A 2–3°F difference between floors is normal physics. A 6–10°F split that makes bedrooms unusable is a solvable problem. Here's what's actually causing it, in the order we check.

Heat rises — but that is only half the story

Warm air stratifies upstairs, and a sun-baked roof radiates heat into the top floor all afternoon. But well-designed systems compensate for this. When they can't, the culprit is almost always delivery, not physics: the upstairs simply isn't getting its share of cold air.

Undersized or leaky upstairs duct runs

The most common cause we find. Long duct runs to the second floor lose pressure through leaks, kinks, and poor insulation — especially runs through a 130°F attic. Sealing and insulating those runs is often the single biggest fix, and a duct cleaning visit includes the full assessment.

One thermostat, two climates

A single thermostat downstairs satisfies itself while the upstairs roasts. Zoning — separate dampers and thermostats per floor — solves this properly. A good interim step: a smart thermostat with upstairs remote sensors that averages temperatures instead of trusting one hallway.

Attic heat pressing down

Poor attic insulation and ventilation turn the upstairs ceiling into a radiator. If your attic floor has less than 12 inches of insulation or your ridge vents are blocked, the AC is fighting the house itself. Air-sealing plus insulation often cuts the split in half.

An aging or undersized system

A system that cooled fine at installation but can't keep up anymore may be low on refrigerant, running a weak blower, or simply past its prime. And some two-story homes were always under-equipped — in which case a right-sized replacement or a ductless mini-split for the upstairs solves it permanently.

Fixes, cheapest first

  • Close downstairs vents partially (never fully) to push more air up — free, try tonight.
  • Run the fan on "circulate" to keep mixing air between cycles.
  • Add remote thermostat sensors upstairs for smarter averaging.
  • Seal and insulate duct runs through hot attics.
  • Zone the system with dampers and a second thermostat.
  • Add a ductless mini-split for the hottest rooms — the permanent fix.