Somebody notices the attic runs hot, or the shingles are curling early, or there were ice dams last winter. The advice is always the same. Add more vents.
So a ridge vent goes on. Or a couple of turbines. Or a powered fan. And a surprising share of the time nothing improves, and once in a while it gets measurably worse.
Attic ventilation isn't a volume problem, it's a circuit. Air has to come in low, travel up under the deck, and leave high. Break that path and it doesn't matter how many holes you cut.
The math nobody shows you
Building code, meaning the International Residential Code that Ohio's residential code follows, sets the minimum at 1 square foot of net free ventilating area for every 150 square feet of attic floor. That relaxes to 1:300 when the ventilation is split roughly evenly between low intake and high exhaust, or when there's a proper vapor retarder. Most modern roofs get designed to 1:300.
Two things about that number trip people up.
Net free area isn't the size of the hole. A vent's NFA is the open area left after you subtract the screen, the louvers and the baffle. A soffit vent measuring 16 by 8 inches has 128 square inches of face and maybe 50 square inches of NFA. Manufacturers publish the figure, so use theirs instead of a tape measure.
And half of it has to be intake. This is the part that gets skipped. The code language calls for the ventilation to be split, with the lower portion at the eaves or soffits. Exhaust without matching intake doesn't move more air, it just pulls harder on whatever openings it can find.
Put it on a real house. A 1,600 square foot attic at 1:300 needs about 5.3 square feet of NFA, call it 768 square inches. Split evenly that's 384 square inches of intake at the eaves and 384 of exhaust at the ridge. If the soffits are painted shut or packed with insulation, which we find constantly on homes built before the 1980s, you have zero intake and the ridge vent is doing almost nothing.
Where the air comes from when intake is blocked
This is the failure that makes things worse rather than just useless.
If a ridge vent or a powered fan can't pull air from the soffits, it pulls it from somewhere else, and that somewhere else is your house. Conditioned air comes up through recessed lights, bath fan housings, the attic hatch, and the gaps around plumbing stacks and wiring. In summer you're paying to air-condition the attic. In winter you're dragging warm humid indoor air into a cold cavity where it condenses on the underside of the deck and drips back into the insulation.
We have opened up attics in January where the sheathing was black with mold and there was frost standing on the nails, on houses where somebody had fixed the ventilation the previous fall. Powered attic fans are the worst offenders, because they can move enough air to depressurize a house and in the wrong situation backdraft a water heater or a wood stove. We don't install them.
Don't mix exhaust types
A ridge vent and box vents on the same roof plane don't add up. They compete.
A ridge vent works on a pressure difference along the peak. Put box vents or a turbine partway down the slope and the ridge vent will happily pull air in through the nearest one, because that's a short path with far less resistance than the long haul up from the soffits. That's short-circuiting. The stretch of attic between the box vent and the ridge gets a busy little loop of air going nowhere, and everything below it gets nothing at all.
Same story with a gable vent left open alongside a new ridge vent. The gable becomes the intake, the far corners of the attic go stagnant, and the soffits sit unused.
Pick one exhaust strategy per roof plane and commit to it. When we install a ridge vent we close off the old box vents and usually block the gables, which looks like we're removing ventilation. We're restoring the circuit.
What to check on your own house
Twenty minutes gets you most of this.
Go outside and look up under the eaves. Are there soffit vents at all, continuous perforated soffit or individual rectangles? Solid soffit with no openings means no intake, full stop.
Look at the paint while you're down there. Older homes get repainted and soffit vents get rolled right over. Hold a light close and see whether you can see through.
Then go in the attic and look at the eaves from the inside, which is the one that surprises people. Insulation blown in during an energy upgrade very often gets pushed straight into the eave cavity, sealing the soffit vents from behind. The fix is baffles, rigid channels that hold the insulation back and keep an air path open from the soffit up over the top plate. They're cheap and they get skipped constantly.
While you're up there, look at the underside of the sheathing. Dark staining, rust rings around the nail shanks, or any frost in winter all mean moisture is getting in and not leaving.
And count your exhaust types. More than one kind on the same plane is a red flag.
Why this matters more here
Our climate does both of the things that punish bad ventilation hardest. Summers are humid enough that an unventilated attic runs well over 140 degrees and cooks the asphalt out of shingles from beneath, which is a big part of why a 30-year shingle sometimes doesn't make twenty. And winters run through dozens of freeze-thaw cycles, where a warm attic melts snow on the deck that refreezes at the cold eave. That's an ice dam, and it's a ventilation and insulation problem wearing a roofing costume.
When to fix it
A roof replacement is the moment. The deck is off, the soffits are reachable, and adding baffles and cutting a proper ridge slot costs a fraction of what the same work costs later as a standalone project. We check attic ventilation on every roof we inspect, because we'd rather tell you the ventilation is the problem than sell you shingles that will fail early for the same reason the last set did.
If your attic runs hot, the upstairs won't cool, or you had ice at the eaves last winter, have us take a look. Half the time the answer isn't a new roof.
