The difference in the two spaces is the dry bulb temperature.
Reduce moisture in attic.
The federal housing administration has recommendations for the amount of ventilation you will need based on the size of your home.
Sealing these can greatly reduce the moisture drivers.
Ventilating this space can also reduce heat that radiates up through the roof and down into the living areas of the home.
Check seals sweeps weather stripping around doors and windows make an assessment if the equipment may be significantly oversized.
These conditions prevent warm damp air from migrating to the attic in the first place.
Proper ventilation can help circulate the air reduce the heat build up in the attic and keep down humidity levels.
Regularly airing out your attic is a very simple and effective way to prevent the formation of attic moisture.
So the first step is to seal the air leaks.
Here is a link to an article that describes the work.
Proper precaution in an uninsulated attic such as the department of energy s recommended r30 to r49 insulation for warm climates r30 to r60 for moderate climates and r49 to r60 for cold climates.
Right now we just have eave vents and gable vents.
If you raise the attic temperature the rh in both places would be the same.
Look for can lights gaps around boots into the space holes in walls between the attic and crawl space and the living space etc.
Open windows and use ceiling fans to keep the air circulating in your home and avoid stuffiness.
By adding moisture to the lower floor you increase the partial pressure of the water vapor which pushes it to areas of lower vapor pressure like the attic.
Air sealing an attic.
You want to use a ridge vent that goes down the peak of the roof.
During winter months humidity from this escaping air can condense on cold attic surfaces leading to mold.
Limit warm air reaching the attic one of the best things you can do to limit attic humidity is to keep the rest of your home cool and dry.
So they equalize pretty quickly.
High humidity in an attic is usually caused by air leaks in the ceiling of the top floor of your house.
Simply by sealing the various possible paths of air leakage already discussed infiltration will be reduced therefore restricting the movement of air and eliminating the stack effect.
This can reduce moisture as well as the temperature in the attic which can make it far more difficult for mold and mildew to gain a foothold in the space.
Do you have a ridge vent right now.
Once a week open your attic door and leave it open for several hours.
If you live in a warmer climate you may want to air out your attic twice a week.