In September I added blown cellulose to my attic. I then covered the cellulose with plywood in order to have a usable space for storage. I have a walk-up stairway that does not seal very tightly. When I went in the attic to bring Christmas decorations down, I noticed it was damp. I installed a humidistat, and it read 90% humidity. I have a ridge-vent, and I installed sofit vents before blowing in the insulation, so I am not sure where the humidity is coming from, or why it is staying in there.
Your problem is warm air leaking into the (now) cold attic. In the past, air leaks existed but the attic was warm because of the lack of insulation. When the air is warm it holds a lot of moisture, so the “relative” humidity was low and there was no moisture problem.
The solution is to air-seal between the attic and the heated space below. Seal around the chimney and all penetrations, including the access door. If you stop the air leaks you will solve the problem and save lots of energy.
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