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The problem might be the result of an inadequate vapor barrier between the insulation and the inside air. The warm air inside is more humid because warm air can hold more moisture than the cold outdoor air. Humidity also can get trapped inside a well-insulated and sealed home. Without an adequate vapor barrier, moisture condenses somewhere inside the insulation as the warm, humid air comes in contact with the colder, outdoor air. Sort of like a mini weather system inside your walls. The same thing happens on windows, especially unventillated windows: condensation occurs on the windows as the warm inside air hits the cold window. The vapor barrier is more necessary inside walls than in attics, because the moisture has nowhere to go if it condenses inside a wall.
You mentioned that you have paper-backed insulation. The paper backing on some brands of roll insulation serves as a vapor barrier, but it is not as good a barrier as, say, a big, solid sheet of 6 mil plastic. The paper on your on your insulation might not be a vapor barrier. It would be specified on the label and product description if it is a vapor barrier.
Another potential reason might be that the roll insulation was not stapled down when it was installed. Usually, the roll insulation gets stapled on the inside of vertical walls, which is easy because the vapor barrier faces out when rolled into walls. Stapling the insulation to the walls is somewhat necessary to keep the insulation in place. However, in an attic installation, the barrier is laid on the bottom, making it tougher to do. It is also eaisier to overlook this step because you do not need to staple it to keep it in place. The angle is tough for stapling, so the builers could have just rolled it in and not secured the vapor barrier down to the sides of the joists. If the paper was not stapled down to the sides of the attic rafters/joists, then gaps could form in the barriers along the sides of each joist, allowing the warm, humid air to penetrate into the insulation.
Finally, all these problems should be solved, though, with proper ventillation in the attic. Even if it condenses, the moisture should evaporate into the drier outdoor air that circulates through the attic. The ultimate problem is that moisture is condensing faster than it can evaporate. Additional ventillation should be the easiest solution.