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I have a balloon frame house with an unfinished basement. The air handler and furnace are in the basement too. Last fall, we had the duct work removed because it was wrapped in asbestos tape that was deteriorating. We DIYed the replacement just like the previous, except I used some online tools to make some airflow calculations and made one of the returns bigger. 100% of the duct work is metal box or round pipe, and every seam from the plenum to each register boot is well sealed with mastic.

We did not insulate anything, because the laundry and a couple work benches and a video game area are down there, and we decided some radiant heat from the ducts would be nice, but I have since read that we should have insulated the supply ducts to help control condensation. Also, I learned that insulating ducts is required in an unconditioned space, and I think my unfinished basement with no HVAC registers meets this definition.

This is going to be our first cooling season with the new work, though we haven't noticed any condensation yet, so...

Does code require to insulate the supply ducts in this space? If so, how does one insulate big square trunk duct? (All the tutorials/materials I see are for 6" round)

Could we turn this into a "conditioned space" simply by adding a register in the unfinished basement?

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    as long as you have air exchange, you shouldn't have condensation. a register would seem to accomplish this. i would wait and see if it needs it before spending time and effort to "fix". – dandavis May 21 at 17:53
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They make insulation that is like a foil and fiberglass blanket that you wrap square ducts with, if you are in a high humidity area you may find the duct will sweat if not insulated because your ductwork close to the air handler will be 20-30 degrees lower than the room temp causing the condensation. I might wait and see. Just google duct insulation and you will see the different types.

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Insulation is the best way. Add a outlet vent to area. And if not needed ,close vent. open when needed. They even have vents that are hooked up and keep room at the temp you want.

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