Duct Leakage


Duct leakage affects heating & cooling system efficiency and effectiveness and may adversely impact smoke removal for water heaters, furnaces, and fireplaces.

Problems are made worse when return air paths back to a central heating & cooling system are restricted.

Depending on the type of duct leak and where the lost air goes, there can be a significant impact on comfort, indoor air quality, and moisture control.

Discussions in this article include

  • Current State of Duct Joint Seals
  • Contributing Issues Leading to Duct Leaks
  • Diagnosing Duct Leakage
  • Duct Sealing Recommendations
  • Duct Dynamics: Duct Pressure Differences
  • Customer Performance Targets for Duct Sealing
  • Improving Duct Performance: Air Seal Ductwork

When a duct system takes air from and provides air to the conditioned spaces of the house, performance is predictable.

A house with a properly installed, continuous, tight, and well insulated duct system attached to heating & cooling equipment operating at peak performance has an efficient and effective heating and cooling system. It gets even better when air leaks through the walls, floors, and ceilings are reduced.

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.

Because of their impact on the heating & cooling system, the services of an HVAC contractor are required.

Current State of Duct Joint Seals

Most duct systems lose 25-40% of their Energy (DOE) 30% of energy lost is due to duct leakage (EPA) 1 sqin hole in ductwork = 30 sqin hole in an exterior wall Duct leaks draw in outdoor heat, dust, pollutants, & humidity Duct leaks force air out the house Duct leaks can cause backdrafting of appliances & fireplaces Duct leaks can cause premature failure of equipment

Contributing Issues Leading to Duct Leaks

Homeowner Understanding. In short, homeowners don’t understand why it’s important, so they’re unwilling to pay for it. It isn’t intuitive for us to directly relate costly utility bills to duct leaks.

When we’re uncomfortable, we think there is something wrong with the size or performance of the equipment, or there isn’t enough air supplied to a room.

Homeowner Education. On the contractor’s side, it’s a business decision, but an unfair one. If we’re unwilling to pay for a good duct system, we won’t get one.

However, it takes a lot of education to explain why it’s needed. We don’t understand it and the contractor is not likely to explain it.

Low Profit Margin. In addition, sealing ductwork is a lot of work and there isn’t much profit in it. However, rather than try to explain it and possibly confuse customers, the subject of duct leakage is ignored.

Contractor Understanding. The other contributor is that the HVAC contractor may not understand the effects of duct leakage themselves!

Home Energy Audit: Diagnosing Duct Leakage

Tip: Diagnosing duct leakage depends on the Duct Dynamics.

The process for reducing duct leakage follows this path:

  • Interview Homeowners
  • Conduct Visual Assessment
  • Perform Diagnostic Duct Tests
  • Select Duct Sealing Options
  • Correct Air Duct Pressure Issues
  • Air Seal Ductwork
  • Post Duct Sealing Testing

Interview Homeowners. Improving home performance is about providing a healthier, longer lasting, more comfortable, and energy efficient house.

As they relate to air sealing, the following questions are asked during the homeowner interview

How often do you need to dust? Who in your family has allergies? Who in your family has asthma? Are there any hot or cold rooms? Is there anything that looks to be mold or other microbial growth? Is there anything else you feel is relevant?

Answers to these questions help us evaluate the system in terms of moisture control, indoor air quality, and energy transfer: the golden home performance triangle.

Conduct Visual Assessment. Where it is safe and possible to do so, the ductwork is inspected at the equipment, attached plenums, trunklines, distribution boxes, ducts, duct-to-duct connections, register boots, and at connections to floors, walls, and ceilings. The entire duct system should be intact with every joint sealed.

Much of the inaccessible ductwork is inside the house, in conditioned space. There shouldn’t be leaks here, but unless the ducts are broken or disconnected, they can be tolerated. Ideally these leaks should be sealed too.

There are times when ductwork is visible but practically inaccessible.

Perform Diagnostic Tests. Duct leakage is measured twice during the course of an audit to determine total duct leakage and duct leakage to outside.

Measurement. Because of complexities in truly measuring duct leakage, it approximated by using the square footage of the space the heating & cooling system is expected to condition.

Location. It’s one thing to know the ductwork is leaky, but it’s quite another to determine where. Pressure pans are used to cover registers one at a time to detect suspect duct runs. Sometimes, theatrical fog is used to find it. On rare occasions, infrared thermal imaging scans are used.

Pressure Testing. Sometimes, it’s worth finding out whether there are supply leaks or return leaks dominate by comparing space pressure to outside.

Select Duct Sealing Options. The emphasis to place on duct sealing depends on how tight and where duct leaks are. It also depends on other improvement priorities.

The primary consideration is how much duct leakage there is to outdoors. However, people like their heating & cooling system to make them comfortable in whatever room they happen to be in.

Duct Sealing Basis. In general, Home InSight recommends sealing ductwork based on total leakage, rather than leakage to outside, because the heating & cooling system behaves better where there are intentional air pathways.

However, the option is always available to ignore duct sealing on ductwork inside the conditioned living area—as long as the leaks are just around seams and not due to busted ductwork.

Duct Sealing Options. There are three options. Two options for duct sealing are presented: mastic or Aeroseal. They cost about the same. In some cases, it may be more cost effective to replace parts or all of the ductwork, while sealing it.

Mastic Option. Mastic is messy--and takes awhile when rigid ducts are used. Applying mastic is like putting tar or mud on each joint.

First, the insulation needs to be removed to expose the joints, which occur about every five feet. All seams need to be sealed too. Then, insulation needs to be put back on.

When flexible ducts are used, there are fewer joints to seal.

Aeroseal Option. Aeroseal is like using Fix-A-Flat to inflate car tires with holes in them. Sticky beads are allowed to flow through the system. The beads collect where there are holes, which ultimately seals the holes. This works when holes are small. Big holes need to be fixed first.


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