Attic Fan Runs During Winter
Attic Humidity
Is My Thermostat Crazy? My Attic Fan Runs During Winter!
Your attic fan runs during winter because you probably have a humidistat control on your attic fan so it runs when attic relative humidity is too high. The idea is for the fan to draw in dryer air from outside to get rid of water vapor that has risen up into the attic from the rooms through gaps in the vapor barrier or worse through improperly vented bath fans. Sometimes the problem is your furnace humidifier setting.
How do I Set the Humidifier?
Some humidifiers have a thermocouple that senses outside air temperature and changes the setting of your humidifier’s humidistat so you don’t have to. But, if you have to set it yourself, here is how to set it.
Jet Fan Recommends setting your furnace whole house humidifier humidistat by this table to avoid attic dampness from excess room humidity leaking into your attic and condensing on wood and metal conduit. Water-vapor is lighter-than-air and can leak through light fixtures and your wall’s top-plate penetrations. Insulation alone will not stop vapor. Vapor barriers help but usually have gaps. Your attic fan is running in the winter because there is excess attic vapor which is triggering the humdistat to turn on the fan to draw in dry air from outside through static vents in eaves or soffits, roof vents, or attic wall louvered vents.
Whole House Humidifier Humidistat Settings Which Will Help Keep Your Attic and Windows Dry.
Settings for Different Outside Air Temperatures
| Above 40° F | 40% – 50% Relative Humidity |
| 32° F | 30% – 40% Relative Humidity |
| 20° F | 20% – 30% Relative Humidity |
| 10° F | 10% – 20% Relative Humidity |
| 0° F | OFF |
If you see condensation on your window glass your attic is probably already damp; especially if you have double pane glass.
Winter Attic Fan Running is Okay?
Your attic fan runs during winter regardless of the temperature to bring in dry outside air to replace damp attic air and prevent mold from starting in your attic. Your Jet Fan Attic Fan Humidistat should be set to 70 -75% relative humidity. Fan runs less with higher setting. If you hear your fan is running and running for long periods; here are some possible explanations:
What Causes Excess Attic humidity Anyway?
- House humidifier set too high.
- Bath fans blowing into attic instead of ducted out.
- Kitchen fan blowing into attic.
- Gaps in ceiling and vapor-barrier that allow vapor to rise into attic.
- Area water-table is high, and vapor is penetrating house through porous concrete foundation or crawl-space floor, rising into attic.
- Not enough attic intake venting to provide dry air from outside into attic through static vents in eave, roof and attic wall vents.
- Unusual atmospheric change has resulted in low temperature and high vapor in attic…fan will shut off when weather normalizes.
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