Why your upstairs is 95° — and what your attic has to do with it
If you've got a two-story house in central Oklahoma and the upstairs runs 10 degrees hotter than the downstairs in July, the AC is rarely the actual problem. The problem is what's sitting on top of the AC: an attic that hits 140 to 160 degrees by mid-afternoon and stays there until well past sundown.
Heat doesn't politely stop at the attic floor. It drives through the insulation into the bedroom ceilings below, and your second-stage compressor runs all evening trying to undo what the sun did to the roof.
What's supposed to happen
A working attic exchanges air constantly. Cool intake air comes in low (soffit vents along the eaves), heated air leaves high (ridge vent or gable vents). The minimum target is 1 square foot of net free vent area for every 150 square feet of attic floor, split roughly evenly between intake and exhaust.
That's the building code minimum. Oklahoma sun earns you more than the minimum.
What's actually happening on most houses we inspect
- Soffit vents painted shut from the last exterior paint job. We see this constantly.
- Soffit vents blocked from above by insulation that got pushed into the eaves.
- A ridge vent that was installed but never had the underlying decking cut open. The vent is decorative.
- Gable vents that "balance" against each other instead of with the soffits, creating a short-circuit that never reaches the hot air over the bedrooms.
- A powered attic fan that pulls conditioned air out of the house through ceiling gaps because there isn't enough intake to feed it.
Any one of these turns a ventilation system into a closed box with a heat source.
What to check yourself
Stand under the eave on a hot afternoon and put your hand near a soffit vent. You should feel a steady draw of air going in. If the air is still, your intake is blocked or your exhaust isn't pulling.
Then go in the attic (early morning, never midday) and look at the soffit area from the inside. You should see daylight at the eaves between the rafters. If you see insulation packed against the roof deck, the air can't get in.
What we do about it
Re-cutting a ridge vent, installing baffles to keep insulation out of the soffits, and replacing painted-shut soffit panels with new vented soffit is usually a one-day job. It does not require tearing off the roof. On most houses we've corrected, the upstairs runs 6 to 9 degrees cooler within a week and the electric bill drops noticeably in August.
If your upstairs has been miserable every summer and you've already replaced the AC, get someone to look at the attic before you spend money on the unit again.