When people think about attic ventilation, they look up: ridge vents, box vents, turbines, fans. But the part that's easiest to overlook is down at the eaves. An attic can only push out as much air as it can pull in. Here's how to figure out how many soffit vents you need, and why intake comes first.
Why intake matters
A vented attic works like a chimney. The roof consultants at IIBEC describe it in Attic Ventilation 101: "Outside air enters the attic space through soffit or eave vents, rises through the attic space as it warms, and exits through vents that are positioned at or near the top."
Take away the intake and the system stalls. IIBEC describes a common reroofing case: a new ridge vent on a house with no soffit or eave vents. "Since there isn't any provision for the intake of air, the ridge vent is basically ineffective."
What the code says
The minimum is set in IRC R806.2: "The minimum net free ventilating area shall be 1 / 150 of the area of the vented space."
An exception allows 1/300 if the vents are balanced high and low. It requires that "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," with the rest "located in the bottom one-third of the attic space." (Climate Zones 6, 7 and 8 also need a vapor retarder.)
In plain English: at least half of your vent area belongs down low as intake.
How to calculate your soffit vents
Here's the math, using the example IIBEC walks through for a simple gable roof.
- 1. Measure the attic floor. A 20 ft. by 50 ft. attic is 1,000 square feet.
- 2. Pick your ratio. At 1/300, you need 1,000 ÷ 300 = 3.33 square feet of net free area. At 1/150, you need 6.67 square feet.
- 3. Convert to square inches. Multiply by 144: 480 square inches at 1/300, or 960 at 1/150.
- 4. Split it between intake and exhaust. IIBEC's 1/300 example puts "240 sq. in. of NFVA located at the ridge and 120 sq. in. of NFVA located at each soffit." That's 240 square inches of intake in total.
- 5. Divide by the vent's rating. Every soffit vent has a net free area (NFA) rating, usually printed on the vent or its package. Divide your intake total by that number.
Round up, never down. If you're unsure which ratio applies, ask your building department.
Err on the side of more intake
IIBEC notes that the exception lets you put more than half the vent area at the intake level, but not more than half at the exhaust level. The reason: "Intake area should always be equal to or more than exhaust area, or intake air may be pulled from the interior of the building/residence through openings in the ceiling (e.g., can lights, attic access doors)."
Building scientist Joe Lstiburek is blunter in Lstiburek's Rules for Venting Roofs: "Put more vents down low than up high." He adds: "Attics should be ventilated with air from the outside, not the inside."
That's why "bigger is not better." A high-CFM fan in an attic that's short on intake will pull air from wherever it can: the ridge vent, a gable, the chimney or the house below. We covered that in Why Solar Blaster Bets on Balance, Not Brute Force.
Placement matters too. Lstiburek says "the entire perimeter of the roof needs to have air inlets, meaning continuous soffit ventilation." A few vents bunched at one corner leave the rest of the roof deck in dead air.
Make sure the vents you have actually work
Plenty of soffit vents let no air through. IIBEC says to check that they aren't:
- "Blocked by attic insulation because insulation baffles weren't used"
- "Clogged with debris"
- "Installed over a solid substrate"
IRC R806.3 requires "Not less than a 1-inch (25 mm) space" between the insulation and the roof sheathing at the vent. The U.S. Department of Energy's Building America Solution Center adds in its ice dam prevention guide that "Baffles and insulation dams should be installed to keep insulation out of the soffit vents and to direct cooling air flow from the soffit vents up along the underside of the roof deck to the ridge vents." For other clues that your attic is starved for air, see Signs of Poor Attic Ventilation.
Where a solar fan fits
A solar fan doesn't replace intake. It depends on it. Solar Blaster's fans assist the passive system you already have, intake low and exhaust high. In a 1,000 cubic foot space, the Solar Mega RoofBlaster replenishes the air about every 10.4 minutes "with proper intake ventilation," and the standard Solar RoofBlaster about every 28.4 minutes. What Size Solar Attic Fan Do I Need? walks through the sizing.
Sheds and shipping containers usually have no soffits at all. For those, we recommend our 2.5" Intake Vents near the floor on the shadiest side, four for every solar vent. They can also be used on a house's eaves.
Being honest about the limits
- Code minimums are minimums. Local code, roof design and climate can change the numbers. Your building official has the final say.
- Vent ratings vary. Use the actual NFA rating, not a guess.
- Some houses have no room for soffit vents. Roofs with little or no overhang may need other intake options. A roofer can tell you what fits.
- Intake won't fix air leaks. If moist house air leaks into the attic around lights, pipes and the hatch, seal those gaps first.
- More vents don't repair damage. Rotted wood, wet insulation and mold need a pro. Good ventilation helps keep things drier going forward.
The bottom line
Count your intake before you add any exhaust. Figure your net free area, put at least half of it down low, spread it along the eaves and keep insulation out of the way. Then add exhaust that's matched to the space.
As Daniel Rheaume of Solar Blaster puts it, "Our products move air, which is a natural cleanser. Clothesline fresh."
Questions about your attic? Call us at (480) 747-7097 or email sales@solarblasterfans.com.
