Heat in the attic: why the upstairs is ten degrees hotter

Climb into an attic on a July afternoon and you will understand the second floor. Under a dark roof in full sun the air up there runs 130 to 150 degrees, and that heat has two ways into the house: through the ceiling, and through the ductwork that in much of the country runs right through the middle of it. A bigger air conditioner does not fix either one. It just fights harder.

Where the heat comes from

The roof absorbs the sun and radiates it down into the attic. With too little ventilation the hot air has nowhere to go and sits there, heating the top of the insulation. Where the insulation is thin, or where it has been pushed aside by wiring and storage, the heat comes through the ceiling into the rooms below.

Then the ducts. A supply duct in a 140-degree attic, carrying 55-degree air, loses a lot of that cold on the way to the register, and if the duct leaks, which most do, it loses the air itself. Leaky attic ducts are the most common reason a house cannot cool the upstairs, and one of the cheapest to fix.

The fixes, in the order they pay

• Seal the ducts. A Pro tests them with a blower and seals the joints with mastic, not the cloth tape that was misnamed duct tape and fails in a year. Ten to thirty percent of the cooled air can be going into the attic; getting it back costs a day.
• Insulate the ducts. Any duct in an unconditioned attic should wear a thick sleeve of insulation, and most old ones wear a thin one.
• Air seal the ceiling. The same gaps that let heat into the attic in winter let it out of the attic in summer: the hatch, the recessed lights, the top plates, the plumbing and wiring holes.
• Add insulation. Most older attics carry half or less of what the current code calls for. Blown insulation over a sealed ceiling is the most cost-effective comfort upgrade in the house, summer and winter, and it is what the tax credit is for.
• Ventilate. Continuous soffit vents at the eaves and a ridge vent along the peak let the attic breathe. Powered attic fans are controversial: they move a lot of air, but if the ceiling is not sealed they pull cooled air out of the house, and they can backdraft a water heater. Do the sealing first.
• A radiant barrier, a foil layer stapled under the rafters, reflects the roof's heat back up and can drop attic temperatures substantially in hot, sunny climates. Worth it in the South and Southwest, less so in the North.
• A lighter roof when it is time to replace. A light-colored or reflective shingle or a metal roof runs far cooler than a black one.

The upstairs thermostat

If the house has one system and one thermostat downstairs, the upstairs will always lag. A zoning damper system, or a separate ductless heat pump for the upstairs, solves it properly. Closing the downstairs registers to push air up is the folk fix, and it raises the pressure on the system enough to shorten its life. The right order is sealing and insulation first, then equipment if it is still not enough.

Bath fans and the attic

Look for bath fans that vent into the attic instead of through the roof. They put a shower's worth of moisture into the insulation every morning. Any fan should have a duct to an outside cap, and a roofer or an HVAC Pro adds one in an hour.

An energy auditor with an infrared camera, or a good insulation Pro, can show you where the heat is coming through in a single visit. It is a better use of a service call than a bigger air conditioner.