HVAC Duct Insulation—Making Sound Choices
Aug. 08, 2018
by Bill Ronca
Generally, when the words duct insulation and acoustics appear in the same sentence, I usually think duct liner and sound absorption. If I am having a more cerebral moment, I may think insertion loss or friction loss. But that is not all there is to HVAC duct insulation. While duct liners certainly play an important role in providing thermal and acoustical insulation for HVAC systems, duct coverings or duct wraps also provide important alternatives.
In its simplest form, duct lining (traditionally fiber glass), would be installed in rectangular ducts as part of an automated process while the duct is being fabricated on a full coil line. This process has the lowest manufacturing costs, and you can make large quantities of duct quickly. The cost and speed advantages may be reduced because every transitional piece (elbows, reducers, and rectangular to round) must be fabricated and insulated manually. The biggest disadvantage of lining ducts with a coil line is that there are very few insulation materials that can be applied on coil lines. This means specified design criteria—such as closed cell or antimicrobial properties—may not be met with this limited selection. In these cases, the desired insulation material must be applied by hand, which is very labor intensive and increases the cost significantly. Round and oval ducts always must be insulated manually, which again, increases the difficulty and cost of insulating the duct.
There is also the issue of double wall duct construction for round or rectangular ducts. Double wall duct has the benefit of isolating (solid inner layer) or semi-isolating (perforated inner liner) the duct liner from the air stream, which is desirable for a number of reasons (e.g., keeping particulates out of the air stream and protecting the liner from harsh environments). However, double wall duct fabrication and field installation has its own set of disadvantages, such as added initial cost, added weight, and a more difficult installation. Additionally, a solid inner layer means that no sound absorption is taking place.