The outside humidity is 96%. The attic humidity is at 60%. When the attic fans turn on, the humidity in the attic begins to increase. Is this normal?

The attic insulation is batts, not blown-in. There are baffles installed as well. I cannot tell if air flow is moving through the baffles.

  • 4
    Why is this not entirely expected considering the situation? You explain it yourself in your opening paragraph.
    – isherwood
    Jul 28, 2023 at 13:08
  • 1
    Wait, is "the attic fan" venting the attic, or the house?
    – isherwood
    Jul 28, 2023 at 13:09
  • Is the attic fan just for the attic or does it move house air into the attic (whole house fan). Check where the air is coming into the fan, if you homes moisture is going up outside air must be entering it, maybe an open window???
    – Gil
    Jul 28, 2023 at 14:49

2 Answers 2


This is a function of what relative humidity means.

The hotter air in the attic can hold more water than the cooler air outside, so for the same absolute amount of water-in-air, the warmer/hotter attic has lower relative humidity than the cooler outside air with the same absolute amount of water-in-air. When the fans pull in cooler outside air, the relative humidity will rise as the attic cools off. The absolute amount of water-in-air is unchanged.

One measure of the absolute amount of water-in-air is the dew-point. In a ventilated attic, the dew-point will be the same as the outside air, because the outside air moves through the attic via ventilation.

  • Why does the air outside have the same amount of water?
    – DonQuiKong
    Jul 28, 2023 at 19:30
  • 3
    Because, in a ventilated attic, it's the same air, due to ventilation. Entering an attic vent does not magically remove water. Nor does exiting an attic vent add water. Even with no fans, the air is the same as the outside air, other than whatever heating is happening in the attic space due to sunshine.
    – Ecnerwal
    Jul 29, 2023 at 0:24
  • then an attic "fan" is actually an air conditioner? A fan alone doesn't cool air, it heats it up.
    – DonQuiKong
    Jul 29, 2023 at 7:34
  • 1
    @DonQuiKong The attic air will be hotter than the outside air. The fan brings in cooler outside air, which cools the attic. No air conditioning here. The whole "fans don't actually cool things!" thing is overused; quite often a fan really does cool the air in or around a space, by bringing cooler air or dryer to it (or by removing hot/humid air, from another perspective). Jul 29, 2023 at 17:14
  • @RadvylfPrograms but did you read the comment above mine? Your comment makes no sense in context ...
    – DonQuiKong
    Jul 29, 2023 at 18:44

Logic would say it is normal.

When the fans turn on they are drawing in the outside air.

Outside air is higher in humidity than attic air.

Low humidity air is being replaced with higher humidity air.

Unless there is another device to change humidity, you simply are exchanging the air. You are not changing the humidity in the air, simply mixing it.

  • 1
    The amount of moisture in the air - the absolute humidity - is pretty much the same for the attic & the outside air. What's different is the relative humidity which, for a given amount of moisture in the air, goes down as the temperature rises.
    – SteveSh
    Jul 28, 2023 at 15:13

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