Roof and Attic Ventilation in Aurora, IL

The Maths Aurora Actually Uses

It is three steps and you can do it yourself in about two minutes.

1. Attic area = attic length × attic width. Multiple attic spaces connected without fire separation get calculated separately and added together.
2. Required net free vent area = attic area ÷ 300, giving you a figure in square feet.
3. Convert to square inches = that figure × 144.

Then the split. Aurora wants exhaust at 40 to 50 percent of that total and intake at 50 to 60 percent. Intake slightly exceeding exhaust is deliberate: a positively pressured attic pushes air out through the ridge rather than pulling conditioned air up out of your house.

Worked example, a 1,500 square foot attic

Step	Calculation	Result
Attic area	50 ft × 30 ft	1,500 sq ft
Required NFVA	1,500 ÷ 300	5 sq ft
In square inches	5 × 144	720 sq in
Exhaust (40–50%)	720 × 0.4–0.5	288–360 sq in
Intake (50–60%)	720 × 0.5–0.6	360–432 sq in

Separate attic spaces need separate vent sheets. An attached garage attic needs its own sheet if the space is conditioned. A detached garage does not, unless it is conditioned.

What Aurora Says Goes Wrong Without It

Straight from the city's own roofing handout, this is the published list of what unbalanced ventilation causes: mould in the attic, ice dams, moisture build-up, rust, uncomfortable indoor temperature, snow blown into the attic, and damage to the roofing materials themselves.

That last one is the expensive one and it is invisible. A roof over a hot attic cooks the asphalt from underneath. A twenty-five year architectural shingle over an unvented attic is an eighteen year shingle, and the manufacturer's warranty will not cover it because inadequate ventilation is an installation condition rather than a product defect. You paid for twenty-five years and the paperwork says you got what you deserved.

The Failure We Find Most Often

A ridge vent with nothing feeding it.

Somebody re-roofed, fitted a nice continuous ridge vent, ticked the exhaust box, and never looked at the intake. Meanwhile the soffit is either solid panel, or vented panel with blown-in insulation packed tight against it from the attic side, or vented panel that was painted over at some point. Exhaust with no intake does not move air; it just pulls conditioned air up out of your living space through every ceiling penetration, which costs you money on the heating bill as well.

The fix is usually baffles at every rafter bay to hold the insulation back off the soffit, plus opening up or replacing the soffit venting itself. It is not expensive and it is far and away the highest-return roofing work we do.

Second most common: mixing exhaust types. A ridge vent and gable vents on the same attic, or a ridge vent plus a powered fan. Instead of drawing from the soffits, the ridge vent pulls from the nearest gable vent and short-circuits the whole attic. If you have gable vents and are getting a ridge vent, the gables usually need closing, or the gable vents need to sit at least three feet below the ridge to count as intake, which is the exception Aurora's own guidance allows for.

Vent Types, and Which Ones Actually Work Here

Ridge vent, continuous, along the peak. Best exhaust for most roofs because it draws along the whole ridge line rather than a point. Wants a proper baffle on the exterior to stop wind-driven snow, which matters in Illinois more than it does in Tennessee.

Soffit and eave vents, your intake. Continuous vented soffit, or individual vents. Aurora's guidance counts vented panel by ratio, one in every three, four or five panels vented, or all panels vented, and the ratio changes the NFVA credit substantially.

Gable vents can act as intake if installed at least three feet below the ridge. On their own, as the only ventilation, they are weak, because they only work when wind blows across them.

Box and turtle vents, fine as exhaust where a continuous ridge is not possible, on a hip roof with very little ridge for instance.

Powered attic fans: we generally advise against them. They can depressurise the attic enough to pull conditioned air from the house, they use electricity, they fail, and in a cold climate they can pull moist air up into the attic in winter. A properly balanced passive system does the job with no moving parts.