Posted by: SherlockHomesSY | 8 Oct 2010

Solution Basics: Duct Sealing

DUCT SEALINGDuct Sealing Joints

Duct sealing is a special case of air sealing.  The difference is that the holes are not created equal—especially when the fan is running.

People say, from an energy perspective, that holes in ductwork doesn’t matter when the ductwork is inside the shell because the air just leaks into the house.

Don’t kid yourself.  Holes in ductwork do matter because a fan is forcefully moving the air.  The energy loss may be near zero, but the hole can wreck havoc on the pressure differences in the house, leading to static electricity, moisture intrusion, backdrafting furnaces and water heaters, as well as other crazy behavior.

Nope.  Holes in ductwork are not created equal.

When ductwork runs through the attic, crawlspace, or garage the holes behave like any other hole in the house—until the fan runs.  When it does, then you unwittingly have a polluted air delivery system.  You still get static electricity, moisture intrusion, backdrafting—only much worse.  Add pollen, dust, soil gas, vehicle exhaust, fertilizer, pesticides, herbicides, and the like to get quite a potentially toxic stew!

Nope.  Holes in ductwork are not created equal.

Ideally, the air distribution system does not leak air.  Would you tolerate leaky plumbing pipes? No!  That’s a water disaster.  Leaky ductwork is an indoor air quality disaster.

Every joint in the air distribution system needs to be sealed.  At the air handler (inside unit), the door needs to be taped shut and holes sealed up.  Don’t forget to cap the vent to the p-trap on the condensate drain.

All plenum and distribution box seams need to be sealed.  They include the edges, collars, and connections to the air handler.

All ducts need to be sealed at every connection, such as collars, extensions, and register boots.  Don’t forget the long seam that holds the tube together.

All register boot seams need to be sealed.  Why they don’t come that way from the manufacturer, I’ll never know.  Boots have lots of seams—some of them sharp, so watch out for cuts.

Where the register boot goes through the wall, floor or ceiling, that joint needs to be sealed too.

Watch out for returns made of or through house framing.  HVAC contractors who do very often connect the return ductwork to the attic or crawlspace by “accident”.  Be sure these pathways are sealed too.

A special kind of return made with framing is called a panned return.  These are made by fastening sheet metal to wood cavities, such as in floor joists, ceiling joints, or wall studs.  If you have them, I’d prefer you got rid of them.  If you’re going to keep them, make sure all the joints in the cavity are sealed.

There are two general ways to seal the ductwork: with mastic or Aeroseal.  Mastic is my choice because I can see the joints I’m working on.  However, instead of being tarred and feathered, I usually end up masticed and insulated.  Aeroseal is a popular alternative that works much like Fix-a-Flat does.  From what I here, it costs about the same.

Nope.  Holes in ductwork are not created equal!

The mechanics of diagnosing duct leakage and duct sealing are found on my home performance website.  Another source is Southface Energy Institutes fact sheet on Air Distribution System Installation and Air Sealing.

The pressure dynamics of ductwork is explained on my home performance website.

Eventually, I’ll find a web source explaining the nitty-gritty details of duct sealing.  If not, I may create it myself.  I got my total duct leakage to less than 3% by talking to professionals, reading books, reviewing lessons, and trying things.  There’s gotta be an easier way to learn what to do!  If you go this route, be sure to get your HVAC technician to re-tune (commission) your heating & cooling system.

Duct sealing is relatively to easy and one of the most cost effective thing you can do to improve the energy efficiency of your house.  Future articles will discuss this topic in detail.

As always, if you have any questions or comments, share them with the rest of us by leaving your comments.  Otherwise, email me at

Next time, a related subject: Insulation!

Don’t forget to heed our warning though . . .

WARNING! Do not implement any of these solutions without considering the impact on IAQ, moisture control, & heat transfer. A combination of solutions are usually needed to improve health, building durability, comfort, and energy efficiency. Ignoring this warning may lead to disease, deterioration, high energy bills or worse!


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