is similar to that seen in smoke damage after a fire, because similar forces are at work. Because of the billowing effect of smoke and the airstream in a fire, soot and ash tend to concentrate and be deposited first at the juncture between the ceiling and the wall. Impaction and attraction, together with gravity, are the main forces involved in this characteristic staining deposition (see Staining Patterns). Stains also may outline furniture or other objects on the floor, and windows or pictures on the walls.

How Old Is Old? Wall and whole-house streaking happens in both newer and older homes. In new homes, the problem typically appears after the air handler has been operating for between two and five years. But, I have seen new residential air handlers with a deteriorating insulation problem as early as three months from installation.

Older insulation seems to hold up longer than some of the newer products. Staining problems in older homes don't usually appear until after some type of work has been done on the existing air handling system. Typically, a new furnace is installed but old ductwork, with old insulation, is left in place. The old insulation is disturbed by the installation process or by the operation of the new furnace, and its surface coating begins to flake off. This continues until the insulation is removed or until all the coating material is gone. In these cases, the house develops very noticeable black streaking within a few months.

Who's to Blame? All products age and at some point reach the end of their useful life. I believe that some of the newer air handler insulation products are being made with less attention to the length of time they are expected to last.

Manufacturing processes differ from company to company and from product line to product line. Lower-end products may use lower-quality plasticizers to hold the carbon-black or polymer coating onto the fibrous glass batting. A product that binds poorly or that is highly volatile will off-gas faster than a product that binds tightly and is less volatile. And the faster the product off-gases, the faster it deteriorates. The specifics of how and why the breakdown occurs, and how to avoid it, need more study on the part of insulation manufacturers.

In the meantime, it's a good idea to check air-handler insulation frequently and replace it when it shows signs of deterioration. Care is also needed when cleaning ducts--using a brush or vacuum on degraded insulation can make the coating flake off. In some cases, rather than attempting to clean the ducts, it may be better to simply replace them.

Other Sooty Stain Sources

What happens when you have soot staining in a location that has only electric baseboard heaters? The stain patterns are the same as those in locations that have air handlers with internal insulation. 

Chances are the problem is with metal alloy deterioration, or metal fume, which is formed by heating the metal to a high temperature. If it is a baseboard unit, the metal is being burned off the surface of the metal fins. If it is an electric-resistance forced-air system, metal fume can be coming off the heating element. 

To verify the presence of metals in the soot stains, have the samples analyzed by a laboratory for a welding fume profile. This will reveal the presence of aluminum, combined with other metals, such as cadmium, chromium, manganese, nickel, iron oxide, zinc oxide, lead (from solder), copper (from the pipes), and titanium dioxide (from paint). Stains from metal fume look black, rather than metal colored, because of the small diameter of the particles. The light scattered from these tiny particles is seen as black by the human eye.

This radiant baseboard heater has copper pipe, aluminum fins, and a galvanized cover. Notice the distinct, black, sootlike lines on the cover (which lies open on the floor). The lines match up with the positions of the fins. This black sootlike material appeared on a wall above a baseboard heater.

Staining Patterns and Physical Forces

The deposition of sooting stains from insulation deterioration is very characteristic in a house. They are caused by three main forces: impaction, attraction, and gravity. Impaction Impaction is a physical forces that plays a key role in particle deposition. The air in a building is fluid and moves through the occupied space in a distinct, usually circular, pattern. typically this is a pattern of convection, in which warm air rises in the middle of the room, cools while moving toward the walls, and falls again. When the air turns to go down a wall, some particles in the airstream, the larger ones, keep moving outward instead of down. Because of their larger size, and thus greater momentum, inertia keeps them moving in a straight line until something stops them and the particles impact on the wall's surface (see figure). Attractive Forces The attractive forces include static charge and thermophoresis.

Static charges--and therefore, sooting stain depositions--often occur on the television screen, plastic pipes, plastic dishes, vinyl window and door casings, and plastic appliance cases. They are also often seen inside the refrigerator, especially on the egg-keeper and butter door. This is due to the fact that many plastic items have an electronic, or static, charge buildup on their surfaces because of the chemical nature of the material and the way the item is manufactured. Polyethylene and polypropylene are two plastics that typically build up static charges.

Static charge results when there is a an overall net possitive or net negative charge on the surface of an item. This happens when two surfaces are pressed tightly together and then pulled apart, or from friction. One of the surfaces will have more electrons (net negative) and the other will have more protons (net positive). You can also create ions from air molecules when the air is passed through an electric field or a magnetic field. A classic example of static charge is the shock a person gets when they touch a metal object. The person has discharged themselves with a micro-lightening strike.

Opposite charges attract whether it is the surface it was just removed from or a different surface. If you have a surface that is positivly charged and you have another surfac that is negatively charged, they tend to be attracted to each other. The result is a dirt accumulation on the other surface.

Thermophoresis, or thermal precipitation, is a physical settling effect in which particulates in the air stream plate out on surfaces that are colder than the airstream. Different materials acquire or lose heat at different rates; therefore, some items will always be colder than other surrounding items. Particles will stick to colder surfaces and will become noticeable as a sooty stain.

Gravity Soot staining often outlines such things as furniture, pictures, and window treatments, and may be found on the top side of ceiling fan blades when the fan is turned off-anywhere there may be a pocket of still air. This is because the still air allows the particulates time to drop down, pulled by the Earth's gravity, and settle on the closest surface. 

Lila Albin, Ph.D., is an Industrial Hygienist at Purdue University and an Industrial Hygiene Consultant with Barron and Associates in West Lafayette, Indiana.


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