Humidity level in Attic vs outside

attichumidityroof

My attic humidity level is 85% but on the outside when I can my location in the weather app the humidity is 85%. My question is, is it normal the humidity level in my attic same as the humidity level outside? I know that humidity level in the attic should stay around 50%. My main concern is my attic might now have enough airflow that's why humidity is high even its on my attic.

-Thanks

Best Answer

You are asking about relative humidity, which is a measure of the total amount of moisture in the air relative to the amount of moisture the air could hold at its current temperature. Humidity measured this way will change if the temperature changes even if nothing else is different. I.e. the same exact slug of air will have different relative humidity, depending on its temperature.

Attics are typically vented to the outside, pulling fresh outside air in while warmer air in the attic is exhausted, usually via vents on the gable or the rooftop, but sometimes with a fan.

Unless there is an incorrectly installed vent from the house (e.g. bathroom vent), the air in the attic is basically the same air as outside, but possibly at a different temperature.

In the winter, it will be a little warmer, because of heat losses from the house coming into the attic, warming the attic air. In the summer it can often by much warmer, because of heating from the sun on the roof radiating into the attic space. Either way, this heat difference provides the convection that drives the air movement through the attic, drawing fresh air from vents under the eaves or soffits, and exhausting air through the gable and/or roofs.

Bottom line:

  • The absolute amount of moisture in the air should be the same in the outside air as in the attic air. If it's not, something in your house is exhausting humid air into the attic, and that should be fixed.
  • Assuming the absolute amount of moisture in the air is the same as the outside air, then if the temperature is different, the relative humidity will also be different. Since the attic is typically warmer than the outside air, the relative humidity in the attic will typically be lower than the relative humidity of the outside air.

So to answer your question:

is it normal the humidity level in my attic same as the humidity level outside?

Exactly the same? No, I wouldn't say that's normal. Humidity in the attic generally will be at least a little lower than outside, because the attic air temperature is generally higher than outside, while the content of the air is generally the same as outside.

The exception to this being in weather where the air in the attic is at the same temperature as outside. This is more common in the Fall and Spring, but can also happen in the Winter in zones where the outside temperature is relatively mild (i.e. when the attic air temperature is as cool as the outside air temperature).

It's not clear why you believe that 50% relative humidity is "ideal" or "typical" for an attic. The actual humidity in the attic can vary a lot, and there's no reason to believe it's going to always be right at 50%. In Arizona, it may never ever get that high, and in Florida it might be higher than that all year round.

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