Flex vs. Metal vs. Duct Board: Differences and Use Cases

Subscribe and follow my Podcast on Apple or Spotify!
Apple: apple.co/32AOwgU
Spotify: spoti.fi/3FXNg4X
Sign up for our twice-weekly newsletter:
buildshownetwork.com/newsletter
To sponsor a video or advertise with us visit:
www.buildproductions.com
Want to learn more about building? buildshownetwork.com/
Build Show Network on Instagram: thebuildshow
Huge thanks to our Show sponsors Builders FirstSource, Polywall, Huber, Rockwool & Viewrail for helping to make these videos possible! These are all trusted companies that Matt has worked with for years and trusts their products in the homes he builds. We would highly encourage you to check out their websites for more info.
www.bldr.com/
www.Poly-Wall.com
www.Huberwood.com
www.Viewrail.com
www.Rockwool.com

[@RobinBos909]

I'm a HVAC tech in the Netherlands, we only use solid metal ducting in our installations. I work with one of the few companies who specializes in air heating(/cooling) for residential buildings. Even if we need to cross load bearing beams or so, we make custom conduit in al kind of shapes and sizes. Only when there are no options for solid, we use flex. Never would we use some kind of fiberboard!

[@danielashton5697]

As a former HVAC installer, I spent an inordinate amount of time repairing/ replacing duct board and flex duct with metal because they just don’t hold up over time and really adversely affected system performance

[@Steven-mm7gb]

They don't mention aluma flex which stands up good unless someone crushes it, but it does suffer from turbulence from its ridges but it is a good alternative to wire flex as it wont degrade from the inside out. But yes rigid is the best

[@mattp4079]

Det. John McClane taught me everything about the utility and durability of metal ducting. Ever try low-crawling with a .45 and a Bic lighter through fiber board or flex?

[@burtfamilyhistory347]

Just reducted an 1875 farmhouse, added vents in rooms that were missing and upgraded the return from one 4x6 in the front room to a real return on both floors. Best upgrade ever. Nice to know they’ve used the right products in the right way!

[@crabkilla]

3:15 that is the most beautiful ductwork I have seen installed 🤩

[@TerryKashat]

Please DON’T ever use Duct Board. Ever. I put some other posts in this thread. You will see my reasoning.

[@Anthony.Delagarza]

When he said you guys know what to do, who else yelled out, “Oooooon the build show!”

[@1agaragedoorssacramento]

Great video Matt!

[@hoopie1022]

Duct board has no use case for being a duct. Exterior insulation sure, a duct that airflow goes through absolutely not. Way too much fiber shedding and I’ve seen way too many home owners have respiratory issues when duct board is in the home.

[@lrc87290]

I think you are talking about the old duct board with the raw fiberglass on the inside. Our 35 yr. old condo community I help manage has this horrible stuff. It is disgusting after 35 years and honest duct cleaning companies won't clean it. The board they are showing has a coating of some kind.

[@hoopie1022]

@@lrc87290 The newer duct board does has a black liner on it which I agree is a million times better but it also starts to breakdown over time but this system they have built with the Dehus is also pretty advanced so in this VERY RARE case it might work out okay.

[@compasteedee]

I agree with this 100%. It is a horrible product, even in its current iteration. It is a disposable product in a permanent application. There are plenty of other problems, such as combustion and fire conduction, and any one of them should be enough to have it outright banned. In Canada there are no building codes that allow it, from the largest producer of pulp and fiber in the world; that should tell you something.

[@brettbret8836]

As a wellness expert who specializes in environmental toxins i can tell you that duct board is going to contaminate the air with toxic materials.

[@TerryKashat]

I can personally vouch for this. An office building I used to work in - I noticed I had respiratory challenges when I spent too much time there. Wouldn’t clear until a full day or two of not being there. After I did my own inspection - duct board everywhere.

[@DrMJJr]

FOR ME, I’d do metal duct everywhere no matter what; there’s just no substitute for the ability to repeatedly clean them on a regular basis.

[@hoopie1022]

Facts

[@TerryKashat]

100%. The other stuff is not just bad in performance - it’s bad for your health.

[@MrElemonator]

You need to replace your filter on a regular basis

[@gregorybergere]

How many ventilation systems are designed for cleaning? I gues none with flexible trunking.

[@dannym1772]

I actually just got my air ducts cleaned and did not know this. My air ducts were really old and now they completely broke them all over the place. I have to do a full replacement now since i had non-metal ducts and the cleaning company ruined them.

[@1stWorldProblemsSolved]

Good to know my older all metal ducts are still the best option.

[@MrElemonator]

Also good to know metal is great at transferring heat so there is a energy factor where the heat might take longer with metal ducts.

[@xslabcabxhearsex]

I have been doing HVAC/sheet metal mechanic for over 33 years.if you can invest in a custom metal duct system done by a reputable professional company.it will cost more but worth it.

[@ecospider5]

My house had a lot of flex ducting when it was built. I didn't know this till after I had my ducts cleaned and the cleaners tore multiple of the runs going under the house.

[@electroluxwash]

Ouch how it wasn’t too expensive to fix 🙏🏼

[@ecospider5]

@Electrolux Wash I was able to get to all of them and fix them with tape. I do wonder if that is a hidden problem for the new owners who have been there more than a decade.

[@CoconutPalmPictures]

I was waiting to hear about insulating metal ducts but never heard anything. How is that accomplished in an un-AC attic or crawlspace?

[@Tom-pc7lb]

Always get your ducts in a row.

[@Rocco101]

Metal duct all the way , seal all seams really really well and it will last a lifetime time. Great video Matt Appreciate you

[@buildshow]

Right on. Totally agree

[@TerryKashat]

100% Metal duct all the way. The cheaper options should not even be available for something as important as our air.

[@jakubsebor4756]

@@buildshow that's why you used pvc duct hose in your house. Do you remember the Zehnder system?

[@lukemarks9132]

@@jakubsebor4756 what’s wrong with the zehnder ducts

[@alonzosmith6189]

Learn so much from this channel. I was told to stop finding issues in my house by the wife.

[@DrMJJr]

That means you’re LEARNING!! 👏🏽👏🏽👏🏽👏🏽

[@Smallduckenergy]

Good job inacho

[@boby115]

In my neck of the woods (St Louis metropolitan area) well over 90% of the homes are built with basements, sheet metal ductwork was always used and continues to be used to this day. Sized, supported and installed correctly sheet-metal ductwork can withstand the test of time with few problems. The problem I’m starting to see is the lack of skilled tradesmen in this field, the Legacy sheet metal men are starting to retire and die off (I am seeing less pants legs and cheeked or rounded corners used , i’m assuming the contractors don’t want to pay for the extra time or have the expertise).

[@goobygoober455]

Over here in Lancaster County, PA and surrounding areas we mainly use fiberglass ductboard instead of metal. Not many of the guys know how to do metal anymore

[@bquedenfeld]

43 yrs. in the HVAC business I delt with a 3 and more I would never ever use duct board in my home it's not allowed in Hospital and Schools so why should anyone want that in their home.

[@TerryKashat]

I didn’t know it wasn’t allowed in Hospitals & Schools but that simply backs up my HIGH scolding of this product. Shouldn’t be on the market.

[@ddivinia]

How about noise concerns. Sound thru the ducts from room to room. Metal ducts has got to be louder. In that conditioned space - what is your target humidity level? Love the show, D.

[@quacktony]

I never knew duct board could be use to make ducts. The insulation properties were there but I assumed stainless steel pipes or flex duct was used. Good to know

[@EOTE_TX]

I am building a custom home with a gambrel roof. I decided to custom build my ducts out of 1" aluminized insulation board in a triangular shape fitting to the lower section of the lower gambrel of the roof. The duct is compact, and was easy to build and install. I taped it together with aluminum tape and zip tape. I also created the inlet and outlet plenums out of the same materials. I used 6" flexible duct to provide air to the registers from the second floor to the main floor ceilings. Since the home is in an "L" shape I used 12" flexible ducts between the floor trusses to transfer air between ducts on both sides of the gambrel roof. The temperature difference between different parts of the home only varies about 1/2 of a degree from day to day. Then again, because the roof is 6-1/2" PolyIso on the exterior and 6" of fiberglass batt insulation on the inside and the walls are 3-1/2" PolyIso on the outside and 6" of fiberglass batt on the inside the house varies only 1/2 degree from day to day. Coupled with an HVAC system and dehumidifier the duct work performs flawlessly. The amount of insulation ensures that the house temperature does not change significantly and the load on the HVAC system is negligible. We had a night where the temperature dropped to 9 dgrees Fahrenheit and we purposely turned off the HVAC unit overnight for 12 hours. The inside temperature only dropped 3 degrees. This is a 3800 sq. foot home with 2 floors and only a 5 ton HVAC system. I call it my big Yeti!

[@imtheonevanhalen1557]

In my area R-8 duct board is required.....2 inch. I used R-8 duct board for the 20' plenum, and came off with 6 inch flex duct. Works very well and passed inspection with no problem.

[@gerwynsamuel]

Matt, how do mini-ducts in a high velocity small duct system compare?

[@MrStreaty122]

I thought it was best practice to use solid metal ducts for the majority of runs, and then flex for the last 5-10 ft if absolutely necessary

[@goobygoober455]

I think code is 14 feet max for flex

[@oldschool6798]

Good, to the point program. Thanks much.

[@jaxchristian6408]

Flex duct most common because easiest and cheapest to install. Easy to crush crawling around in the attic

[@Steven-mm7gb]

Surprised they did not mention aluma flex which I would say is one step below sheet metal hard pipe, yes it does crush easy but it wont degrade over time in the inside like wire flex , and will last just as long as hard pipe if not molested by someone crushing it. And yes, it does have ridges so some turbulence.

[@louiem4134]

One of the first few to see as soon as it posted noice.

[@erhvacramirez8023]

For me the better way always is BUILD system or combination between metal round and flex !

[@andrewnaber520]

Hey Matt, you ever see or come across Alumaflex? Thoughts on that?

[@larryseibold4287]

An end to end metal duct is loud compared to flex duct or duct board, so there is a trade off beyond cost, even if oversized a bit.

[@joshuaellison01]

We are searching for a solution to our in slab duct repair issue. There appears to be areas which are deteriorating and the spray encapsulation, like Duct Armor, seems like a good solution. Anyone have experience or feedback on this method? Replacing what's in the slab does not appear to be an option, neither is installing split units or overhead duct work.

[@DonTruman]

How do you clean ducts? I presume you have to pull it apart, then run some sort of brush-vacuum through it.

[@lesliehatun2044]

My concern is noise at the vent, intake. With increased capacity flowing thru a flex is probably quietier?

[@georgek.5817]

Higher quality products always cost less long term

[@townsendliving9750]

Hydronic heat and cooling, per room home runs to cassette systems. Each room can be temperature controlled, can be installed by the most rookie DIY homeowner, easy to clean and maintain.

[@michaelshettig7805]

If you're building a million-dollar house, I would get an upgrade alternate price on metal ductwork and see if it fits into your budget.

[@terrencesauve]

Well that the main problem with new homes. Many pick the kitchen upgrade or the third bathroom over insulation and better HVAC. You only want to pay for what you see, not what comes out of your pocket every month...

[@hotrodpaully1]

If you're building a $1000 dollar house I still would use metal

[@MultiYlin]

a quick question: I see Germans begin to use rigid plastic for duct work.

[@Abdullah34610]

My initial concern would be warping when hot. Also how porous/vulnerable to mold it is. Probably depends a lot on the material.

[@hoopie1022]

And you can clean flex duct as long it’s not brittle. You can clean and incapsulate lined ductwork or duct board according to the IICRC and NADCA standards per the NADCA ACR 2013

[@juligrlee556]

Does anybody know the physics of sizing metal ducts to best guide heat from the furnace to the extreme other end of a series of rooms at distance from the source? Discussion of products without proper location and sizing is at least as important as materials. Also, what options you use to insulate your metal pipes has a lot to do with efficiency, especially when the furnace is downwind of prevailing winds outside winds or thermal issues in walls. Everyone believes they are the perfect duct system designers although they know little about volume of moving air inside of the ducts themselves. Additionally, where to place furnace outflow to rooms and where to place cold return is the field of HVAC designers for each room. There is a lot to know in planning your HVAC ductwork and very few qualified HVAC flow designers capable of providing even heat to a 3 BD house with large living quarters, entertainment centers, office space, play room etc.

[@kellyhutson8988]

Good point, that's why engineers make the medium bucks.

[@terrencesauve]

That's why we use hydronic heating and mini split a/c. Theres so much head loss moving air through joist space that it doesn't make sense anymore. We used to only have hydronics half a century ago, why not go back to systems that still works 50 years later. All these shitty lined cardboard will have to be ripped out in 20 yrs.

[@patora911]

I need the breakdown of how easy these are for ninjas to crawl through…

[@TerryKashat]

I was concerned about Solid Snake - but you make a good point too 😂

[@redneckgoatfarmer]

Sheet metal duct is the only ninja approved duct. Due to higher cost sheet metal ducts are utilizing thinner metal which can bare less weight and it is noticeable noisier. Making it much harder on ninja.

[@Regnarrussell]

My home has duct board and flex duct. It’s horrible and I will swap it out for metal. I may even spray foam the surface to insulate.

[@pleasenomore100]

I'm having a new home built, and I'm afraid that my HVAC contractor is going to pencil whip the DALT test on my ductwork. How to I keep my HVAC contractor honest with duct air leakage testing (DALT), it is energy conservation code in NC. I'm certified through AABC and performed numerous DALT tests in a commercial setting using equipment from Retrotec and Oriflow.

[@Jameson4327]

You haven’t put out a video on cleaning ductwork. I just cleaned my house and I was shocked at the amount of dirt,hair? whatever.

[@hoopie1022]

kzbin.info/www/bejne/mIimo2lsm72SoZY

[@loctobert9421]

Love that "Tesla" has become equivalent to "High-end" or "Great quality".

[@turbo3922]

Yeah that doesn't make a lot of sense to me. There are plenty of other cars that are better known for build quality and high cost. Tesla has a reputation for poor quality control (big panel gaps, misaligned parts, etc).

[@02au17]

​@@turbo3922true but non car people probably dont know that

[@1oldbluesheepdog]

Is the metal ductwork insulated or does it have to be in a controlled space?

[@kellyhutson8988]

It comes uninsulated but can be wrapped in insulation if used in an unconditioned space. You just have to check the insulation type and specs for the space you're using it in and make sure it's installed correctly.

[@danielstoscup755]

Hey guys I have a 30 year old house with metal ducting and I hear it creak and pop every time my furnace kicks on and off. I assume it’s from the metal expanding and contracting, any advise for how I can fix it?

[@hoopie1022]

Sometimes you can find the spot that pops and just stuff foam or insulation around that area to keep it tight…I know that’s a gangster recommendation but it’s the easy cheap option. If you want the expensive option get new duct that is thicker metal.

[@Casmige]

Wow!, Nacho certainly has his ducts in order…!

[@kenreynolds1000]

Corrugated pipe kills flow. Anyone who has studied head loss will tell you round and smooth is best. Sharp corners are bad too. Hard to do and mostly overcome with more horsepower.

[@williammauzy7918]

Metal duct work everywhere, sprayed with foam. Only way to go.

[@TerryKashat]

100% Metal Duct. I would advise against the foam and use sealing tape. It’s healthier from a biological standpoint.

[@audreye7078]

@@TerryKashat insulation?

[@TerryKashat]

@@audreye7078 the foam is not biologically healthy from a chemical standpoint. Insulation is technically fine - the challenge wise that is you want NO fiberglass insulation exposed to the air leaks or interior of your duct do that the fiberglass never gets in the duct pathway.

[@macoveialex6823]

What about HDPE?

[@dsdragoon]

The best ducts are the ones running throughan air condition attic space.

[@1galste]

after several test of air flow the amount of gain of metal is not justified by the higher cost. Also with time the metal duct became noisier. Last consideration is that if you put a piece of flex along a metal built system the efficiency people is talking about is really less, even using a short amount o flex. Metal is better but it is not all gold as they say.

[@gordonlee5761]

I use rigid galvanized steel for most parts of my ductwork. I only use duct board for return air and registers, connecting with 7ft long flex ducts for noise control. I have a MERV13 media filter and a whole house dehumidifier system so I don’t think it gonna cause IAQ issues.

[@Loadrunner620]

Flex duct always installed incorrectly 60% inefficient. Fixed metal duck is the best. Avoid ninety-degree turns elbows. Use a slight curve elbow. Insulate around the metal

[@frankminisplitking]

Wouldn’t it be cool, no pun intended, if there was an in-between board and sheet metal… I think as systems get more efficient and complicated… why haven’t we come up with something better? An “in between” would be awesome, as more people care about the “lungs of their house” and their investment. Obviously it would be great if we can use the same tools we already use for duct board… but a smoother, more resistant finish on the interior… maybe a neoprene layer, call it duct board pro…

[@tonycosta2455]

Double wall metal ductwork. Very expensive but, you get an inner liner of metal a inner layer of insulation then an outer metal layer.

[@frankminisplitking]

@@tonycosta2455 I’m sure that’s more awesomeness, but looking to offer an in-between metal and duct board for the budget minded. I saw someone post about plastic something from Germany, sounds interesting… hopefully we can use the same tools!

[@1hiphopnow]

Even if there were the experts would advise against it cuz it’s not 100% 🤷🏼‍♂️

[@tullgutten]

Flex should be illegal for any closed space like in wall or attic as it is garbage for air flow, it collects dust innside that leads to mould and is impossible to clean after some years without destroying and need to replace it all (it is really a 5 to 10 year max product before replacement is needed/reccomend. Steel is great for everything except working with it as it takes more time but as long as it doesn't get water on it it is a 100 year product that only needs inspect/clean for dust now and then and it is easy clean, will not get damadged from cleaning at all, fire resistant (still a fire hazard if flames get into it without any fire dampener) it is silent since it is smooth and heavy. The foam board one i haven't seen before but it should have many of the same benefits as metall just not nearly as easy to clean

[@tullgutten]

As a homeowner and doing full renovation on my house im going for steel duct in and out to my HRV with aluminium semi flex expandable duct the last meter for long bends and it is also a "lifetime" product. Half meter flex out from the HRV to give easy access for cleaning and it all being in a "closet" for easy access to replace only used for noise reduction. Then steel and possibly semi flex Aluminium the rest. Still making it so not done. but i am limiting plastic flex only to 1 meter to and from the HRV since it is some thin flimsy shit only for vibration free connection. Also going to hang the HRV on acoustical spring hangers to isolate vibration from the house frame (not any noticeable vibration tho as it is not the rotating ceramic core type

[@TerryKashat]

Matt - I highly suggest you highly advise against Duct Board and advise against Flex Duct. These things are cheap and cause respiratory issues due to the fiber glass insulation entering your space. Additionally - the porous nature of Duct Board allows for moisture build up and mold. Go look at any installation and you WILL FIND MOLD on the Duct Board and that gets injected into your air. I speak from personal experience. It should have never made it to market.

[@terrencesauve]

These are great points. Carrying out or moving moist air with lined cardboard is just a terrible idea.

[@goobygoober455]

Fiberglass duct is pretty resistant to moisture unless you pour water on it when installing or leave it in the rain

[@TerryKashat]

@@goobygoober455 and when there is a leak in the cieling? Or major humidity challenge in the building. Game over - both things happen and it is hard to detect once installled.

[@drunkennewfiemidget4251]

This is just straight up wrong.

[@Ed-jg3ud]

Is it me or is the audio on this seem weird and kinda off? Like they re-recorded after the fact and dubbed it in?

[@concreteguyjd]

What, no FRP ducts?

[@winkel08]

I would strongly discourage the use of duct board due to poor air quality

[@lrc87290]

See my answer above

[@moabfool]

The loss of velocity alone is a good reason to never do flex. Metal or don't bother.

[@gregorybergere]

It is worrying how much of a new house is made up of complex chemicals, especially plastics in many forms. Do we know what is slowly leaching out over the years into these tightly sealed boxes? We usually only find out after people get sick.

[@cosmicallyderived]

There’s an air exchanger Matt likes to have installed. Fascinating devices that balance pressure and cycle air. Air still comes in but heat and cold is dealt with and particulates too.

[@wendellsatterwhite50]

Why couldn't you use PVC?

[@mitchdenner9743]

You could if your independently wealthy.

[@stephen7938]

Static charge build up, possible fire hazard.

[@shannabolser9428]

I've seen it used for underground system

[@wendellsatterwhite50]

Didn't Matt have a show where it was used in an underground system in a house in Colorado?

[@nathanmrudd]

Good content, but I demand more than 5mn. 😂

[@duanehundley]

Which one is the quietest?

[@winkel08]

Using flex is the quietest I would only use duct board if necessary given you live close to the ocean and concerned with the main duct rusting out otherwise use metal for main duct and flex going to vents if you use all metal it's a good bit noisier you can do 3 to 5 foot of flex and the rest metal going to the vent

[@hoopie1022]

I think lined metal is quieter than anything but duct board is pretty quiet

[@kenreynolds1000]

Anything to decouple a solid pipe vibration to the vent. That’s why they use a short section of flex.

[@winkel08]

That may be true but I don't care for lined metal bc of the fibers in the insulation their is a reason hospitals don't allow lined metal or duct board in their facilities

[@WastedBuff]

@@winkel08 Just added a note for my builder to do exactly this based on your comment. Thanks!

[@TinyGiraffes]

Their Mic's sound off. I've heard Matt enough to know the higher parts of his voice aren't getting recorded. (It's happening on all of them; so I doubt it's intentional.)

[@ridiculous_renovations]

Duct board where it's appropriate for only a 20% bump in price of the ductwork seems like a no-brainer.

[@edwardlucero9665]

😂 only if this were a real message. You notice it is a message recording, not even sleepy Joe would leave a voicemail.😂

[@virgil3241]

New builders using flex duct should be sued and then banned from building. Cheap and fast is the only reason they do it, and then charge a normal price or more anyway. Should be banned from building, since they probably hide it as well from home owners that dont know any better

[@tjrooger1092]

This is all a bit misleading. When a HVAC system is installed, there's a duct flow calculation done. It doesn't matter what the % reduction is. The ducts would be upsized if the loss was significant enough to matter. Like you'd use a 7" flex instead of a 6" metal. The double the cost must just be for materials only. The labor alone is probably 5 times. Flex duct can be a 20+ foot run without worrying about seams.

[@1hiphopnow]

So duct board and flex is useless ✍🏻 steel it is

[@oatlord]

"Equates" or rather "e-queets"

[@TerryKashat]

Flex Duct and even worse Duct Board have NO BUSINESS being used. They should be banned - taken off the market. It’s so cheap and so BAD for you in any application. I speak from personal experience on Duct Board. Nobody is ever going to replace them once installed.