Attic ventilation: intake vs exhaust

Powered attic ventilator manufacturers publish an intake requirement because their product does not work without one. Broan’s formulation is to multiply total ventilator CFM by 0.48 for the minimum soffit vent area in square inches. The Home Ventilating Institute states the same relationship the other way round: one square foot of inlet per 300 CFM of certified fan capacity. The two are identical — 144 square inches divided by 300 is 0.48.

Those capacities are real products: a Broan gable ventilator at 760 and 1,020 CFM, a solar roof fan at 1,150, a Maxx Air gable unit at 1,600, and a whole-house fan on low and high at 2,361 and 4,280.

A fan moves the air it can get. If the soffits cannot supply it, the attic goes slightly negative and the fan starts drawing air from the next easiest source — and in most houses that is the conditioned space below, through recessed lights, attic hatches, plumbing chases and gaps around wiring.

Three consequences, in order of how expensive they are:

1. You pay to cool air that goes into the attic. The fan runs on the hottest afternoons, which is exactly when your air conditioner is working hardest, and some of what it removes is air you just paid to cool.
2. The attic gets less ventilation than the fan promises. Rated CFM assumes free intake. Starved of it, the fan moves substantially less air and the attic stays hot anyway.
3. Combustion appliances can backdraft. In a house with an atmospherically vented water heater or furnace, depressurizing the building is a genuine safety concern rather than an efficiency one.

This is the core of the argument against powered attic ventilators generally, and it is why the honest comparison with a whole-house fan in attic fan versus whole-house fan turns on the building, not the machine.

Attics ventilate naturally by stack effect: hot air rises and leaves at the ridge, drawing cooler air in at the eaves. That works only if the openings are in the right places, and it is why the conventional design splits vent area roughly evenly between low intake and high exhaust.

• Intake low — soffit or eave vents, ideally continuous along both eaves.
• Exhaust high — ridge vent, roof vents or a gable vent near the peak.
• Do not mix exhaust types. A ridge vent plus a powered gable fan lets the fan pull air backward down the ridge vent, which is a short circuit: the fan ventilates a few feet of roof deck instead of the attic.

That last point is the most common installation error in this category. If a house already has a functioning ridge vent, adding a powered gable fan frequently makes the ventilation worse rather than better.

1. Count the soffit vents and read the NFA stamped on them, in square inches. Multiply by the count.
2. Look for blocked ones. Insulation pushed into the eaves is the usual culprit, and baffles are the fix. A vent with insulation against it contributes zero.
3. Compare with 0.48 times the fan’s CFM. If you are short, you are choosing between more intake and a smaller fan.
4. Check the exhaust path is a single type. Ridge, or gable, or roof vents — not a powered fan fighting a ridge vent.

If the answer is that your soffits cannot be improved — a house with no eaves, or closed soffits that would need rebuilding — then a powered attic ventilator is the wrong purchase, and the money is better spent on insulation or on a whole-house fan, which takes its air from inside the house by design rather than by accident.

A solar attic fan removes the running cost and the wiring, which is a real advantage. It does not reduce the intake requirement by a square inch: a 1,150 CFM solar roof fan needs the same 552 square inches of net free intake that a mains-powered fan of the same capacity would. The category is covered in solar attic fans.