Air that leaks out of a duct in an unconditioned attic or crawlspace never reaches the room it was sent to.
Leaky ducts in an unconditioned attic or crawlspace lose conditioned air before it ever reaches the room, and sealing is usually the fix that actually changes comfort rather than adjusting the thermostat again.
Leaks concentrate at joints, takeoffs, boots and plenum connections — the places duct sections meet each other or meet the equipment. These are sealed with mastic or tape at installation, and that seal is what fails over time, particularly in an attic that swings between summer heat and winter cold every year.
Air leaking into a conditioned basement is still inside the thermal envelope of the house. Air leaking into an unconditioned attic or crawlspace is gone outright — it was heated or cooled and then lost before delivery. This is why houses with a lot of attic or crawlspace duct running often show uneven comfort room to room: the rooms at the far end of a leaky run are the ones shorted.
It's common for a homeowner to chase an uneven house by upgrading the furnace or air conditioner, when the actual cause is leakage between the equipment and the registers. Sealing the runs that pass through unconditioned space is usually the cheaper option and the one that targets the actual problem rather than making a bigger unit push more air through the same leaks.
Describe the problem and we'll tell you whether it's cleaning, sealing, repair, or not a duct problem at all.
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