When one room is hotter than the rest of the house, it usually comes down to two things: the room isn't getting its share of cool air, or it takes on more heat than the air it does get can carry away. Often it's both, which is why the bedroom over the garage or the finished room under the roof tends to be the worst in the house. The system itself, especially where the thermostat sits, can make either problem look worse.
A quick first test tells you where to start. With the air conditioning running, hold your hand or a tissue at the room's supply register and compare it with a register in a comfortable room. If the flow is weak, start with airflow. If it's strong and the room still runs warm, look harder at heat gain.
Airflow problems: the room isn't getting enough cool air
Long, undersized or damaged duct runs
The room farthest from the furnace or air handler often sits at the end of the longest duct run, with the most turns and the most resistance. If that duct was undersized to begin with, it simply can't deliver as much air as the rooms near the trunk. Ducts in an attic add another layer: flexible duct can sag, kink or get crushed under stored boxes, and a joint that has pulled apart can dump cool air straight into the attic. Even intact attic ducts pick up heat on a summer afternoon if they're poorly insulated. ENERGY STAR estimates that in a typical house, 20 to 30 percent of the air moving through the ducts is lost to leaks, holes and poorly connected sections, and it lists rooms that are hard to heat and cool as one of the signs.
Closed or blocked registers
Check the obvious first. A register whose damper lever got pushed closed, a sofa sitting over a floor register, or long curtains draped across one can cut the airflow to a room more than you'd expect.
No path for air to get back out
Air delivered to a room has to return to the system. In many homes, especially older ones, bedrooms don't have their own return grille, and the air finds its way back under the door. Close that door and the room pressurizes, which means less cool air can get in. You can sometimes feel the door pulling shut or hear air whistling through the gap. The fixes are a dedicated return, a jumper duct (a short duct through the ceiling connecting the room to the hallway), a transfer grille through the wall, or a larger gap under the door.
Dampers set wrong
Many duct systems have manual balancing dampers: small levers on the branch ducts, usually near where they leave the main trunk in the basement or attic. They let you send more air to some rooms and less to others. If they were set to favor the first floor for winter heating, the upstairs can be starved in summer. Some installers mark summer and winter positions on the duct; if yours aren't marked, note where each lever sits before you change anything.
A dirty filter or a blower set too low
Anything that reduces total airflow hits the far rooms first. A clogged filter does it, and so does a blower speed set for heating that's too low for cooling. Air conditioning generally needs more airflow than heating, and setting the blower speed for each mode is a job for a technician, who can measure the system's static pressure while doing it.
Heat gain: the room takes on more heat
Some rooms get plenty of air and still lose the battle, because they're gaining heat faster than the rest of the house.
Sun on the windows. West-facing windows take the afternoon sun at the hottest part of the day, and south-facing ones collect heat for hours. A room with large windows on those sides can need noticeably more cooling than a north-facing room of the same size.
Rooms over garages. The floor of a room above a garage sits over an unconditioned space that bakes in summer and freezes in winter, and that floor is often poorly insulated and leaky.
Finished attics, bonus rooms and knee walls. In a Cape or any story-and-a-half house, the upstairs rooms sit directly under the roof, and the short knee walls along the eaves separate the room from unconditioned attic space. Those knee walls and sloped ceilings are often thinly insulated and poorly air sealed, so the roof's heat comes straight through. Bonus rooms added over an existing house or garage often have the same weakness.
What's in the room. A home office with computers and monitors, or a bedroom with a TV and two people in it, adds its own heat on top of everything else.
System causes: how the house is controlled
A single thermostat can only respond to the temperature where it hangs. If it's in a first-floor hallway, which is often the coolest spot in the house in summer, it turns the system off once the hallway is comfortable, whether or not the upstairs bedroom is. In a two-story house on one zone, the upstairs usually runs warmer in summer because heat rises and the roof heats the top floor, while one system and one thermostat try to serve both.
Equipment size is another common source of uneven cooling. An oversized air conditioner reaches the thermostat's setpoint in short bursts and shuts off before air has circulated to the distant rooms, while an undersized one can't keep up on hot afternoons, and the room with the most heat gain falls behind first. Our guide to what size air conditioner you need explains how a proper load calculation sorts that out.
Some people set the fan to On instead of Auto to keep air moving and even out temperatures. It can help, but in humid weather a fan that runs nonstop can blow moisture from the wet indoor coil back into the house, so it isn't a cure for a muggy upstairs.
What you can check yourself
- Make sure every register in the room is open and that furniture, rugs and curtains aren't covering any of them.
- Check the filter and replace it if it's due; our guide to how often to change your furnace filter covers the timing.
- Do a door test. With the AC running, measure the room's temperature with the door open, then close it for half an hour or more and measure again. A much warmer reading with the door closed points to a return air problem.
- Compare supply temperatures. Hold a kitchen thermometer in the airflow at the hot room's register and at a register in a comfortable room. If the hot room's air arrives noticeably warmer, the duct is picking up heat or leaking along the way.
- Find the balancing dampers, if you have them, and note their positions.
- If you can get into the attic safely, look for ducts that are crushed, kinked or disconnected. Step only on framing, not on insulation or drywall.
Fixes, from simple to bigger
Balancing the airflow. Adjusting the branch dampers, or partly closing registers in rooms that are too cold, pushes more air toward the hot room. It's free and worth trying first. Go easy on it, though: closing too many registers raises the pressure in the ducts, which the blower doesn't like and which can make leaky ducts leak more.
A better return path. A jumper duct or transfer grille is often a modest job, and it can make a real difference in a bedroom that's comfortable with the door open and stuffy with it closed.
Sealing and insulating ducts. Sealing joints with mastic or foil tape and insulating attic ducts means more of the cool air you're already paying for actually reaches the room. Replacing a crushed or undersized run is a natural part of the same work.
Attic insulation and air sealing. For rooms under the roof, this can do more than any change to the HVAC system, and it helps in winter too. At the time of writing, Mass Save offers a no-cost home energy assessment and 75 to 100 percent off approved insulation improvements. Towns served by a municipal light plant run their own programs, so check with your utility first.
Window treatments. Closing blinds or cellular shades on west and south windows during the afternoon cuts heat gain, and exterior shading such as awnings works even better, because it stops the sun before it reaches the glass.
Zoning. Motorized dampers and a second thermostat let one system cool the upstairs and downstairs separately. Zoning works best when the ducts are designed for it and the equipment can handle reduced airflow when only one zone is calling, which often means a two-stage or variable-speed system, so it's a design job rather than an add-on.
A ductless unit for that room. When the ducts can't be fixed at a sensible cost, or the room is an attic conversion or addition the system was never designed to serve, a single-zone ductless mini split gives it its own heating and cooling with its own temperature setting. Because most mini splits are heat pumps, it handles winter as well. If you're weighing that against changes to the central system, our comparison of mini splits and central air lays out the tradeoffs.
The same room is often the cold one in winter
The room that's hottest in July is very often the coldest in January. The same thin insulation, leaky knee walls, long duct run or missing return works against it in both seasons. That's why the fixes that address the room itself, such as insulation, air sealing, duct repairs or a dedicated ductless unit, tend to pay off twice. If you're comparing options for a room like this, it's worth weighing them as a year-round problem, not just a summer one.
If the simpler fixes haven't solved it, or the room is a finished attic or addition the ducts were never meant to reach, our mini split installation page explains how OniFix plans a ductless unit for a single room, with the price before any work starts.
Sources: ENERGY STAR, duct sealing; Mass Save, home energy assessments.