Attic fan thermostat and humidistat settings

A powered attic ventilator usually arrives with a thermostat, sometimes with a humidistat, and almost never with a clear explanation of what to set either to. The two controls exist for different seasons and different problems.

This is the one everybody uses, and the published adjustment ranges are wide: 50 to 120°F on the units we hold data for, and 50 to 122°F on one.

That range is much wider than the useful range, which is the source of most bad settings. Consider what each end does:

The honest position: there is no certified optimum, and anyone quoting one to a single degree is guessing. What we can say is that an attic reaches temperatures far above outdoor air on a sunny day, so a setting near the bottom of the dial produces near-constant running for little benefit, and a setting near the top produces no running at all.

Start around 100°F, then adjust. If the fan runs most of the day in mild weather, raise it. If the attic is clearly baking and the fan is idle, lower it.

This is the part most people have backwards, and it is the reason a humidistat is worth having at all.

In winter, warm moist air from the house rises into the attic through every gap — light fittings, loft hatches, pipe penetrations. It meets cold roof framing and the underside of the deck, where it condenses.

Sustained condensation up there produces exactly what you would expect: damp insulation that no longer insulates, staining on ceilings, mold on the sheathing, and eventually rot. It is invisible until it is expensive.

A humidistat runs the fan when attic humidity rises above its setpoint, exchanging the moist air for drier outdoor air. Published adjustment ranges are 30% to 90% relative humidity, with an off position.

• A mid-range setting is the usual starting point, because the goal is to prevent condensation rather than to dry the attic out.
• Set it too low and the fan runs constantly, including in humid summer weather where outdoor air is wetter than attic air and running it actively makes things worse.
• Turn it off entirely in a humid summer climate. This is why a genuine off position matters. Bringing in 85% outdoor air to fix 70% attic air is not an improvement.

The underlying fix for winter attic moisture is air-sealing the ceiling plane so the moist air never gets up there. A humidistat manages the symptom, well, and it is not a substitute.

One of the controls here publishes a fire-safety shut-off that stops the fan above 182°F. It is worth understanding why that exists.

A powered ventilator moves a large volume of air through the roof space. In a fire, that is feeding oxygen to it and accelerating spread. A thermal cutout that kills power well above any temperature an attic reaches in normal weather removes that contribution.

If you are replacing a control, this is worth having and costs nothing extra. It is not a substitute for smoke alarms.

Attic fans and whole-house fans are confused constantly, and the settings on this page apply only to the first.

• An attic ventilator moves air through the roof space only. The house stays closed. It lowers attic temperature so your ceiling is radiated at less.
• A whole-house fan moves air through the living space with windows open, then out through the attic. It is a night-purge device that genuinely lowers indoor temperature — whether one suits your house.
• A bathroom or in-line fan removes moisture from one room and must duct to outside, never into the attic — bathroom extraction.

There is no ENERGY STAR program for attic ventilators or whole-house fans, so none of these products carries a certified efficiency figure. Every airflow number on this page is a manufacturer claim, and the CFM-per-watt figures are our arithmetic on published numbers — the two compared directly.