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Attic ventilation: the 1/150 rule and the 1/300 rule

The code's attic vent ratio is 1/150. 1/300 is an exception, and it applies only when its conditions are met: in climate zones 6, 7 and 8 a Class I or II vapour retarder on the warm side of the ceiling; 40% to 50% of the required area in vents no more than 3 ft below the ridge; the rest in the bottom third of the attic. Answer the checklist below to see which figure is yours.

Short answer
1,500 sq ft attic at 1/150
1,440 sq in
1,500 sq ft attic at 1/300, conditions met
720 sq in
Upper vents under the 1/300 exception
288 to 360 sq in
Lower vents under the 1/300 exception
360 to 432 sq in
Upper vents no lower than
3 ft below the ridge, measured vertically

2021 IRC R806.2 and its exception, for an attic over a flat ceiling; the 2018 and 2024 editions read the same. The vapour retarder sentence names only zones 6, 7 and 8: this tool takes it as asking for nothing in zones 5 and lower, but it can be read as limiting the exception to zones 6 to 8, so confirm with your building official. Where framing is in the way, the code permits the upper vents lower than 3 ft below the ridge. Local amendments can remove or change the exception.

Your space
What are you venting?
Area of the attic floor
sq ft

The attic floor is the flat ceiling below it: measure the house outside wall to outside wall, and include a garage or porch under the same attic. The code says "the area of the vented space" and gives no measuring rule, so take the larger figure. Work out each separate attic on its own. For an L-shaped house, add the rectangles up and enter the total.

Not for a vaulted or cathedral ceiling fixed to the underside of the rafters. There the vented space is each rafter bay, measured along the slope, and a floor area measured flat would come out too small. This tool does not work that case.

Can you use the reduced 1/300 ratio?The code's figure is 1/150 of the attic area. It allows half of that, 1/300, only when all of the following are true. Leave a question on its first choice and the answer stays at 1/150.
1. Climate zone and ceiling vapour retarder

The exception's sentence names only zones 6, 7 and 8. This tool takes it as asking for no vapour retarder in zones 5 and lower. It can also be read as limiting the exception to zones 6 to 8, so confirm the reading, and your zone, with your building department. Class I is 0.1 perm or less; Class II is up to 1 perm.

2. Where will the upper vents be?

Measure straight down from the ridge or the highest point of the attic, not along the roof slope. The upper vents must carry 40% to 50% of the required area; this tool checks that from the vent sizes you enter below.

3. Where will the lower vents be?
sq in
sq in

Use the figure printed on the vent's spec sheet or carton as NFA or net free area. It is not the vent's size: one maker lists a roof vent about 16 in by 20 in overall, with an 8 in opening, at 50 sq in. The numbers filled in here are examples; replace them with your vents' own. For ridge or strip vent, enter the net free area per linear foot and read the count as feet.

Whose figures are the two vent sizes above?

The 40% to 50% split depends on the real size of your vents. Until you choose the second answer, a 1/300 result is shown only as conditional on that split.

ft
Used only to draw the top 3 ft and the bottom third to scale.

Uses the 2021 IRC: Section R806.2 for an attic over a flat ceiling, Section R408.2 for a vented crawl space. Other editions, state and local amendments can differ, and they govern: check which code your area has adopted.

Result
Enter your space

Required areas are rounded up, caps are rounded down, and vent counts go up to the next whole vent, so a figure shown here is never short of the exact one.

Rule: 2021 IRC R806.2 (attic) and R408.2 (crawl space) · Tested against 21 worked examples, 7 printed in a published source · How this is worked out →

Questions people ask

How many roof vents do I need?
Divide the attic floor area by 150 and multiply by 144 to get the required net free area in square inches, then divide by the net free area of one vent and round up. Under the 2021 IRC a 1,500 square foot attic over a flat ceiling needs 1,440 sq in. With example upper vents of 50 square inches and lower vents of 36 square inches each, this tool lays that out as 12 upper and 24 lower vents. Your count depends on the net free area printed for your vents.
When can I use 1/300 instead of 1/150?
Only when both conditions of the exception in 2021 IRC R806.2 are met. First, in climate zones 6, 7 and 8, a Class I or II vapour retarder is on the warm-in-winter side of the ceiling; this tool takes that as asking for nothing in zones 5 and lower, which is a reading to confirm with your building official. Second, 40 to 50 percent of the required vent area is in vents no more than 3 feet below the ridge, measured vertically, with the rest in the bottom third of the attic. Otherwise the ratio is 1/150. Local amendments can differ.
What is net free area, and is it the size of the vent?
No. Net free area is the open area air can pass through after louvers and screens are taken out, and the manufacturer publishes it for each vent. One maker lists a roof vent about 16 by 20 inches overall, with an 8 inch opening, at 50 square inches of net free area. Use the published figure, not a measurement of the vent.
How should attic vents be split between high and low?
The 2021 IRC sets a split only as a condition of the 1/300 exception: 40 to 50 percent of the required area high, within 3 feet of the ridge, and the rest in the bottom third. At 1/150 it requires cross ventilation of each separate space and gives no split. A vent manufacturer read for this tool advises that intake area should meet or slightly exceed exhaust area.
How many crawl space vents do I need?
Under 2021 IRC R408.2 a vented crawl space needs 1 square foot of net opening area for each 150 square feet of under-floor area, with one opening within 3 feet of each external corner. For 1,500 square feet that is 1,440 sq in, or 29 example vents of 50 square inches each. With an approved Class I vapour retarder over the ground the area may be reduced to 1/1,500 and the openings placed for cross ventilation instead.