Improving Duct Performance:
Air Duct Sealing

Controlling air leakage is one of the most things to do to improve home performance.  Duct air leakage is a special case of building air leakage.  When the heating & cooling system is off, duct leaks can add to building air leaks.  When the system is running, leaks here are made 2-5X worse. 

Customer Performance Targets for Duct Sealing

There is no real standard established for sealing ductwork.  However, various home performance programs do set their own targets.  The sponsor for incentive programs dictates with the performance improvement for duct leakage has to be to qualify.

Guidelines. Without a standard, it's worth reviewing what some home performance programs have established.  The primary goal is to limit duct leakage to outside to avoid energy loss.  Indoor duct leaks should be sealed too because they can be indirectly responsible for other energy losses.

Some programs set targets of 6-10% duct Leakage to Outside while maintaining Total Leakage to less than 15% for new ductwork. 

Another program looks for a 40% improvement in duct Leakage to Outside when existing ducts are sealed.

Duct Leakage to Outside is air lost outside conditioned spaces.  Energy is truly lost!

Total Leakage accounts for duct leaks inside the house too.  To get duct leakage to inside, subtract Leakage to Outside from Total Leakage.  Energy is not lost from the house through indoor air leaks.

Note: Percentage leakage is determined by comparing the rate of airflow through the leaks to the square footage of the area conditioned.  It’s not a true percentage because units are mixed (cfm/sqft), but it is the standard the industry uses for measuring it.

Preparing the House for Sealing Ductwork

Correct Air Duct Pressure Issues. Heating & cooling system performance depends on proper air pressure for air delivery to where it’s needed. 

For ductwork the areas of concern air infiltration through floors, walls, and ceilings, as well as ensuring water heaters & furnaces behave properly.  Air infiltration adds unconditioned air and humidity that must be overcome by the heating & cooling system. 

Caution. Sealing ductwork can also cutoff unintended combustion air supplied to water heaters & furnaces that they been using since they were installed.  Limiting air leakage and providing combustion air satisfy these needs.

Air Duct Sealing

Once work begins, before sealing the ductwork, the first step is to put the components of the system together after designing it. 

Correct Airflow First. Every effort should be made meet room and system airflow needs after sealing the ductwork.  This may involve installing, replacing, or modifying the duct system.

Duct Seal All Joints. The entire air distribution system needs to be sealed, including the air handler cabinet (indoor equipment), plenums, trunklines, distribution boxes, splitters, ducts, splices, boots, and the boot’s connection to the floors, walls, and ceilings.

Duct Sealing Priorities. Priorities are set to allow for cost effective duct sealing to help meet budgets in this order:

  1. Catastrophic leaks (disconnects, broken ducts, etc.)
  2. High pressure leaks (leaks close to the air handler cabinet)
  3. Return leaks that depressurize areas around combustion appliances
  4. Plenum connections, including trunklines & distribution boxes
  5. Duct to duct connections, such as splices and splitters
  6. Duct to register boot joints
  7. Boots to walls, floors, and ceilings

In the process, any joints and seams in rigid metal ductwork need to be sealed too.  All sealing is done with mastic or Aeroseal.  Duct tape doesn’t work because the adhesive (glue) dries out.

DIY Cautions. Unlike other parts of the heating & cooling system, duct sealing can be a do-it-yourself project.  However, before attempting to seal ductwork, verify other parts of the system or house will not be adversely affected by it. Don't start without a home energy audit with diagnostic testing!

Also, don’t forget to have a professional retune the system afterward for better efficiency and to prolong equipment life.

Post Duct Sealing Testing.  When the contractor is done and the heating & cooling system is re-tuned, final testing is needed, sometimes by a third-party who didn’t do the work.

For home performance contractors, final testing is mandatory when combustion appliances are present.  The concern is ensuring the areas fireplaces, water heaters, and furnaces are in are not depressurized to the point of causing smoke to backdraft into the house.

Where there are incentives or certificates offered, the sponsor (e.g., utility company) usually requires final testing too.  Their considerations are to verify better equipment was installed, duct leakage meets or exceeds performance targets, or system performance is improved.

We are ready, willing, and able to perform third-party testing.

In Conclusion . . .

In the home performance improvement process for energy improvement, the top three things are to air seal the building, air seal the ducts, and insulate the ducts and building.

Sealing ductwork is key for better heath, longer lasting building, comfort, and energy efficiency.  Reducing duct leakage leaves more pollutants outside.  Sealed ducts help manage moisture intrusion into the house.  When duct airflow is right and humidity is maintained, the house is comfortable.

Sealing ductwork along with air sealing the building significantly reduces energy loss, especially when the ductwork goes through attics, crawlspaces, and unfinished basements.

Here’s to better indoor air quality and energy efficient house!


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