Commercial air ducts, as in an office setting, are often lined with interior insulation for the purpose of noise attenuation and the reduction of heat transfer. Ducts with this insulation on the inside (as opposed to exterior-insulated, or having no insulation at all) are often referred to as "lined ducts." Interior insulation reduces the amount of fan and other system noises heard moving through the ducts, and additionally allows air that is heated or cooled to maintain its temperature while making its way through the ductwork. The twofold benefit is a quieter system for building occupants, and greater overall system efficiency, reducing energy costs.
How lined ducts differ from unlined in terms of cleaning
Because the surface of interior-insulated ducts is more porous and less smooth than uninsulated, they tend to collect more debris and need specialized or more frequent cleaning. Interior insulation is often pin-welded, and at the location of the pin, or where the seams meet, particulates—often fiberglass particles--tend to slough off and spread throughout the ductwork over time, dirtying the ducts alongside more run-of-the mill dust and debris. Below is an image before cleaning of lined ducts in a 10-story commercial office setting in Minneapolis.
How to clean interior-insulated ductwork
The two primary methods of commercial cleaning of lined ducts are crawl-through and air whipping. The first method is just as it sounds. If the ducts are large enough, a technician dons personal protective equipment and crawls through the ductwork (variously on his stomach, back, and sides) with a 3-inch dust-brush head attached to a 100-ft hose, which connects to a HEPA-filter vacuum on the ground below. If the ductwork is big enough to stand in, he may use a 14-inch long dust-brush head. Below is the same ductwork after having been cleaned with the crawl-through method.
If the size of the ductwork is prohibitive, the alternative is to use air whips, along with compressed air and forward and reverse skipper balls. The process begins with the insertion of an industrial vacuum hose to create appropriate negative pressure. The technician will then cut 8-, 10-, 12-, 14-, or 16-inch access holes (depending on the size of the ductwork) down the line. He'll use this access to insert a multi-tentacled air whip (such as the Viper Clean Sweep or the Octopus Predator) starting from the furthest point upstream, slowly reinserting the whip and moving progressively downstream toward the vacuum.
Some commercial technicians will use bristled brushes rather than rubber-tentacled whips for this process. In this case it is vital that the bristles not be too stiff or left in place for too long lest they damage the insulation.
Finishing up with antimicrobial coating
After cleaning, an assessment will be made regarding the integrity of the insulation and whether an antimicrobial coating is necessary. Some of the various products applied are Foster 40-20, IAQ 8000, or Sentinel 24-7. These coatings are applied using an airless paint sprayer for even application.
When complete, the coating provides a smooth and durable surface that inhibits odors as well as the growth of mold and bacteria. (In the case of a previous fire event in the building, interior insulation tends to hold the smell of smoke, which then circulates throughout the building until it's been cleaned and coated.) These products have the additional effect of sealing the surface of the insulation to inhibit the sloughing off of fiberglass particles into the airstream.
Below is the same ductwork as in the previous two images, after an antimicrobial coating has been applied.