My attic has high humidity. I put a combo temp-humidity monitor and it was in the high 70s. It’s 30 degrees outside. I’ve read about what could cause the high humidity and two key things came up:

  1. Poor insulation between the attic and living space so heat and humidity rises into the attic. Key things that may cause leaks are recessed lights and AC register vents.
  2. Something leaking humidity into the attic such as a bathroom fan vent pipe used during showers.

I have all of those things: AC vents in the attic, recessed lights, vent pipes form all of my bathrooms. The vent pipes seem to come together in the attic and go into a single pipe out the top of the roof.

The reason this matter to me is I’ve noticed a lot of rusty nails from my shingles and when the temps are below freezing they cause icicles to form due to the high moisture. When the sun heats my roof in the day those rusty icicles become drips of nasty rust on my boxes in the attic.

A few questions:

  1. During my home inspections years ago I was told my ridge vent wasn’t wide enough. What is the proper width for a ridge vent?
  2. How can I determine why is causing the high humidity? Is this something I call a weatherization company to investigate? Or a general contractor?
  • too small they are probably right. they may have determined this by sighting signs of condensation drips. a venting is the way to prevent humidity. who to call, well you need someone who understands roofs. enlarging the ridge vent is not the only option.
    – Jasen
    Commented Nov 23, 2018 at 23:39
  • 1
    What is the temperature in the attic? Temp and (relative) humidity are tightly related. For instance taking warm air and merely chilling it will raise its (relative) humidity. Warm air has more capacity to hold water than cold air. Commented Nov 23, 2018 at 23:45
  • Last I checked, outside temp was 24, attic temp was 36 and attic humidity was 70
    – J Dilly
    Commented Nov 24, 2018 at 3:22

1 Answer 1


Yes, 70% RH is too high for a 36 degree attic. And your advice is on the money: stop humidity from entering, and remove humidity from the space. First, add vapor barrier caps to the lights, seal the hatch opening, and use duct seal or tape on all the ducting.
Second step, get some air changes happening. You could install an attic exhaust fan towards one end of the space, and put an intake grille at the opposite end. The fan can exit air through a soffit vent, a gable vent, or a roof jack. Wire the fan with a line-voltage dehumidistat mounted in the attic not too close to the hatch.

  • Hello, and welcome to Stack Exchange. You should try to improve this answer because as it stands it is being flagged as low quality and is in danger of being deleted. Possibly add a more complete description of how the humidity reduction works in the attic.
    – Michael Karas
    Commented Nov 29, 2018 at 10:58

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