I'm in the process of finishing my basement and am currently planning out my HVAC. I live in Minnesota so I'm trying to do what I can to maximize heat delivery/retention down there so I don't end up with an uncomfortably cool living space in the winter.

I'm wondering if it would be a good idea to insulate the trunk duct which will be enclosed in a soffit that runs along the middle. I'm also considering the same thing for the branch ducts. Would it be a good idea to insulate those ducts as they run through the floor joists? If so, would it make more sense to only insulate the supply ducts that are delivering to the basement and leave the existing supply ducts for the upper floors uninsulated? Perhaps leaving those ones uninsulated may allow some extra heat to stay in the basement? But, I'm guessing the vast majority of that would just be waste that would just be staying in the interfloor space.

Just hoping to get a sense for what steps may actually be beneficial vs what would be a waste of time/money.

  • Is your basement expected to be kept at a very cool temp?
    – isherwood
    Oct 21, 2020 at 16:18
  • Heat doesn't just disappear. If it's radiating (or leaking) into the interfloor space it still contributes to heating adjacent spaces to the same extent.
    – isherwood
    Oct 21, 2020 at 16:22
  • No, ideally I'd like to have it maintain the same temperature as the upper floors, though I'm guessing it's inevitable that it will tend to be a somewhat cooler.
    – grin0048
    Oct 21, 2020 at 16:45

1 Answer 1


All ducts are now required to be sealed (at the seams), but you don't normally insulate unless:

  • they pass through a cold/hot environment such as an attic or unheated crawl space
  • you have an extraordinarily long run and need to reduce heat loss along the way

If you have a typical home, don't bother. Exhaust temps should remain acceptable unless you're skirting the fringes of performance with an old or inadequate furnace. The key is to have enough vents for each room (based on square footage, windows, and other factors) and good return flow.

Use foil tape and/or silicone caulk to seal all joints and be happy.

  • Ok, that's good to know. House was built in 2014--nothing atypical about it.
    – grin0048
    Oct 21, 2020 at 16:59

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