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Attic ventilation calculator

Enter the attic floor area and pick the code ratio. Add the net free area printed on your vent datasheet to turn the total into a count of vents.

1/150 of the attic floor IRC R806.2 sets net free vent area at 1/150 of the vented space, or 1/300 when both conditions of its exception are met. Confirm the edition your jurisdiction adopted.

The model code asks for 1 square foot of net free vent area per 150 square feet of attic floor. A 1,200 sq ft attic needs 1,152 sq in, split between intake at the eaves and exhaust near the ridge.

Divide each half by the net free area on the vent’s datasheet to get a count.

Net free area, then product counts

Type the datasheet figures to count vents. Nothing is stored or emailed.

An example to start. The floor of the vented space, not the roof area.

From the product datasheet, not the opening size.

For ridge vent, the figure per linear foot gives feet of vent.

— Total net free area
— Upper portion (50%)
— Eave intake (50%)
— Exhaust units (or feet of ridge vent)
— Intake vents

What this assumed —

Arithmetic on R806.2, not a design. Confirm your adopted code and the datasheet. HyreRoof does not install ventilation.

The rule, and the exception that halves it

The model code’s baseline is 1 square foot of net free vent area for every 150 square feet of vented attic floor.

It halves to 1/300 only when both conditions of the exception are met. In Climate Zones 6, 7 and 8 there must be a vapor retarder on the warm side of the ceiling. And 40 to 50 percent of the vent area must be high in the attic, with the rest as intake low down.

HyreRoof analysis: if a bid assumes 1/300 on a house where the exception does not apply, the attic has half the vent area the code asks for. The calculator runs both ratios so you can see the gap.

Net free area is not the size of the hole

Vent typeWhere its net free area comes fromWhat can reduce it
Continuous ridge ventDatasheet, per linear footThe width of the slot cut in the deck under it
Continuous soffit ventDatasheet, per linear footInsulation packed into the eave behind it
Single soffit ventDatasheet, per ventLayers of paint closing the perforations
Box or louver ventDatasheet, per unitScreens and louver shape
Gable ventDatasheet, per unitPaired with a ridge vent it can short-circuit the airflow
TurbineDatasheet, per unitIt relies on wind to turn

Net free area is the open area after screens, louvers and baffles. We publish no typical figures: use the one on the product you are buying.

Why it is worth getting right

Attic ventilation touches the shingle warranty and how moisture behaves in the attic in winter.

  • It can be a warranty condition

    Some shingle warranties set ventilation requirements. Read yours before the roof goes on, not after a claim.

  • Winter moisture is the quiet failure

    Warm, moist air reaching a cold deck condenses on the sheathing. It can look like a leak and not be one.

  • Ice dams start with heat

    Heat escaping into the attic melts snow that refreezes at the cold eave. Air sealing and ventilation work on the cause; heat cable works on the symptom.

  • Unvented attics are a different design

    An attic insulated at the roof deck is a legitimate approach with its own code rules. This calculator does not apply to it.

The 1/300 exception, quoted in full

The exception that halves the requirement has several parts. Short summaries can mention only one.

The baseline. IRC 2021 R806.2, Minimum vent area: “The minimum net free ventilating area shall be 1/150 of the area of the vented space.”

The exception. “The minimum net free ventilation area shall be 1/300 of the vented space provided both of the following conditions are met:

“1. In Climate Zones 6, 7 and 8, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling.

“2. Not less than 40 percent and not more than 50 percent of the required ventilating area is provided by ventilators located in the upper portion of the attic or rafter space.

“Upper ventilators shall be located not more than 3 feet (914 mm) below the ridge or highest point of the space, measured vertically. The balance of the required ventilation provided shall be located in the bottom one-third of the attic space.”

Read the exception closely: it says “both of the following conditions.” The vapor-retarder condition applies in Climate Zones 6, 7 and 8. The 40 to 50 percent upper-vent condition applies everywhere. The three-foot rule and the bottom-third rule are part of the exception too, and short summaries of it can leave them out.

The framing relief. “Where the location of wall or roof framing members conflicts with the installation of upper ventilators, installation more than 3 feet (914 mm) below the ridge or highest point of the space shall be permitted.”

It allows lower upper vents where framing is in the way. It does not waive the rest of the exception.

Why it matters. If the exception does not apply, the requirement doubles. A 1,600 sq ft attic needs 768 sq in of net free area at 1/300 and 1,536 sq in at 1/150.

This is model code. Adoption and amendment are local, and your jurisdiction may have adopted another edition. Confirm with your building department before relying on it.

Why more exhaust on a blocked roof can make things worse

Ventilation is a circuit. Exhaust is the half you can see from the street, so it is the half that gets added. Intake sits at the eave, the same edge the gutter calculator sizes.

The code line on insulation

IRC 2021 R806.3, Vent and insulation clearance: “Not less than a 1-inch (25 mm) space shall be provided between the insulation and the roof sheathing and at the location of the vent.” And: “Blocking, bridging and insulation shall not block the free flow of air.”

HyreRoof analysis: this line can be broken by accident. Blown insulation goes down across the attic floor, runs out to the eave where the floor ends, and buries the soffit intake. Nothing looks wrong from the ground or the attic hatch. The vents are all still there. They just no longer connect to anything.

What happens when intake is shut

A ridge vent does not push air. It gives air a way out. If no outside air can come in at the eave, the attic draws replacement air from the house below.

That air comes through the attic hatch, around recessed lights and up plumbing chases. In winter it carries indoor moisture onto a cold deck, where it condenses. The damp sheathing can look like a roof leak.

A powered fan on that attic pulls harder, so it moves more house air, not more outside air.

Why the fault is hard to see

From outside, the soffit vents all look open and the ridge vent is new. The fault only shows from inside the attic, at the eave, looking for daylight above the wall plate.

A reroof replaces exhaust. The intake is not on the roof, so a new roof leaves it as it was.

Symptoms that look like something else

Ventilation faults show up as other problems. Here is what to ask about before paying to fix the wrong one.

What you seeWhat it gets calledWhat it can be
Dark staining or frost on the underside of the sheathing in winterA roof leakIndoor moisture condensing on a cold deck. A leak tends to follow a path from a penetration; condensation spreads across bays.
Ice at the eave and in the gutter each winterA gutter problemHeat escaping into the attic, melting snow higher up, refreezing at the cold eave. Air sealing and ventilation work on the cause. Whether an ice barrier is required is a separate question, covered in the ice barrier study linked below.
Curled or brittle shingles on the sunniest slopeA bad batch of shinglesHeat from below. A poorly ventilated attic can keep the underside of the covering hot for longer each day.
A powered attic fan that runs all the time and changes nothingA fan that is too smallToo little intake. The fan may be moving house air, and a bigger fan moves more of it.

HyreRoof analysis. We hold no dataset of attic inspections and do not inspect roofs. Use this to ask better questions of someone standing in the building.

How to check your intake without going on the roof

Every step here is done from the ground or inside the house.

  1. 1
    Count what is there from the ground

    Walk the perimeter and photograph the soffit: continuous strip vent, single vents, or none. Note the length of soffit that actually has vents. A porch, garage or extension can have long unvented stretches.

  2. 2
    Look at the vents, not past them

    Paint can close perforated aluminum vents. A zoomed phone photo from below shows whether the holes are still holes.

  3. 3
    Go into the attic on a bright day

    Turn off your flashlight, let your eyes adjust, and look toward the eaves. Open bays show a thin line of daylight at the outer edge. Count the bays with none.

  4. 4
    Stay on the walkway

    If there is no floor out to the eave, do not crawl out. Photograph along the bays from where you can stand safely.

  5. 5
    Check for baffles

    A baffle is a rigid chute between the rafters at the eave. It holds insulation back and keeps the one-inch airspace R806.3 requires. Blown insulation added without baffles can bury the intake.

  6. 6
    Follow the bath and kitchen fan ducts

    Each should leave the building through a roof jack, wall or soffit fitting. A duct that ends in the insulation puts shower steam into the attic, and venting will not fix that.

  7. 7
    Never finish the job from the roof

    OSHA rules bind employers, not homeowners, but the line still applies. A trained roofer may not work above six feet without fall protection. Look from the ground, a window or the attic, or use a photo or a drone.

If your attic has no safe access, stop and hire an inspection. HyreRoof does not inspect and takes no fee from anyone who does.

What a good ventilation line on a bid says

On some roofing bids ventilation is one word. Ask for these instead.

  • The ratio it was designed to, and why

    1/150 or 1/300, named, with the reason the exception does or does not apply to your house.

  • Net free area, not product length

    Forty feet of ridge vent is not a ventilation figure. Forty feet at the datasheet’s net free area per foot is. Ask for the datasheet and run the numbers above.

  • A line for intake

    If the bid adds a ridge vent and says nothing about the soffit, ask what the bidder found at the eaves.

  • Baffles where they are missing

    A bid that prices baffles per bay has been in the attic.

  • A powered fan as the fix

    On an intake-starved attic a fan makes the fault stronger. The intake numbers should come before any fan.

  • Gable vents left open next to a new ridge vent

    The ridge can draw from the nearest opening. If that is a gable louver, the air may never cross the attic floor. Ask how the bidder will handle the gables.

  • A promised energy saving

    Ventilation is a moisture and durability measure. A specific percentage off your cooling bill is a number nobody can support for your house without measuring it.

What this calculator cannot do

The ventilation calculator sizes vents from the code ratio and the area you enter. These are its limits.

  • It does not apply to unvented attics

    An attic insulated at the roof deck is a different design under different code sections.

  • It does not know your attic floor area

    The vented space is not the house footprint. Kneewalls, cathedral sections and rooms in the roof all change it.

  • It does not check that your intake is open

    It sizes what should be there. The steps above check what is.

  • It does not know your adopted code

    Everything here is model text. Your jurisdiction may have adopted another edition or amended the exception.

  • It is not an air-sealing plan

    Ventilation works above a tight ceiling. If the ceiling leaks, seal it first.

Ventilation terms

Net free ventilating area
The open area left after screens, louvers and baffles. A published property of the product, and the unit the code sizes in.
Vented space
The attic or rafter space the ratio applies to. Each separate attic is sized on its own.
Intake
Low vents, at the soffit or in the bottom third of the space.
Upper portion
The code’s term for the exhaust side: 40 to 50 percent of the required area, within three feet of the ridge where the 1/300 exception is used.
Baffle
A chute between rafters at the eave that holds insulation back and keeps the airspace open.
Short circuit
Air that leaves through a high opening without entering at a low one, such as a gable vent feeding a ridge vent.
Vapor retarder
A layer that slows moisture through the ceiling. In Climate Zones 6, 7 and 8 it is one of the two conditions of the 1/300 exception.

Embed this calculator

You can put this attic ventilation calculator on your own site, free. Paste the code below into your page’s HTML. Keep the credit link under it. The tool runs in the reader’s browser and stores nothing.

Questions this calculator answers

How much attic ventilation do I need?
IRC R806.2 starts at 1/150 of the attic floor, as net free area. A 1,200 sq ft attic needs 1,152 sq in. The 1/300 exception applies only when both of its conditions are met. Confirm your adopted code.
What is net free area?
The open area left after screens, louvers and baffles. It is printed on the vent’s datasheet and is smaller than the opening the vent sits in.
Why does the calculator split it 50/50?
Under the 1/300 exception, 40 to 50 percent must be in the upper portion. A 50/50 split sits at the top of that band, and it is the default this tool uses everywhere.
Where do I find the net free area per vent?
On the manufacturer’s datasheet or the packaging. Ridge vent is given per linear foot. Type that figure in and the calculator counts units.
Does a new roof fix attic ventilation?
Not by itself. A reroof replaces the exhaust at the ridge. If insulation or paint has blocked the soffit intake, the new roof leaves that as it was.
Can I add more ridge vent to fix a damp attic?
Check the intake first. Exhaust without intake draws air from the house below, which can carry more moisture into the attic.

Sources and methodology

Figures dated September 5, 2026. Last reviewed .

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