Is rigid plastic allowed for mid-efficiency furnace duct in attic?

Is plastic allowed to be used as a duct pipe for a mid-efficiency furnace in an attic?

I thought only Hi-effciency Furnace only had PVC.

No not for a duct in the attic. I’ve seen plastic ducts installed underground.

Check your local codes.


Are you talking furnace duct or vent?

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The manufacturer should provide the most reliable information.

Edit: Are we talking about supply/return or exhaust?

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I’m not sure whether it was a supply or a return duct, but it was not a fresh air supply.

Thank you. We’re under the 2015 IRC now.

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It was a supply or return, but not an exhaust.

I am currently unable to find installation instructions for this furnace due to its age.

Most ducts in the attic in my area are constructed of a thin plastic flexible insulated material.

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I see that quite a bit, as well, but not usually rigid plastic.

Thanks for clarifying about the type of plastic that’s used for furnace ducts.

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Adam, it looks like they may have replaced the thin plastic flexible insulated material with hard plastic because it is in a door/walkway and maybe got tired of the thin stuff getting squashed…not saying it is right.

Just a thought…


So, when I see these things…I try to consider consequence.

So, IMO opinion there are two possible consequences:

  1. Condensation due to lack of adequate insulation.
  2. Loss of energy due to lack of adequate insulation.

They might be an honorable mention, but probably not if the home has bigger fish to fry.

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It was a substitute to the hard metal duct that was more vulnerable to getting crushed. This furnace mainly had hard metal ducts. The home was built in 1942.

My mind was originally thinking about fire hazard, but I agree that the more relevant consequences are condensation that could create moisture intrusion and energy loss. Thank you for bringing those up, Brian!


I did not find code to be very helpful. It would require a deep dive.



I think that it’s just a return duct. Sorry for the confusion. Either way, it needs insulation.

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What is it’s age? Mid-efficiency furnaces have been around, and still are, for a very long time.

I wouldn’t be too concerned about it, especially if it is return air. If you feel you must mention it, use the term “Non-Standard Practice” and don’t make a big deal.


The age was not listed on the manufacturer’s tag, and I couldn’t decode the serial number through Building Intelligence Center. On Monday I’ll try calling Coleman to ask if they know. The house was built in 1942.

“Non-standard practice”–and not alarming.

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