I've finished dry-fitting a new branch (6-inch round) from the main furnace duct to a new register. I've supported each section of duct with metal strap and have begun taping all connections with foil tape. The tape and strap seems quite sufficient to hold everything together. Are screws required for every junction? The other branches in this (older) home are screws-only; no tape.

up vote 2 down vote accepted

What I've learned thru doing residential HVAC for a couple of years in Michigan is that for it to be up to code you need at least three screws for under 8 inch diameter ducts. Anything larger deserves at least four. Keep in mind it's not always possible to evenly distribute the screws, so this is why you also want to use hangers of some sort.

Finally, use mastic sealant or aluminum tape; be sure to degrease your duct when using tape as the duct has oil on it from production and tape will not always stick.

Yes, now a days screws and a sealant of some sort are required. Use aluminum tape, not the gray plastic crap with cloth backing, it falls apart in a few years. There are brush on sealants also.

  • "required" by who? – Tester101 Feb 21 '15 at 1:39
  • Required to be a good installation (I've made some easy money putting screws into ducts that came apart). +1 for mastic, tape is only for the 'temporary' joints on your equipment. – Mazura Feb 21 '15 at 2:06
  • I vote also for the screws. It really takes no more time to prepare for and install screws that the other idea to use "pop rivets". – Michael Karas Feb 21 '15 at 4:27
  • @Tester101 Basically, I am curious if in most areas of the US, a building inspector would give ductwork a pass in a residential home without screws. – Paul Feb 21 '15 at 6:13
  • @Tester101, the requirement I speak of is the building inspectors, they as you know, enforce the local building codes. All of the US is following the IRC or IBC code since about 2005, give or take a few years. I got rusty with the codes after CABO lost favor. I mainly go by what all the HVAC subs did on the sites I ran, asking questions to learn by others, not nothing I read for myself. Every job was done consistent job after job, over a great number of years, I guess I could be off a bit... – Jack Feb 21 '15 at 8:26

Self Screwing Sheet Metal Screws

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Air Duct Sealant/Caulk

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Aluminum Foil Tape

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Duct Wrap (if in unconditioned space)

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  • Did you mean self "tapping"on the first photo? – TomG Feb 21 '15 at 13:55

An alternative to a screw is to use a rivet like a pop rivet. You need a diamond hole cutter to make the hole for the rivet.

Using rivets will result in a tight, permanent connection that will not work loose and will not have any sharp protrusions.

  • 3
    Pop rivets do make a nice neat connection but they are a ripping pain when you come at a future time and have to open the ducts for what ever reason. – Michael Karas Feb 21 '15 at 4:25
  • Yes they are a great product, but a PITA even for gutter and downspouts, more so for ductwork. – Jack Feb 21 '15 at 8:28

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