How to Prevent Attic Condensation

Add a roof edge sheathing intake vent. This is a vent that is louvered, and is installed beneath the shingles near the peak. It is best to add this when replacing the shingles on the roof.

Install a membrane that's waterproof (but takes nails) on the bottom 2 feet of the roof. One example is by WR Grace: the Ice and Water Shield. Placing this membrane on the roof will help keep ice dams from forming, or ice melt leaking into the building.

Put insulation in your attic. If the attic is unfinished, add some to the attic floor. Be certain there are no air spaces. If you install insulation under the roof, ventilation can be prevented (lack of air flow), and the risk of mold forming increases.

Stuff insulation between the rafters, then cover it with a vapor barrier if you are dealing with a finished attic. Reflectox is a vapor barrier, which can be found at a hardware store or building supply store. It is a reflective sheet which helps keep moisture from penetrating the insulation. On top of the vapor barrier, put up some drywall to finish the interior of the roof.