In reference to maintaining the integrity of an air barrier, best practice requires all penetrations be properly air sealed. 

Specifically, when HVAC ducts penetrate the air barrier between conditioned and conditioned space, both code and Energy Star require the annular space between the hole and the duct be air sealed. 

Since flex duct seems to be the among the HVAC community favorite material, most of these penetrations is made with flex duct. And in all cases, I see the air sealing being made around the outer jacket or scrim of the duct. The attached graphics even give a thumbs up on the practice.

Of course the outer jacket is NOT the air barrier. If so, then why do ADC guidelines require sealing the inner liner to all duct connections? It is my understanding the outer jacket is simply to retard vapor diffusion.

Commercial code does not allow this. The penetration must include a metal sleeve through the air barrier with flex connected on each end. Then, the air sealing would be effectively sealing to the air barrier of the duct. 

If the air barrier is also called a draft or fire stop, is it code compliant to seal the outer jacket of the flex duct to meet the fire stop requirement? 

Apparently, I am the only person in the U.S. who ever questioned this. Am I being just too picky? Or maybe I am in "Passive House" mindset where a local 17,000 ft3 project tested at 30 CFM(50). That would have never happened with such poor air sealing processes.

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metal sleeve is only way IMHO but they do give a way to "comply" with ES under description tab

air barrier is rarely a "fire stop" though a fire barrier maybe considered par of the air barrier system

If said duct does pass through a fire stop, I know some inspectors that would let it slide while most would be going uh he11 no which might even include the use of flex at all - just depends on area & type of structure

I agree Sean. Unfortunately we are in the minority. Even the site you provided shows sealing the outer scrim of the flex duct. Look at the fourth graphic under. "How to Air Seal a Flex Duct Chase"


If flex duct/duct board/foam board  duct goes though a fire wall for air flow must change to steel and then fire damper at fire wall.    I have been though this a lot with powers that be known as code pros.   Now I do my best to keep all duct in the building and in same space as the air handler.   This has killed more than one of my jobs.   One job in Lawrence KS trades man spent 47.5 hr in jail over this and not pulling permits, work comp, and being a jerk, cussing out  code person.   Most of time code person just writes it up and will not let job go.  But when a life can be taken then more things can happen.   I have see fire go right though flex. 

That’s always been a requirement where I live as well. But the concern I have is a penetration through a non fire rated draft stop/air barrier.


When we find one of these (more and more rare these days) we use single part foam around the perimeter and are very generous with the amount. The hope is that it will compress the insulation enough to reduce any air leakage through that area. But, yes, it is something to twist a few arms to change.


I think this is one of those areas where compromise is used to have a type of "solution" and move on to bigger issues.

But back to you specific comments -

By code, flex duct alone CANNOT pass thru a fire rated barrier. If it does, that is no longer a fire rated barrier. In even a small low intensity fire, the flex will melt and expose a major penetration of the barrier. Off the top of my head, I think the plastic is flex will melt at a lower temperature than the ignition temperature of most wood building materials. To retain the fire rating, the flex (or duct board) has to connect to a fire damper at the penetration.

In my opinion, sealing the air barrier to the outer jacket of the flex duct will provide decent integrity of the air barrier. Is it perfect? No, it's not. But to improve it with something like a double wall metal pipe with 1" or 2" of insulation in the annular space would increase the cost significantly with little to no chance of a return on the added cost. And it would create two more joints with chances for duct leaks at each joint.

If somebody has a suggestion to improve the air barrier seal at a cost that may be recovered - I would love to see it.


At least in the mid-Atlantic, there is no place in single family detached housing three stories and under that needs to be fire stopped.  There are plenty of areas needing fire BLOCKING, but not fire STOPPING, and single part foam is fine for that duty.  

Picture A and B in the original post appear to be ducts that travel under a kneewall from an attic and these need to be fire blocked at the air barrier where they are shown to be sealed.  To solve the problem of the area around the duct proper not being sealed, the duct should be uninsulated pipe through the air barrier where it is sealed tight, then wrapped with insulation or changed to an insulated flex once outside.  And keeping those ducts down low against the heated ceiling below allows maximum coverage of the duct when attic insulation is blown (it will be blown, right?).

I would suggest that picture C, showing an overhang of the upper floor over the lower floor, is air sealed at the wrong place.  The soffit covering the bottom of the overhang and the surrounding band areas of the overhang (seen through the window) are the proper location for the air barrier.  Where air sealing is shown would be a belt-and-suspenders approach, but not mandatory even though squash blocks may be required in some cases.  Any insulation in the overhang would be placed in contact with the soffit and the band areas, and easiest to fill that area completely.  Also, the ducts should be as high in the cavity as possible, preferably touching the floor above, so that the maximum amount of insulation could be installed below them on the cold side, and give the warm side of the duct an opportunity to heat the floor above.  I would even request that those ducts be uninsulated to get them closer to the floor above and allow more thickness of insulation below.



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