Fiberglass Duct Board Vs. Flex Duct

Fiberglass Duct Board

• Fiberglass duct board is a rigid material used for home insulation. It’s made of flat sheets and has an aluminum foil-like outer lining that serves as an air barrier. The duct board panels are cut with a knife blade so the panels can wrap around the outer perimeter of metal ductwork pieces. They're secured to the outside of the metal ductwork with adhesives or metal tape. The addition of the duct board helps block leaks of warm or cool air.

Flex Duct

• Flexible ducting is typically made of plastic, which is stretched over metal wire coils into long and round snake-like tubes. The plastic is surrounded by a layer of polyester fiberglass or glass wool to insulate the flexible duct tubes, and is covered with aluminum. The material's flexibility makes it easy to attach to the stationary metal ductwork channels of most residential HVAC systems.

Similarities

• Fiberglass duct board and flex ducting are only suitable for interior use, such as insulating ductwork around the heat vents and air conditioning units inside the home, and crawl spaces in an attic. According to the U.S. Department of Energy, homes can lose anywhere from 10 to 30 percent of the energy used for heating and cooling if the metal ductwork isn't properly insulated. As a result, adding duct board or flex duct helps reduce energy loss and improve energy efficiency.

Differences

• Insulation materials are given an R-value rating for their energy efficiency. The R-value rates the insulation’s resistance to heat flow; the higher the R-value, the greater the effectiveness of the insulation materials. Fiberglass duct board panels range in R-value from 4.3 to 8.7, depending on the thickness of the panel. For example, a 1-inch-thick fiberboard panel has an R-value of 4.3 and a 2-inch-thick panel has an R-value of 8.7. Flexible ducts range in R-values from 4.2 to 8. As a result, the R-values of both materials are relatively the same, with the thickest fiberglass duct board being only 0.7 points higher than the best flex duct.

Considerations

• If air quality is a concern in addition to insulation properties, flexible ducting has advantages over fiberboard. Dust, dirt, mold and mildew can develop and be trapped under fiberboard panels. In time, the particles can seep through the metal ductwork and eventually vent into rooms inside a home. This can pose problems for those who suffer from allergies and have breathing-related health issues. Dust and airborne particles are less of a problem with flexible ducting.