The rule in one line
Section R806.2 of the 2021 International Residential Code (IRC) says the minimum net free ventilating area is 1/150 of the area of the vented space. That is the baseline. The exception lets you halve it to 1/300 when the vents are arranged in a specific way, described below. This guide uses the attic floor area as the area of the vented space and quotes the IRC as adopted in Colorado; your state or city may adopt a different edition or add amendments, so check with the local building department.
The 144 converts square feet to square inches, because vent products are rated in square inches. The area converter does the same conversion for any unit, and the square footage calculator adds up the attic floor if the house is not a simple rectangle.
When 1/300 is allowed
The reduced ratio applies only when both conditions in the R806.2 exception are met:
- Vapor retarder in cold zones. 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.
- Balanced high and low vents. Not less than 40 percent and not more than 50 percent of the required ventilating area is in ventilators in the upper portion of the attic or rafter space. Upper vents must be no more than 3 feet (914 mm) below the ridge or highest point, measured vertically. The balance goes in the bottom one-third of the attic space.
Where framing gets in the way of the upper vents, the code permits them more than 3 feet below the ridge. If you cannot meet the split, you are back at 1/150.
Square inches by attic size
Every value below is net free area. The last column is the range the upper vents must supply under the 1/300 rule; the lower vents supply the rest.
| Attic sq ft | 1/150 | 1/300 | Upper at 1/300 |
|---|---|---|---|
| 600 | 576 sq in | 288 sq in | 115 to 144 |
| 800 | 768 sq in | 384 sq in | 154 to 192 |
| 1,000 | 960 sq in | 480 sq in | 192 to 240 |
| 1,200 | 1,152 sq in | 576 sq in | 230 to 288 |
| 1,500 | 1,440 sq in | 720 sq in | 288 to 360 |
| 1,800 | 1,728 sq in | 864 sq in | 346 to 432 |
| 2,000 | 1,920 sq in | 960 sq in | 384 to 480 |
| 2,500 | 2,400 sq in | 1,200 sq in | 480 to 600 |
The numbers scale in a straight line: each 100 sq ft of attic adds 96 sq in at 1/150 and 48 sq in at 1/300. Because the rule is a ratio, it works in metric too: a 100 m² attic needs about 0.7 m² at 1/150.
Intake and exhaust
R806.1 requires cross ventilation for each separate attic space, and the 1/300 exception sorts the vents into two groups:
- Upper vents (exhaust). Ridge vents, roof vents near the peak or gable vents high on the wall, within 3 feet of the ridge measured vertically to count toward the upper share.
- Lower vents (intake). Soffit or eave vents and other openings in the bottom third of the attic.
Under the 1/300 rule, the upper share is capped at half. So intake always supplies at least half of the required area, and up to 60 percent. A layout with a long ridge vent and a few small soffit vents may meet the total but still fail the split.
Where 3 feet below the ridge lands
The 3-foot limit is a vertical drop, not a distance along the roof. On a low-pitch roof that reaches far down the slope; on a steep roof it does not. The roof pitch calculator gives the pitch from rise and run.
| Pitch | Horizontal from ridge | Along the roof |
|---|---|---|
| 4/12 | 9 ft | 9.5 ft |
| 6/12 | 6 ft | 6.7 ft |
| 8/12 | 4.5 ft | 5.4 ft |
| 10/12 | 3.6 ft | 4.7 ft |
| 12/12 | 3 ft | 4.2 ft |
Worked example: a 1,200 sq ft attic
Using 1/300: 1,200 ÷ 300 × 144 = 576 sq in of net free area. At the 50 percent maximum, the upper vents supply up to 288 sq in.
Suppose the ridge vent is labeled 18 sq in of net free area per linear foot and each soffit vent 65 sq in. These are example ratings only; use the figure printed on the product you buy.
- Ridge vent: 288 ÷ 18 = 16 linear feet, which gives 288 sq in.
- Lower vents must cover the remaining 288 sq in: 288 ÷ 65 rounds up to 5 soffit vents, 325 sq in.
- Total: 613 sq in, at least the required 576. The ridge supplies 50 percent of the required area, inside the 40 to 50 percent window.
Without the balanced layout, the same attic needs 1,152 sq in at 1/150, twice as much.
Openings, screens and insulation
- Opening size. R806.1 says ventilation openings must be at least 1/16 inch and at most 1/4 inch in their least dimension. Larger openings need corrosion-resistant screening with openings in that same range. Vents must open directly to the outside air and keep out rain, snow, birds, rodents and similar animals.
- Insulation clearance. R806.3 requires at least 1 inch (25 mm) between the insulation and the roof sheathing at the vent, and blocking, bridging and insulation must not block air flow from eave or cornice vents.
- Installation. R806.4 says ventilators are installed according to the manufacturer's instructions, with roof vents following section R903 and wall vents section R703.1.
Because the code counts net free area, a screen or louver reduces what a vent contributes. That is why the rating on the label matters more than the size of the hole in the soffit.
Checklist before you buy vents
- Measure the attic floor area in square feet.
- Decide whether you can meet the 1/300 conditions, including the vapor retarder in Climate Zones 6 to 8.
- Compute the required square inches and the 40 to 50 percent upper share.
- Add up the net free area ratings, upper and lower separately.
- Keep at least 1 inch of air space above the insulation at every eave vent.
- Confirm the adopted code edition with your building department.
Frequently asked questions
What is the 1/300 rule for attic ventilation?
The International Residential Code requires a minimum net free ventilating area of 1/150 of the area of the vented space. Section R806.2 lets that drop to 1/300 when 40 to 50 percent of the required area is in upper vents no more than 3 feet below the ridge and the rest is in the bottom third of the attic. In Climate Zones 6, 7 and 8 a Class I or II vapor retarder on the warm-in-winter side of the ceiling is also required.
How many square inches of attic ventilation do I need?
Divide the attic area in square feet by 150 or 300, then multiply by 144 to get square inches. A 1,500 sq ft attic needs 1,440 sq in of net free area at 1/150, or 720 sq in at 1/300. Those are minimums from R806.2; your local code may have amendments.
What is net free area?
It is the open area air can actually pass through, after louvers, screens and grilles are taken out of the gross opening. The code sets the requirement in net free area, so add up the net free area ratings of the vents rather than their outside dimensions. Ventilators must be installed according to the manufacturer's instructions.
Can I have more exhaust than intake?
Under the 1/300 exception, no more than 50 percent of the required area may be in the upper vents, and the balance must be in the bottom third of the attic. If you use the plain 1/150 rule, R806.2 does not set a split, but the 1/300 figure is only available with the 40 to 50 percent split.
How much space do I need between insulation and the roof deck at the eaves?
Section R806.3 requires at least 1 inch (25 mm) between the insulation and the roof sheathing at the location of the vent, and says blocking, bridging and insulation must not block the free flow of air where eave or cornice vents are installed.
Sources
- UpCodes: 2021 International Residential Code as adopted in Colorado, section R806.2 Minimum vent area (1/150 of the vented space; 1/300 exception with the vapor retarder condition in Climate Zones 6, 7 and 8; 40 to 50 percent upper vents not more than 3 feet below the ridge; balance in the bottom one-third)
- UpCodes: 2021 IRC as adopted in Colorado, chapter 8, section R806 (R806.1 cross ventilation, openings 1/16 to 1/4 inch, screening, opening directly to outside air; R806.3 1-inch insulation clearance; R806.4 install per manufacturer's instructions)