Every attic ventilation calculator uses 1/300. That is the exception, not the rule, and it has two conditions. One of them caps how much of your venting may be at the ridge, so more ridge vent can double what you need.
Attic ventilation
sq ft
%
sq in
sq in
ft
Both R806.2 conditions are tested, not assumed. Net free area figures are yours, off the product, because the screen and the louvre are what make them smaller than the hole.
Specification summary
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Result
The window R806.2 leaves you
Worked example
A 1,500 sq ft attic in Climate Zone 5, with 45% of the venting up at the ridge.
Zone 5 is outside 6, 7 and 8, so condition 1 is satisfied by default. The vapour retarder requirement only bites in the three coldest zones, which is why most of the country reaches the exception without installing anything.
45% is inside the 40 to 50 band, so condition 2 is satisfied too. Both conditions met, so the exception applies: 1/300, which is 720 square inches of net free area.
Now move the split to 60% at the ridge. That fails condition 2, because the band has a ceiling. The ratio reverts to 1/150 and the requirement becomes 1,440 sq in. More ridge vent, twice the work.
The ridge check catches the consequence. At 60% the upper share alone needs 48 ft of ridge vent and there is only 40 ft of ridge, so the design does not fit its own roof.
And in Zone 6 with no vapour retarder, 45% is not enough either. Condition 2 passes, condition 1 fails, and the requirement doubles again to 1,440.
The question that decides everything here is not how big the attic is. It is whether both conditions are met, and one of them is a rule about proportion that most people have never read.
The formula
One division, and everything hangs on which denominator you are entitled to:
net free area = attic area × 144 / ratio, ratio = 300 if both conditions, else 150
attic area
the ceiling area of the vented space, in square feet
144
square inches in a square foot, because vents are sold in square inches
condition 1
a Class I or II vapour retarder, but only in Climate Zones 6, 7 and 8
condition 2
between 40 and 50 percent of the area in the upper portion, and it is a band
There is no middle ground between the two ratios. You either meet both conditions and halve the requirement, or you do not and you do the full amount. Which makes condition 2 worth designing to deliberately rather than discovering afterwards.
Why the usual figure is the wrong one
1/300 is quoted everywhere as if it were the rule. R806.2 opens with 1/150 and introduces 1/300 as an exception, conditional on two things. Reversing that is not a small error: it halves the answer.
Condition 1 is easy and usually free. It only applies in Climate Zones 6, 7 and 8, the coldest end, so most of the country satisfies it by geography rather than by installing anything.
Condition 2 is where designs actually fail, and it fails upwards more often than downwards. Continuous ridge vent is easy to specify and generous soffit venting is harder to achieve, because soffits are narrow and get blocked. So the natural drift is toward too much high venting, and the code caps it at 50 percent.
The reason is not paperwork. Airflow through an attic is limited by whichever opening is smaller. A big ridge vent with small soffits does not move more air, it moves the same air and takes the makeup from wherever it can get it, which is the house below through every ceiling penetration. That is warm moist indoor air arriving in a cold attic, which is exactly what ventilation exists to prevent.
Read from Seattle's adopted code, which prints IRC Chapter 8 in full, because codes.iccsafe.org returns HTTP 403 to any automated request.
Prices for roof coverings and tear-off
The published figures for roof coverings and tear-off are on the roof replacement cost calculator, with the byline, the publication date and the caveats that go with them. They are not repeated here, because the same table on two pages is worth less than one table you can find.
One fifty is the rule. Three hundred is an exception with two conditions
Every attic ventilation calculator on the internet uses 1/300, because it is the smaller number and it is what everybody has heard. Here is what R806.2 actually says:
The minimum net free ventilating area shall be 1/150 of the area of the vented space. 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.
The rule is 1/150. On a 1,500 sq ft attic that is 1,440 square inches of net free area. The exception halves it to 720, and you only get the exception if both conditions are met.
Condition one is a vapour retarder, and it only applies in Climate Zones 6, 7 and 8. Outside those zones this condition is satisfied by default, which is why most of the country can reach the exception on the second condition alone.
Condition two is a distribution rule, and it is where people fail. Between 40 and 50 percent of the required area has to be in the upper portion, with those ventilators not more than 3 feet below the ridge. The rest goes in the bottom third of the attic.
Miss either and the requirement doubles. Which means a calculator that hands you 1/300 without asking about your vents is giving you half the answer, in the literal sense.
The 40 percent rule has a ceiling, and that is the surprise
Read condition two again: not less than 40 percent and not more than 50 percent. It is a band with a top as well as a bottom, and that is not how anybody quotes it.
So more ridge vent is not better. Put 60 percent of your net free area up at the ridge and you have failed condition two, which means you no longer qualify for the 1/300 exception, which means your requirement doubles and you now need considerably more of both.
The reason is physics rather than paperwork. Ventilation works because air enters low and leaves high, and the flow is limited by whichever opening is smaller. An attic with a large ridge vent and small soffit vents does not move more air; it pulls the makeup air from wherever it can find it, which is usually the house below through every ceiling penetration.
That is the failure mode this band exists to prevent. Conditioned indoor air, carrying its moisture, drawn up into a cold attic through the light fittings, where it condenses on the sheathing.
The balanced answer, and the one the code is steering you toward, is roughly half low and half high, with the low half never smaller than the high half.
Net free area is not the size of the hole
Every figure in R806.2 is net free ventilating area, and that is a different quantity from the size of the vent.
Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow. Ventilation openings shall have a least dimension of 1/16 inch (1.6 mm) minimum and 1/4 inch (6.4 mm) maximum.
A vent has a screen, and often a louvre. Both obstruct, and the manufacturer's net free area figure is what is left after they do. A soffit vent measuring 16 by 8 inches is 128 square inches of hole and considerably less than that of net free area, sometimes less than half.
The opening size limits are why. R806.1 sets a least dimension of 0.0625 in minimum and 0.25 in maximum. Below the minimum the opening is not really ventilating; above the maximum it needs corrosion-resistant screening, which is itself an obstruction. Either way the effective area is smaller than the aperture.
So the number to take off the product, not off a tape measure. This page asks for net free area per unit and multiplies; it does not guess at a derating factor, because that factor belongs to the manufacturer and not to a calculator.
The vent that is blocked by the insulation
Where eave or cornice vents are installed, blocking, bridging and insulation shall not block the free flow of air. 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.
One inch, between the insulation and the sheathing, at the vent. This is the single commonest way a correctly specified ventilation system delivers nothing at all.
The sequence is ordinary. Soffit vents go in, sized properly. Later, somebody adds attic insulation and pushes it out to the eaves, because the edge of the ceiling is where heat is lost. The insulation reaches the soffit and the airway closes.
The fix is a baffle, and it has to be installed before the insulation, not after. Once the insulation is in, nobody is crawling to the eaves of a cold attic to check whether the airway survived.
It is worth saying what the failure looks like, because it is not obvious: frost on the underside of the sheathing in winter, then a damp stain at the eaves in spring that looks like a roof leak and is not.
Where this came from, and what it does not cover
Read 2026-09-09. 2018 Seattle Residential Code, Chapter 8 Roof-Ceiling Construction, Seattle Department of Construction and Inspections. Prints IRC Chapter 8 in full, including Sections R806.1 through R806.3. Used because codes.iccsafe.org returns HTTP 403 to any automated request.
Unvented assemblies are a different section entirely. R806.5 covers attics and rafter spaces built with no ventilation at all, which is a legitimate and increasingly common approach and has its own conditions about insulation type and vapour control. Nothing on this page applies to one.
And the climate zone that decides condition one is not on this page. It comes from a map, it is jurisdictional, and zones 6, 7 and 8 are the cold end: most of the country is not in them, which is why the vapour retarder condition is usually satisfied by default rather than by anything you install.
Ask your building department which zone you are in before relying on that.
Frequently asked questions
Is attic ventilation 1/150 or 1/300?
1/150 is the rule. 1/300 is an exception available only if both R806.2 conditions are met: a Class I or II vapour retarder in Climate Zones 6, 7 and 8, and between 40 and 50 percent of the required area in the upper portion of the attic. Miss either and you are back to 1/150, which is twice the venting.
Can you have too much ridge vent?
Yes, and this is the part almost nobody knows. R806.2's second condition is a band: not less than 40 percent and not more than 50 percent of the required area in the upper portion. Put 60 percent up at the ridge and you fail the condition, lose the exception, and your requirement doubles.
Why does too much high venting matter?
Because airflow is limited by the smaller opening. A large ridge vent with small soffit vents does not move more air; it draws its makeup air from wherever it can, which is usually the conditioned house below through light fittings and other ceiling penetrations. That puts warm moist indoor air into a cold attic, which is the failure ventilation is there to prevent.
How much attic ventilation do I need for 1,500 square feet?
At 1/150, 1,440 square inches of net free area, which is 10 square feet. At 1/300 it is 720. Which applies depends on whether both R806.2 conditions are met, and the calculator tests them rather than assuming.
What is net free area?
The effective opening after the screen and the louvre, which is smaller than the hole and sometimes less than half of it. It is printed on the product by the manufacturer. R806.1's opening size limits, a least dimension between 1/16 and 1/4 inch, are part of why: anything larger needs corrosion-resistant screening, and the screening obstructs.
Where do the upper vents have to be?
Not more than 3 feet below the ridge or highest point of the space, measured vertically, per R806.2. The balance goes in the bottom one-third of the attic. So the rule is not just about proportion, it is about position, and a vent halfway up a roof plane counts as neither.
Why is my soffit vent not working?
Very often because the insulation is sitting on it. R806.3 requires not less than a 1 inch space between the insulation and the roof sheathing at the vent, and the usual sequence is that vents go in correctly and insulation is added later and pushed out to the eaves. Baffles have to be installed before the insulation, not after.
Check these numbers yourself
2018 Seattle Residential Code, Chapter 8 Roof-Ceiling ConstructionSeattle Department of Construction and Inspections. Read 2026-09-09. Source of Sections R806.1, R806.2 and R806.3, quoted verbatim, including both conditions of the 1/300 exception and the 40 to 50 percent band.
How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.
Figures on this page last checked against the source documents on 2026-09-09. Codes are amended locally; confirm against the edition your jurisdiction enforces.
Net free area figures are the manufacturer's, not this site's. The screen and the louvre make the effective opening smaller than the hole and no calculator can derate for a product it cannot see. The climate zone that decides condition 1 comes from a map and is jurisdictional. Unvented attic assemblies are covered by R806.5 and nothing on this page applies to one.