Why Intake and Exhaust Must Be Balanced — and What Happens When They Are Not
An attic with more exhaust than intake does not simply underperform. It goes negative, and the make-up air comes from the heated house below — pulling moisture up with it all winter.
Balance is the part of attic ventilation that gets skipped. Exhaust vents are visible from the street and easy to add; intake is hidden under the eaves and nobody looks. The result is a very common Ontario attic: plenty of vents on the roof, almost nothing open at the soffit, and a moisture problem that keeps coming back no matter how many more vents get cut in.
What Balanced Actually Means
Balanced ventilation means the net free area at the intake openings is roughly equal to the net free area at the exhaust openings — a fifty-fifty split of the total, with the intake half at or near the eave and the exhaust half at or near the ridge. Where designers deviate, they deviate toward more intake, not less. A slight surplus of intake keeps the attic neutral or very slightly pressurized relative to the house, which is the safe direction to be wrong in.
Air Has to Come From Somewhere
This is the whole argument in one sentence. If a ridge vent and a row of box vents can move a certain volume of air out of an attic, that same volume has to enter the attic. If the soffits cannot supply it, the attic goes slightly negative relative to its surroundings and the air is drawn through whatever openings remain. On a typical house, the largest available set of openings is the ceiling plane — and on the other side of the ceiling plane is warm, humidified indoor air.
Where the Make-Up Air Comes From
The ceiling of a house is not a solid surface. It is a plane full of penetrations, most of which were never sealed, and a depressurized attic finds every one of them.
- ✓Recessed light housings, which are essentially open cans through the ceiling unless they are gasketed and sealed.
- ✓The attic hatch or scuttle, often an uninsulated panel resting loose in a frame.
- ✓Interior wall top plates, where the drywall meets framing and the wall cavity opens directly into the attic.
- ✓Plumbing vent stacks, wiring runs, and the chases around them.
- ✓Bathroom fan housings, which leak around the body even when the duct itself is run correctly.
- ✓Dropped ceilings, soffits over kitchen cabinets, and bulkheads that are open to the attic at the top.
- ✓Chimney and flue chases, which are often the single biggest opening in a house.
What That Costs You in January
The air being pulled up through those openings is heated air that you paid to heat, and it is carrying the humidity of the house with it. When it reaches sheathing that is at or below outdoor temperature, the vapour it carries condenses — and below freezing it does not condense as water, it deposits as frost on the underside of the deck and on every nail point. That frost sits there until the first mild day, then melts all at once and drips onto the insulation below. Homeowners describe it as a roof leak that only happens after a thaw. It is not a leak. It is the attic returning the moisture it collected all winter.
The Secondary Damage
Wet insulation loses much of its thermal performance while it is damp and can stay compressed after it dries. Repeated wetting of sheathing supports mould growth on the wood and can eventually delaminate plywood or swell OSB at the edges. Nails corrode. And because a depressurized attic is continuously drawing heat out of the house, the roof deck runs warmer than it should, which is exactly the condition that starts an ice dam at the eave.
Short-Circuiting: The Other Way Balance Fails
Even with adequate total intake, mixing exhaust types on one attic can defeat the system. If a house has both gable vents and a new ridge vent, the ridge will draw the easiest air available, and the gable openings are far easier to pull from than a soffit slot at the far end of the attic. The result is a short circuit: air enters the gable, travels a short distance, and exits the ridge, while the eave ends of the attic stay stagnant. The same thing happens when box vents and a ridge vent share one roof plane — the upper vents feed the ridge and the lower part of the attic never moves. Choose one exhaust strategy per attic and commit to it.
The failure mode is worth stating plainly: adding roof vents to an attic that is short on intake increases the amount of warm, humid indoor air pulled into that attic. More exhaust makes an unbalanced attic wetter, not drier.
The Opposite Imbalance
More intake than exhaust is the far less damaging error. The attic ends up neutral or slightly positive relative to the house, so it is not actively pulling conditioned air upward. Airflow is lower than ideal and moisture removal is slower, but the mechanism that drives the worst winter failures is not running. This asymmetry is why the standard advice is to err on the side of intake.
How to Assess Balance on an Existing House
Start with the published net free area of the exhaust you have — ridge vent NFA per linear measure times the vented length, plus the rated area of each box or gable vent. Then do the same for intake, using the manufacturer's figure for the soffit product rather than the visible perforation. Then go outside on a cold day and look up into the eaves with a light. If you cannot see daylight or open baffle above the soffit, the rated area of that product is irrelevant, because the path is blocked. On many houses the intake number ends up being a fraction of the exhaust number, and correcting that at the eave does more than anything else on the roof.
Frequently Asked Questions
How do I know if my attic is depressurized?
The evidence is usually visible rather than measurable. Frost or dark staining on the underside of the sheathing in winter, rusty nail tips, matted or discoloured insulation directly under vents, and ceiling stains that appear after a thaw rather than after rain all point to warm indoor air being pulled into a cold attic.
Should I close my gable vents when a ridge vent is installed?
Generally yes, and this is standard practice. Leaving them open lets the ridge draw from the gables instead of from the soffits, so the eave ends of the attic get very little airflow. The exception is a house where the soffit intake genuinely cannot be opened, in which case the gable vents may be the only intake available and the strategy has to be reconsidered as a whole.
Can I just add more soffit vents instead of sealing the ceiling?
Adding intake helps and is usually necessary, but sealing the ceiling plane is what actually stops moisture from entering the attic. Ventilation removes moisture that gets in; air sealing keeps it out in the first place. Doing only the first is treating the symptom.
Does balance still matter if I am installing a metal roof?
Yes, and arguably more, because a metal roof is a long-term assembly and you want the deck under it staying dry for decades. The panel does not change the physics of the attic. What it changes is your access — the ridge and eave details are far easier to build correctly during the re-roof than to retrofit afterward.
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