Soffit and ridge vents

Intake is the half most attics are short of, and it is the half that decides whether the rest works. Every product below does the same thing. It lets outside air into the bottom of the roof, at the eave, so the air can wash up the rafter bays and leave at the top.

Where the figures come from. Each was read off the maker's own product page on 11 August 2026 and is quoted, not averaged. The SmartVent figure is DCI's own wording: "rated at 9 nfa/ft". Two rows carry no number because we did not read one. Round plug vents and drip-edge vents do publish one. Yours is on the package or the maker's spec sheet, and that is the number to use.

The case that catches people out is a house with no soffit. Some roofs have almost no overhang, and some have a soffit that was never cut open. There is nowhere to put a soffit vent. The retrofit is a vent at the roof edge instead: a strip in the deck under the first course of shingles, an over-fascia vent above the fascia board, or a vented drip edge. DCI states its slit product is for exactly this, and gives 9 square inches a foot. All three are roofing work, not attic work, so they are usually done when the roof is next redone.

Perforated soffit panel can be decoration. Vinyl or aluminum soffit with little holes in it only vents if the framing behind it is open. Fitted over solid blocking, or over a soffit nobody ever cut, it looks like intake from the ground and supplies none. Check it from inside the attic on a bright day. The attic inspection guide covers what that looks like from the hatch.

Exhaust is the half people buy first. It sits high, because warm air rises and because wind across a roof pulls at the peak. Ridge products are sold by the foot. Box vents and gable louvers are sold by the unit.

The four ridge figures are one maker's line, read 2026-08-11. They are here to show the spread, which is the useful part: 20 square inches a foot against 8.5 is more than a two to one difference between products that look alike on a roof. Other makers publish their own numbers. Use the ones for the product you are actually buying.

Powered fans are a different argument, and this site makes it elsewhere. A fan is rated in airflow, so you cannot add it to a net free area total. It also pulls harder than the intake can supply in most retrofits, which is the whole problem on this page with a motor attached. The attic fans page works that case through, including the narrow situations where a fan is defensible.

A vent is not an open hole. It has an insect screen, louver blades angled to shed rain, a rain baffle on a ridge product, and a frame. Each of those takes area out of the air path. Net free area is what is left. It is the only figure worth adding up.

The gap is bigger than most people expect. A 16 by 8 inch undereave vent is 128 square inches of opening. A 16 by 4 is 64. GAF publishes 26 to 50 square inches of net free area across that pair. So even at the top of the published range, well under half the vent is air path.

There is no labeling rule behind this number. R-value has one: the FTC R-value Rule (16 CFR Part 460) governs how insulation R-values are advertised, and the test methods behind a label R are ASTM C518 and ASTM C177. Net free area has no equivalent consumer rule. It is the maker's own published figure. That is not a reason to ignore it. It is a reason to compare products only where each one publishes a number, and to treat a seller who will not give you one as having answered the question.

How to find the figure for the vents you have

1. Name the product. Look for a stamp, a molded part number or a label on the vent itself. Photograph it from a ladder rather than guessing from the ground.
2. Search the maker's site for that part number plus "net free area" or "NFA". It is normally in a spec table with the size.
3. Check the unit. Ridge and strip products publish per linear foot. Rectangular and round vents publish per piece. Some publish per section, like 68 square inches per 8 foot length, which is 8.5 a foot.
4. Total intake and exhaust separately. Two totals, never one. A single combined number hides the imbalance this whole page is about.
5. Take a discount off old vents. A published figure describes the product when it left the factory. Paint, dust, cobwebs and a finer screen retrofitted later all cut it. Nobody publishes how much. Clean them and look.

Start with the total. The Model International Residential Code, Section R806 (roof ventilation), 2021 edition sets net free ventilating area at 1 to 150 of the vented space in section R806.2, and allows 1 to 300 only when both of its conditions are met. On 1,200 square feet that base ratio is 1,152 square inches.

These come from the model International Residential Code. Your state, and often your city, decides which edition applies where you live and whether it was changed. Your local building department is the only authority on that. And these rules cover new building and permitted work, not a top-up you choose to do on an attic you already have.

Then split it. Roughly half low and half high is what a balanced design is normally taken to mean in the trade. That split is convention, not a code provision, and this site records it as an estimate. The model code sets a placement split only for the reduced ratio, where upper vents take 40 to 50 percent and sit no more than 3 feet below the ridge. Half of 1,152 is 576 square inches of intake, and the same again of exhaust. Divide that by each product's published figure and you get the run you need.

Method. Attic floor area divided by 150, times 144 to get square inches, halved by the trade convention above, then divided by the manufacturer's published net free area per foot. The page computes it at build time from the recorded code ratio and the quoted product figures, so no number here is retyped. The rectangular vent is counted in pieces instead: at 50 square inches each you need 12, and at 26 you need 23.

Two things that table tells you straight away. Ridge length is often the limit, because a roof only has so much peak. And intake at 9 or 10 square inches a foot needs a long run, so a handful of small soffit vents rarely gets close.

This is the argument worth reading twice, because it is the way most money on this page gets wasted.

Exhaust cannot create air. Every cubic foot that leaves the ridge has to be replaced by a cubic foot coming in. If the soffits can supply it, outside air washes along the underside of the roof deck, which is the path the design intends. If they cannot, the air still comes. It comes from wherever the resistance is lowest.

In most houses that is the ceiling below. Gaps at recessed lights, the plumbing stack, wire holes, the top plates, the chimney chase and the attic hatch are all easier openings than a soffit packed with insulation. So the ridge vent draws air out of your house. Three things follow, and all three are bad:

• You pay for the air that leaves. Heated or cooled indoor air goes out the roof.
• Household moisture is delivered to the cold deck. That is the exact thing the vents were installed to prevent. In winter it shows up as frost on nails, then staining, then mold. See attic frost.
• The house is pulled slightly negative. Where a natural draft furnace or water heater shares that air, that becomes a combustion safety question rather than a comfort one.

The fix is never more exhaust. More exhaust deepens the imbalance. The fix is at the bottom of the roof: open the soffits, clear the insulation out of the eave, fit baffles, and add intake area until the two totals match.

• A ridge vent means cutting the roof deck. A slot is sawn along the peak, through the sheathing, and it has to miss the truss or rafter tops and be closed the same day. That is roofing work. It is the part of this page most worth handing to somebody who does it for a living.
• Soffit work is ladder work. Falls are how this job hurts people. OSHA's ladder and fall protection rules cover the trade for that reason, and the physics does not change because you are a homeowner.
• Check the combustion appliances. If the house has a natural draft furnace or water heater, changing attic and ceiling airflow can affect how it vents. The U.S. Consumer Product Safety Commission publishes guidance on carbon monoxide alarms. Have working alarms before you change anything.
• Never vent a bath fan or dryer into the attic. No amount of soffit or ridge vent keeps up with a running moisture source. Duct it to the outside.

Air takes the easiest path. That is the whole reason mixing exhaust types is a problem rather than a bonus.

Put a gable louver a few feet from a ridge vent and the louver often becomes the ridge vent's intake. The air then loops across the top of the attic, between two openings that are close together, while the rafter bays down at the eaves stay still. The deck over those still bays is the part that gets damp. The soffits are contributing far less than their labeled area suggests, and nothing outside the house looks wrong.

A powered fan does the same thing harder. A ridge vent near a running fan can become the fan's air supply, so outside air comes in the ridge and goes straight back out again. Box vents low on a slope, mixed with a ridge vent above them, can behave the same way.

The rule to work to: one exhaust type, generous intake. If you have a ridge vent, that is your exhaust. Closing the gable openings resolves it. So does leaving them and confirming the soffits are clear and generous, which is the harder thing to verify. Adding a third exhaust type never resolves it.

The eave is the one place where insulation and venting have to share a few inches. The attic floor runs out to the top of the outside wall. The roof deck comes down to meet it. Intake air enters right there, in the wedge between the two.

Section R806.3 of the model IRC addresses it: where eave or cornice vents are installed, a clear air space of not less than 1 inch is to be kept between the insulation and the roof sheathing at the vent. That inch is the whole intake path.

It gets lost four ways. Loose fill blown into the soffit bays on installation day. Batts crushed into the wedge by somebody reaching the last few feet. Wind pushing material inward over years. And a standard truss that simply does not leave room for full depth and an open channel at the wall. A baffle is what holds that space open. The baffles page covers all four, plus how tall a baffle has to stand and the raised-heel truss that answers the last one.

The timing matters more than the cost. Baffles are cheap and take minutes each while the attic is open. Adding them later means moving the insulation you just paid for. A quote that leaves them out is not saving you much, and it is setting up the failure at the top of this page. If you are adding depth, the add-over guide puts the eave check before the blowing.

Inventory the openings from outside first. Sort them into intake at the eaves and exhaust up high. Then look along the eaves from the attic hatch on a bright day, because a soffit vent that is buried is not intake whatever the outside count said. Then use this table.

Two findings are worth a contractor in the attic rather than a look from the hatch. One is staining or growth across a wide area of the roof deck. The other is an eave you cannot clear from inside because the insulation is packed hard into it. Both usually get rolled into a top-up, so the material only gets moved once.

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What does it cost?

No price for vent work is published here, because none has been measured. This site prices one job: blowing fiberglass onto an attic floor. That runs about $2,600 to $3,200 for a 1,200 square foot attic at R-49. The contractor prices the rest once they have seen it. See how that figure was built.

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We do not sell insulation and we do not install it, which is why this section can tell you to keep your money. Plenty of attics that look short of vents are not.

• The attic is not under-vented. It is leaking warm indoor air. Frost, damp sheathing and a stuffy attic all look like a ventilation problem and are usually a ceiling problem. Air carrying household moisture goes up through gaps and condenses on the cold deck. Vents treat the symptom, and they treat it badly, because a running leak outpaces them. Seal first. Start at air sealing and attic frost, which name the leaks worth finding.
• The totals already meet the ratio and the deck is clean. Ventilation has a target, not a dial. Dry sheathing, no rust on the nail tips, clear soffits, done. More openings buy nothing.
• The complaint is a hot upstairs. That is nearly always depth and air sealing at the ceiling, not vents. If you are within about R-10 of the level recommended for your climate zone, sealing beats both. That R-10 threshold is this site's own estimate, not a measured figure. Measure what you have with the depth checker first.
• The moisture has a source you can just remove. A bath fan ending in the attic is a cheap fix and it is the actual problem. So is an open chase or an unsealed hatch. Fix those and the vent question often goes away.

• Attic ventilation: how much you need, and why
• Baffles, which keep the intake open
• Powered attic fans, and the case against most of them
• Ice dams, where a cold deck matters most
• Mold in attic insulation