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Unfortunately, I have to run an 8 inch HVAC duct through an unconditioned kneewall attic in southern Ontario (climate zone 6). I am trying to decide between two options:

  1. Encapsulate duct with spray foam to a sufficient R-value (image 1)
  2. Bring the duct inside the conditioned space by framing and adding drywall around the duct. Then ensuring continuous fiberglass insulation around the building envelope (image 2)

I'm trying to understand if these are actually different. Isn't option 1 (spray foam) just the same except I avoid the work of framing/adding drywall?

If I spray foam, should I avoid spraying the duct where it is in contact with the conditioned space? And can I spray directly onto the duct surface, or do I need a vapour barrier?

Image 1 Image 2

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  • Your second option seems like a lot of work. I would consider Ed Beal's suggestion of using insulated flex duct. That's what runs through my uninsulated attic, though my climate is not a cold as is yours (mid Atlantic region of the East Coast). – SteveSh May 26 at 1:14
  • The insulation on flex duct would help, but would definitely not be sufficient for here. The recommendation is R60 on an attic floor so that's why I'm leaning towards the spray foam option. I am still unclear what changes vs a duct in a chase inside the conditioned space vs duct on the other side of a piece of drywall. 1 extra side of the chase in contact with an exterior wall seems to be the only difference. – Tenz May 26 at 1:24
  • Ducts don't need the same insulation levels as the condition space since you are just trying to slow thermal losses in air as it passes through the ducting and preventing condensation on the outside. If your concern in the last of insulation below the duct, insulate the attic area below the duct then run the insulated duct on top. – redlude97 May 26 at 17:10
  • I don't understand your comment that "the ducts don't need the same insulation levels as conditioned space". Isn't the thermal gradient higher between the hot air in the duct and unconditioned space, hence a potentially faster location of heat loss? I'm not concerned about insulation below the duct, as the space below is conditioned. – Tenz May 26 at 23:26
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What about using flex duct it is already insulated?

If you want to use metal duct you don’t have to frame around the duct like your pic that leaves air space, get bats of insulation and wrap the insulation over the exterior side of the duct much like your drawing but in contact with the duct.

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  • I prefer the lower resistance of metal duct over flex. Re: insulation, my main issue is understanding what difference there is being inside the building envelope vs being outside it. I am leaning towards the spray foam option because I believe it provides a vapour barrier as well as high R value. – Tenz May 26 at 1:19

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