Ducts are the conduit that circulate conditioned air from your HVAC equipment to the living spaces in your home. They consist of supply ducts that deliver air and return ducts that pull the air back to the HVAC equipment.  They are typically located in unconditioned attics and crawlspaces. The primary issue with ductwork is excessive air leakage in unconditioned spaces.  The EPA’s EnergyStar website says “In a typical house…about 20 to 30 percent of the air that moves through the duct system is lost due to leaks, holes, and poorly connected ducts.”  That translates to at least 20 to 30 percent extra cost to heat and cool a house.  

Completely new duct systems must be tested with a piece of equipment called a Duct Blaster to ensure the system is properly sealed.  Although new systems are required to be well sealed, most regular HVAC maintenance does not address the importance for sealing the ductwork.  

Besides wasting conditioned air, leaky and unbalanced ductwork contributes to pressure imbalances in the home which can exacerbate other issues.

Duct Sealing

Ductwork has many connections that need to be sealed to be air tight.  Duct tape does NOT work because it becomes brittle over time and does not stick well initially to dusty ductwork.  The best product to use to seal duct work is duct mastic that is painted on thick with a brush.  Metal foil tape and mesh tape can be used to cover larger gaps and holes prior to applying mastic.  

Duct sealing should begin at the furnace or air handler where the pressure in the system is the highest and then systematically follow the trunk lines and each duct run to the boot, where it goes through either the floor or the ceiling to enter the house.  Some of the worst leaks we find in the duct system are where the duct plenums are connected to the furnace or air handler by an unconscientious installer.  

Duct boots should be sealed where they enter the house to ensure air is not exchanged between the attic or crawlspace and the house.  Baseboard registers are particularly leaky and require careful sealing.  

Sealing return ductwork is critical to improving the indoor air quality in the home.  Return ductwork is very often found to be leaky and allowing air to be pulled into the system from attics and crawlspaces without being filtered.  Panned returns which use joists in attics or crawlspaces are especially leaky.  

Duke Energy offers a $100 rebate to seal duct systems and ERS of NC can submit the rebate application for homeowners.  The rebate requires additional testing to verify the results.  

Duct Insulation

Duct insulation is important for reducing heat transfer of the conditioned air as it passes through the ductwork, but the greatest problem we find in this climate with duct insulation relates to condensation in the summer.   Ducts that are not properly insulated using insulation with a vapor barrier will develop excessive condensation on the ductwork in crawlspaces and even in attics.  When warm, humid air hits the cold ductwork the moisture condenses on the ductwork and will drip off.  Condensation can also form on duct boots and run back down into the duct insulation causing water to collect in the insulation of duct runs.  

Ducts that are deeply buried in attic insulation without the proper insulation levels and vapor barrier can cause condensation which can appear to show up as a roof leak.  Duct insulation can be added to uninsulated ductwork after sealing, but in some cases the better plan is to replace individual duct runs with insulated flex duct.

Duct sealing is not particularly difficult, but every duct system is unique and requires careful consideration to determine what is the best method to make improvements and reduce condensation, air quality problems, and pressure imbalances.

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