Problems With Fiberboard Ducts

Fiberboard duct board, made of fiberglass is one of the main types of duct board used in heating, ventilation and air conditioning applications. Fiberboard is cut into 4-by-8 foot lengths of fiberglass, which are called "sheets." The sheets are then routed to the proper size. Ducts are either round or square containers that air travels through, so the flat sheets of fiberboard have to either be rolled up or folded over to create enclosed circular or rectangular tubes. This is done by a machine in the shop where the edges are made and the edges are then sealed by duct tape or solder. While there are a number of advantages to fiberboard duct, including low cost, ease of installation and less risk of injury during installation than metal duct, there also are a number of problems. Troubles with fiberboard ducts can occur if they are not properly maintained or installed.

Water Problems

If there is too much condensation or moist air in the duct work as it is blown through by the furnace or air conditioning system, the fiberboard ducts will absorb some of this moisture. The ducts will begin to break down to where they eventually can sag or otherwise fall apart. This problem becomes magnified if the damaged fiberboard duct is in an area that is not exposed for easy access to be repaired or replaced.

Video of the Day

Mold and Mildew

Fiberboard ducts provide a fertile breeding ground for mold and mildew. In fact, according to, mold on fiberglass lined ducts is the most common cause of illnesses related to mold. These illnesses, for example, include chronic sinusitis and asthma. The problem of mold in fiberglass ducts is increased in ducts exposed to moisture and kept in the dark without being properly disinfected. This can cause air quality problems for the residents of buildings that use this type of duct work.


Fiberglass and formaldehyde, two chemicals linked to cancer in laboratory animals, are off-gassed by the usage of fiberboard duct material. This means that as the ducts sit in your home, they are slowly releasing small amounts of both fiberglass and formaldehyde into the air in your home. This can lead to indoor air quality problems for people in those buildings. Some possible health risks of inhaling or coming into contact with fiberglass include rashes, nose and throat problems, and temporary stomach pains, according to the Illinois Department of Public Health Sheet.

The Minnesota Department of Health also indicates that short-term symptoms of exposure to formaldehyde can cause nose and throat discomfort, nausea and dizziness while long-term exposure to high levels of formaldehyde can cause cancer. Groups especially susceptible to potential health risks from exposure to these chemicals include children, older adults or those with weakened respiratory systems.

Flow Problems

By the nature of their rougher interior surfaces–unlike surfaces of similar-sized metal ducts–fiberboard ducts can cause problems with air flow in heating and cooling systems. These problems can arise because fiberboard ducts are not as free flowing. This can mean that the fiber ducts will need to be made larger by your HVAC installer to compensate for the reduced air flow. Increasing the size of your ducts may become a problem if there's not adequate space behind your walls and floors.