Take insulated flexible duct for example. Products like Johns Manville Flex-Glas PC are already formulated for optimal thermal and acoustical performance, so they don’t need to be installed with more insulation around them. 

The Air Duct Council (ADC) notes that some local, state and national building codes allow for encapsulating ducts within additional insulation materials, and some even allow for partially or deeply burying ducts under blown-in insulation. 

While this can improve the overall thermal performance of a system, it can also potentially cause issues. Particularly in certain climates that are conducive to condensation, encapsulating or burying ducts with additional insulation can lead to moisture problems.  

“As an insulation company, of course we’d love to sell you more product, but ultimately, we want to ensure sure you as a homeowner, contractor or distribution partner understand any potential risks. In many situations, more insulation is better, but there are areas of the country where caution should be taken when burying flexible ducts,” said JM Product Manager Riley Peters. 

“When you have cold air traveling through an insulated flexible duct in a hot attic, specifically in parts of the country that are conducive to condensation, moisture problems can occur. In some scenarios, depending on how deeply the duct is buried and which part of the country you’re situated in, blown-in insulation can act as an entrapment for condensation around the flexible duct.” 

Areas at the biggest risk of this issue are in International Energy Conservation Code zones 1, 2 and 3, according to a map from the Department of Energy:

Insulated flexible ducts should be properly suspended in an attic installation to allow free circulation of ambient air all around the surface of the duct, according to the ADC. Avoid directly laying them on attic insulation, especially in areas prone to condensation

The other consideration when burying or encapsulating ducts is that if blown-in wool is not installed correctly, it can put unnecessary weight or pressure on the duct, causing a loss of force through the duct, meaning that it takes longer to air to pass through and heat or cool a home. 

Remember to take your climate zone into account if you’re considering burying insulated flexible ducts so you don’t create future problems with moisture.