Do I need to seal lengthwise ductwork joints?

[meackerman]

Not knowing what "duct seal" is, I'm guessing this is some kind of plastic-like goo you work into the joint? Anyway, at some point (trust me on this) you will want to take that ductwork apart. It could be a new tool needing a new leg, a rearrange of the shop, or (god forbid) a clog. The foil may be  a little easier to break the seal since it's on the outside.

its a brush on mastic.

something like:
https://www.homedepot.com/p/Water-Based-.../100396973

[WaterlooMarc]

I agree with Fred. I might would apply the seal to the long lengths because those snap locks are hard as hell to get apart anyway. But I wouldn’t add it to the actual joint. I average disassembling various parts of mine once a year for various reason and it’s enough a PITA to get the foil tape off. You don’t want to fight with another sealant there too imo.

[daddo]

How big is the shop and how long are the ducts?
If it's exposed duct and not insulated- probably not in typical spaces. In an enclosed hot attic and insulated- yes.

But in a shop where you may often never reach the lower humidity and comfort temperature zone consistently- yes. Even exposed metal duct should be insulated in this case.

What you should be worried about is sweating and dripping ducts.
Don't squeeze the insulation while wrapping and reduce it's thickness. Seal every single air leak- use mastic over tape if you must, and you'll never have a problem. Tape/mastic any holes in the barrier of the insulation- outside air entering means moisture and dripping- the space between the insulation and the metal duct can hold a lot of water.


If you have a smaller shop, just blow air into the shop from the plenum and don't do ducts- problem solved.

[meackerman]

How big is the shop and how long are the ducts?
If it's exposed duct and not insulated- probably not in typical spaces. In an enclosed hot attic and insulated- yes.

But in a shop where you may often never reach the lower humidity and comfort temperature zone consistently- yes. Even exposed metal duct should be insulated in this case.

What you should be worried about is sweating and dripping ducts.
Don't squeeze the insulation while wrapping and reduce it's thickness. Seal every single air leak- use mastic over tape if you must, and you'll never have a problem. Tape/mastic any holes in the barrier of the insulation- outside air entering means moisture and dripping- the space between the insulation and the metal duct can hold a lot of water.


If you have a smaller shop, just blow air into the shop from the plenum and don't do ducts- problem solved.

so now I'm wonder if the OP meant dust collection ducting or HVAC ducting....I thought he meant DC ducting.

[fredhargis]

so now I'm wonder if the OP meant dust collection ducting or HVAC ducting....I thought he meant DC ducting.

Same here........

[daddo]

Well, now I'm not sure.

I think you're right- bet it's DC

[tedrussell]

I assume OP means something like "original poster".  If so, that would be me.

It's ducting for a dust collection system, so insulation isn't a concern.  The HVAC guy told me the duct seal is a bear to get off if you have to, but does the trick for sealing.  Sounds like the only reason to use foil tape instead of duct seal is for ease in disassembly.  I'll talk it over with my son (it's his shop), and see if he thinks it's likely he'll need to move tools around at some point.

Thanks to all for the great comments.

[TDKPE]

Kind of ironic that duct tape is great for just about everything, except sealing ducts.

I believe, or heard at least, that it's actually "duck tape", and was originally invented for sealing ammo boxes in WWII.  "Duck" as in cotton duck, which is what it's made of, or used to be at least.  I remember back in the day, though not back to WWII as I'm not old enough, that my Dad's rolls of duck tape actually were cloth, similar to friction tape.  Now there's a brand called "Duck Tape" with a picture of a duck, but I think that's relatively new. Still some sort of cloth as a base, but with a heavy plastic layer on the outside.

Don't use it for sealing duct work.  The mastic will let go after not too long, and leave a sticky mess behind, while no longer doing its job of sealing.  Use foil tape, designed to seal ducts.

its a brush on mastic.

something like:
https://www.homedepot.com/p/Water-Based-.../100396973

That stuff works really well, but the OP should be advised that it will make a mess.  If you want it to look neat, use masking tape on both sides of the joint, then brush it on.  It's really hard to clean off if you slop it onto places you don't want it.  If you don't care, like if putting the seams on top or behind where you can't see them, then it doesn't matter.  I sealed the joints on my cyclone when I built it, thinking it would just wipe off excess like caulk, but noooooooo, it didn't.  

[Bill Wilson]

I believe, or heard at least, that it's actually "duck tape", and was originally invented for sealing ammo boxes in WWII.  "Duck" as in cotton duck, which is what it's made of, or used to be at least.  I remember back in the day, though not back to WWII as I'm not old enough, that my Dad's rolls of duck tape actually were cloth, similar to friction tape.  Now there's a brand called "Duck Tape" with a picture of a duck, but I think that's relatively new.  Still some sort of cloth as a base, but with a heavy plastic layer on the outside.

Don't use it for sealing duct work.  The mastic will let go after not too long, and leave a sticky mess behind, while no longer doing its job of sealing.  Use foil tape, designed to seal ducts.

Interesting about the WWII ammo boxes.  I had never heard that before.  


I remember helping my Dad install ductwork, back in the mid-70's and it was still used for sealing joints then.  And no, it didn't work very well, at least for very long.

[TDKPE]

I remember helping my Dad install ductwork, back in the mid-70's and it was still used for sealing joints then.  And no, it didn't work very well, at least for very long.

Know where it works too well?  Taping furniture pads that are wrapped around chairs and stuff for moving.  The kind you rent from UHaul.  That tape often doesn't come off the pads without a lot of time and effort.  DAMHIKT.