Intake and exhaust: why a ridge vent alone does nothing
Attic ventilation is the mechanism behind two expensive problems, and it fails in one predictable way.
Nobody arrives at attic ventilation out of curiosity. People get here because their attic is wet, or because they keep getting ice dams, or because a contractor mentioned it. It is worth understanding because it is the mechanism behind both, and because it is usually the cheapest of the three things to put right.
The principle
An attic should be washed continuously by outside air. That air comes in low, through vents in the soffits under the eaves, travels up the underside of the roof deck, and leaves high, at or near the ridge. It is driven by the simple fact that warm air rises and by wind across the roof.
Two things follow, and they are the whole reason anyone cares.
In winter, that flow keeps the roof deck close to outside temperature. A cold deck does not condense the moisture rising from the house, and a cold deck does not melt the snow lying on it — which is what prevents ice dams forming at the eave.
In summer, it carries away the heat that builds under a roof in the sun, which keeps the attic from becoming an oven, keeps that heat out of the rooms below and stops the shingles being baked from underneath and ageing early.
The failure that matters: exhaust without intake
Here is where most real attics go wrong, and it is worth being precise about it because the failure looks like a solution.
Ventilation is a circuit. Air leaving at the ridge has to be replaced by air entering somewhere, and if the soffit vents are blocked, the attic does not simply ventilate less — it takes its replacement air from the easiest source available. In a typical house, the easiest source is the heated, humid interior below, drawn up through every gap in the ceiling.
So a ridge vent with blocked intake actively pulls warm damp house air into the attic. It makes the moisture problem worse while appearing to address it, and the homeowner is left concluding that ventilation does not work.
The usual cause is mundane: blown insulation pushed right out to the eaves, burying the soffit vents. Baffles — simple channels stapled between the rafters at the eave to hold the airway open above the insulation — exist precisely to stop this, and they are often missing or were never fitted. Other causes: soffit vents painted over, blocked with debris or bird nests, or soffit panels that look vented and are decorative.
This is checkable. From inside the attic, at the eaves, on a bright day: can you see daylight at the soffits? From outside, from the ground: are there vents in the soffit at all?
Balance
The rule of thumb in the trade is that intake and exhaust should be roughly equal, with intake at least matching exhaust — and there are code-derived ratios of vent area to attic floor area that a contractor will work to. The practical version for a homeowner is simpler: if there is more exhaust than intake, the difference gets made up from your house.
Why mixing exhaust types backfires
Several high-level vents in the same attic tend to short-circuit. A ridge vent combined with open gable vents is the common case: the ridge vent takes its air from the gable vent a short distance away rather than pulling it all the way up from the eaves, and the bulk of the attic never gets washed at all. The same happens with a powered fan next to a ridge vent, which can even reverse the flow through the ridge.
The general principle: one exhaust strategy, plenty of intake.
Powered attic fans
The most argued-about item in the subject, so here is the honest version. A powered fan moves a lot of air, and if the intake cannot supply it, it takes the rest from the house — pulling out air you have paid to heat or cool, and dragging indoor humidity into the attic. In a leaky attic a fan can cost more than it saves and make the moisture worse. In a well-sealed attic with generous intake it is not doing much that passive ventilation was not already doing.
That is not an argument that they never help. It is an argument that a fan is not a fix for a ventilation problem caused by blocked intake, which is the reason most people are considering one.
Sealing comes first
The last thing, and the thing most often skipped. Ventilation and air sealing are two halves of one job. Ventilating an attic that has an open pipeline of warm damp air feeding it from below is treating the symptom while the cause runs. Seal the ceiling plane first — the hatch, the light fittings, the pipe and wire penetrations, the tops of partition walls — and then make sure the attic can clear what still arrives.
Do it in that order and the two problems this page exists for tend to go away together.
Frequently asked questions
I have a ridge vent, so why is my attic still damp?
Almost certainly because there is no intake to feed it. Air leaving at the ridge has to be replaced, and if the soffit vents are blocked it gets replaced with warm humid air pulled up from the house, which makes the moisture problem worse rather than better.
What blocks soffit vents?
Most often blown insulation pushed out to the eaves and burying them. Baffles are meant to hold that channel open and are frequently missing. Vents painted over, filled with debris or blocked by nests are the other common causes, along with decorative soffit panels that are not actually vented.
Can I have a ridge vent and gable vents together?
Generally it is a bad combination. The ridge vent draws from the nearby gable vent instead of pulling air all the way from the eaves, so most of the attic never gets ventilated. One exhaust strategy with plenty of intake works better than two competing ones.
Are powered attic fans worth it?
A fan cannot fix a ventilation problem caused by blocked intake, which is why most people consider one. If the intake cannot supply it, it takes the difference from the house, pulling out conditioned air and dragging indoor humidity into the attic.
What is the single most useful check?
Go into the attic at the eaves on a bright day and look for daylight at the soffits. If you cannot see it, the intake is blocked, and that one finding explains a large share of attic moisture and ice dam problems.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- US EPA. Improving Indoor Air Quality
- US EPA. Introduction to Indoor Air Quality