A Sign of a Good Duct Installer

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Matt Risinger is a home builder in Austin. He builds great houses and is also famous for his videos. He has over 26 thousand subscribers on his Youtube channel. I was in Austin last week for the inaugural Humid Climate Conference and had some extra time on Saturday morning. So I got a tour of a few of Matt's projects. I was pleased to see that pile of materials below at one of those homes. Can you tell why?

Here's why I liked what I saw there:

  1. He uses rigid metal for his duct fittings. I like flex but prefer it for straight runs. Turning air in ducts should be done in hardpipe.
  2. Those fittings were pre-insulated. Once they're in place, it's not always easy to get insulation on them.
  3. The installers had already sealed the joints and seams in those fittings with mastic.

Beautiful! That's the sign of an HVAC contractor who cares about air distribution. Further, those fittings are part of a system that was designed properly. My friends at Positive Energy in Austin did the full mechanical design for this house.


While I was there, Matt shot two videos with me about ducts. Look for them in a few weeks.

And one final note: You may be wondering why there's OSB on the inside of the walls in this house. That same confusion happened to people who visited the house I built while it was under construction. There's OSB on the inside because the house is framed with structural insulated panels.


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Photo credits:  Duct fittings by Energy Vanguard, Matt and Allison by William Risinger


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Harvey Sachs

I don't often get into houses under construction any more, but the picture hit me in the gut, reminding me of things I've wondered about in the past:
1) Apparently the entire HVAC system (except condensing unit) is inside the thermal envelope, as it should be. That implies relatively uniform temperatures (unless there is zoning, and given room-by-room returns). Is there cost-benefit or a comfort benefit to duct insulation is such construction? I'm not talking about duct leaks, but just the thermal insulation.
2) I'd really like to see duct systems that can't be assembled wrong, unlike steel pipe, flex-duct, etc. Consider the humble PVC sanitary drain system. The joints have some angular and length adjustability, but it's really hard to make leaky connections (and that is a good thing). Lots of reducers and expanders and y-fittings are available, etc. Now, I'm not advocating PVC as such, but as an industry shouldn't we be looking at the Code requirements re smoke/flammability to see what's really needed, so we can move away from labor-intensive craft approaches like sheet metal?
Just a thought about yet another concept I couldn't get to fly...

Allison Bailes

Yes, Harvey, all of the ducts in this house will be inside the thermal enclosure. Insulated ducts does help a bit with comfort because supply air temperatures don't change much inside the ducts. Also, insulation on supply ducts is insurance against condensation problems if the house ever gets too humid while they're air conditioning. And condensation dripping on drywall isn't a good thing.

Great point about using materials that are easier to seal. I haven't seen anything that's going to take the place of flex and rigid duct and accomplish that, though. Also, as I'm sure you know, the inertia of the trades is a really difficult thing to overcome. Even if that new product shows up, it'll have to be easier and cheaper than flex to make a big impact, IMHO.

Dale Sherman

Harvey brings up a good point about wanting easy and tight ductwork. The Swedish HVAC industry uses rubber compression fittings and duct testing protocol that helps raise the quality of workmanship. They even use different chases for supplies and returns in multifamily housing. The ductwork may cost a little more, but they save on the labor of applying mastic.

J Wright

I hope this isn't a stupid question.... why is pvc not used for hvac ductwork? My guess is that it's just too expensive compared to sheet metal. It would seem to me to be the ideal material, based on ability to seal connections so easily, and the smooth inner surfaces. In thinking about my own part of the country, west coast, where the average low on the coldest days of the year is 40°, maybe we could have a system with smaller duct diameter and lower materials cost (still thinking of pvc), but I'm not terribly smart. Comments welcome.

Allison Bailes

Thanks for letting me know about that article, Dale!

Allison Bailes

Not a stupid question at all, J. I'm not an HVAC contractor, but my guess is that PVC isn't used for ducts because of cost and availability. The latter is because it's not so easy to find 12" or larger PVC.

Dennis Brachfeld

What the duct? Go with the flow, when you are in the know. It is a proper fit, when your articles hit. We are trying to do it right, even when our work is out of sight! Will look forward to seeing the videos. When we are green we are growing, when we are ripe we are rotting, Thanks again, for sharing and caring and helping us to keep learning and earning. Sorry, I am still recovering from my rhyme disease.

Allison Bailes

Dennis, maybe you should get some blank verse treatment. ;~)


When you build your next house, instead of using AAC what about poured foam blocks?
The R value is good & it's air tight?

Allison Bailes

Scott, are you talking about ICFs? That's certainly a possibility, especially if I build a basement. Don't know that I'd do it for the above-grade walls, though.

Tom Frymire

In my experience, city code officials do not allow pre-insulated duct fittings. They wanted to visually see the mastic.

Allison Bailes

Tom, at least those officials know to look at the actual duct, which is where the pressure boundary is. Some insist on seeing mastic on the outside of the insulation jacket. Of course, visual inspections don't tell you whether the system is tight or leaky. You have to test for that.


I bet the architect left plenty of room to install a properly sized/designed system. Just this past weekend I was poking around some new luxury townhomes ($660k+) and IMO the mechanical rooms were so short that there really was insufficient room for the flex supply lines.

Wish I could upload photos.

Allison Bailes

Kris, the architect did leave space for the air handlers and ducts in this house. They even came up with good ways to make sure they'd be accessible for servicing. But I know what you mean about tight mechanical closets. I've seen plenty of them.

Jeff Franklin

Just saw a product on This Old House called Aeroseal that is interesting. Ducts are installed and then registers blocked. A blower pressurizes the ducts, measures leakage and them atomizes a sticky polymer into them that can black and seal any openings up to 5/8". The polymer only sticks to the area around leaks and openings, it does not coat the entire inner surface. Developed by Lawrence Berkeley National Laboratory. Seems like a big time saver.

J Wright

I saw that same episode, and got terribly excited about such technology. Wonder what everyone else thinks about it.

David Eakin

I also saw that episode and had a chuckle because Aeroseal has been around for some time now. Their first foray was in the retro-work business of existing duct work needing efficiency improvement. This new foray (sealing new construction work vs manually-applied tape or mastic) also looks interesting (and may even be slightly more expensive right now) but the catch will be Allison's previous response - trade inertia.

Allison Bailes

Yes, Aeroseal is a big time saver. It's been around for a while and is a great product. It's not cheap, but I've heard of some HVAC companies that install all their ducts in new construction and then have Aeroseal come in to seal them. They make more money by moving on to the next job than by hanging around to seal the ducts. Of course, you've got to be making enough on each job to pay for it, but I guess they are.

David Butler

There's no way Aeroseal can compete with conventional air sealing techniques in new construction. In retrofit, sometimes it's the only option, although you gotta at least ask the question whether there's enough savings to be had to justify the considerable cost.

The best solution for new construction is to keep those mechanicals inside the thermal boundary, in which case Aeroseal is clearly overkill.


Aeroseal has been around for years down south. It may be the only way to seal small leaks when you are faced with inaccessible ductwork. It is very expensive to the point where you could start over or hand seal, properly hang, ensure no large holes that aeroseal won't fill (over 5/8") and then better insulate the house with the money left over.


This Old House had a segment on Aeroseal a few years back as well. With a price tag of a few thousand $ there's certainly a diminishing return. Personally, I did all our basement ducts with mastic for $45. Will do the vents in the wall if/when walls are opened for other reasons.


AeroSeal does require air flow to move the goo to the hole so when flows drop in tight ducts the Airsealing stops also. I would like to know from AeroSeal what the flow limit it but suspect it might not work for getting flows much below 12 CFM / 100 sq ft which is still a pretty leaky duct. Can it get much below 12?
Duct sealers can get them below 0.5 CFM per 100 sq ft and we can to encourage this performance since it's the cheapest way to gain performance at almost zero cost. Testing just to pass code means it is still bad; just not atrocious. For 2000 sq ft house, code at 12 cfm/ 100 sq ft is still 240 CFM which is over have a Ton of energy lost to say nothing of pressure imbalances and the iaq and safety problems that causes.


AeroSeal may be the only way to seal ducts where there is no access in existing houses such as joist cavities for example unless you demolish the ceiling. Not applicable in new houses because a license fee is charged per job, set up time is high and mechanicals such as blowers must be sealed off and it only works when there is leakage flow to move the goo to the hole. Ducts can be sealed much tighter in new houses with mastic and cheaper.


I am sure Austin differs from the rest of Texas where we see the lowest compliance rates in the country. Our guess is less than 10% of ducts are tested in Texas overall because it appears that most Code Officials don't bother with ensuring testing takes place. The DOE study showed an even distribution of duct leakage rates from about 8 to 20 CFM per 100 square feet of floor area indicating the ducts were assembled in a reasonable fashion but not tested which would have caused a clumping of results at the 12 CFM mark.

I am interested to know if anyone has similar or different experiences in Texas.

Chris Dorsi

Thanks Alison for showing this example of best practices for duct installation and testing. Both consumers and industry would benefit from widespread implementation of this type of workmanship.

Colin Genge referred to the DOE Energy Code Field Study -- which showed that the Texas ducts tested are all over the map in terms of tightness, suggesting that there is little testing or enforcement. Learn more about that study here:


Or check out the summarized graphs from the study on page 7 of this slide deck from the Compliance Project: