Attic ventilation: in low, out high

An attic should breathe in at the eaves and out near the top. Most attic ventilation problems come down to one of those two being blocked, missing or undersized, and most attic fan problems come from trying to fix that with a motor.

Fire Safe Marin puts the purpose plainly: vents let outside air move through the attic and under the floor so moisture doesn't build up and encourage mildew and rot. Done right, ventilation also carries off some summer heat. Done wrong, it pulls air you paid to heat or cool up through the ceiling.

What the code asks for

Attic ventilation in California is set by section R806 of the California Residential Code. The City of Dublin's building handout, revised for the 2025 code, summarizes it well:

• Each enclosed attic or rafter space needs cross ventilation, with openings protected from rain.
• Vent openings, or the screen over them, must be at least 1/16 inch and no more than 1/4 inch. In wildfire zones the limit is tighter; see ember-resistant vents.
• The minimum net free area is 1/150 of the vented attic floor.
• That can drop to 1/300 when 40 to 50 percent of the required area is in upper vents within 3 feet of the ridge, and the rest is in the bottom third of the attic.
• Where there are eave or cornice vents, insulation can't block the airflow, and there has to be at least 1 inch of space between the insulation and the roof sheathing at the vent.

Net free area is the open area a vent actually lets air through, after louvers and mesh. It comes from the vent maker's data, not a tape measure.

The math for a typical attic

Take the 1,500 sq ft attic on the 1/300 rule: 720 square inches in total, with 288 to 360 of them up high near the ridge and the rest at the eaves.

The screen matters too. Fire Safe Marin gives the net free area of mesh screening as 81 percent for 1/4-inch mesh, 75 percent for 1/8-inch and 71 percent for 1/16-inch. Moving to finer mesh for ember protection trims every vent a little, and on a tight attic that can mean adding a vent or two to stay within code.

When an insulation job triggers a ventilation check

Under California's 2025 Energy Code, adding or replacing attic insulation makes the ceiling an altered ceiling. In every Bay Area climate zone, an altered ceiling has to have attic ventilation meeting R806. The building department verifies it, not a third-party rater. What else an altered ceiling brings in, zone by zone, and the exemptions are on our Title 24 attic insulation page.

So an insulation job and a ventilation check go together, and the cheapest time to fix blocked eaves is before the new insulation goes in. See blown-in insulation.

Problems we look for in Bay Area attics

• Buried or blocked eaves. Old blown insulation drifts into the eave bays and chokes the intake. Baffles at each rafter bay hold the channel open; DOE's Building America guide recommends keeping a 2-inch air space under the sheathing, more than the code minimum.
• Painted or clogged screens. UC ANR notes that paint and debris clog vent mesh. A vent painted shut on the last exterior paint job is a vent in name only.
• Exhaust with no intake. A ridge vent or roof vents with sealed-up eaves pull air from wherever they can, which often means up through the ceiling.
• Bathroom fans venting into the attic. California's Energy Code requires bathroom and kitchen exhaust to vent outdoors on new work, and with good reason: a fan blowing shower steam into the attic is a moisture problem in slow motion. The fix is a duct to a roof or wall cap.
• Moisture from below. Most attic moisture comes from the house, through gaps around lights, pipes and the hatch. Ventilation carries some away; air sealing stops it at the source.

Fans: when they help and when they hurt

Powered attic fans pull air out of the attic. If the ceiling below is leaky or the eaves are blocked, some of that air comes from the house. A 1994 field study presented at the ACEEE Summer Study measured powered attic ventilators drawing an average of 231 cubic feet per minute of conditioned air out of the living space, between 104 and 646 depending on the house, and documented combustion-safety problems where a fan depressurized the space around a water heater or furnace. Seal the ceiling and open the eaves before considering one.

Solar attic fans do the same job and run off their own small panel, so there is no new circuit to wire. The same rule applies: intake and a sealed ceiling first.

Whole-house fans are a different machine. They pull cool evening air in through open windows and push it out through the attic vents, so they need a lot of exhaust area and a wired circuit. The Energy Commission notes that the Energy Code calls for them in some climate zones when a new house is built to the prescriptive path. They make the most sense where summer evenings cool off quickly. The federal aircraft-noise guidelines also point out that a whole-house fan is a sound leak and needs a cover panel when it is off.

Any fan that needs a new circuit or switch involves electrical work, which is its own trade in California. The written quote spells out how that part is handled.

Unvented attics are a separate design

Some attics are better closed than vented, with foam on the underside of the roof deck and the vents sealed on purpose, under specific conditions in the California Residential Code. That's a design change, not a ventilation repair; see our unvented attic conversion guide.

Getting it quoted

Ventilation is counted, not guessed. At a free in-home inspection, or from photos and a short video you send, the job is to find every intake and exhaust vent, note its type and screen, check the eaves from inside the attic, and compare the total net free area with what the attic needs. The written quote lists each vent or fan added or changed, the baffles, and the resulting net free area. Nothing is due before you sign. For a quick range on the whole attic, try the estimator.