Attic condensation can decrease the effectiveness of insulation and be a cause of significant damage and problems in the attic and under-roof areas.
Possible sources of attic moisture:
1) Roof leaks (around chimneys and skylights, eaves and shingles...);
2) Attic leaks (ceiling holes and penetrations)
4) Indoor moisture;
4) Inadequate attic ventilation.
Whatever the source of attic condensation, you should act quickly to prevent more serious damages.
Moisture can damage the attic and infiltrate the insulation materials, reducing its R-value and increasing your home’s energy consumption.
Inspect your roof for signs of water damage. Damaged roofing materials increase the risk of water infiltration. Liquid water - coming from rainwater and snowmelt - will move downwards through openings, or wick through materials.
Look for damaged shingles, but also damaged membranes, or problems with or around chimneys, skylights and vents.
Chimneys are often poorly sealed, but the flashing, the underlayment and details associated with the installation of skylights can also be a source of water leaks (see Skylights and condensation).
Look also for problems with the attic venting system. See, for details: Roof and Attic Ventilation.
Moisture coming from Indoors Can Move into the Attic
Do not underestimate the importance of indoor moisture, and their possible relationship with attic problems.
Showers, laundry, people, firewood and cooking can create large amounts of water vapor, that can end up into your attic space, where it may condense and fall, soaking the insulation or moving into your building's cavities, condensing on contact with cold surfaces. Water vapor and liquid water can move in multiple directions, and penetrate porous materials (concrete, wood, drywall) or use openings and gaps, causing huge damage.
To minimize such problems, you may use exhausting fans in the bathrooms and kitchens, or open the windows (whenever possible) and avoid drying clothes indoors... Or you can install a vapor barrier in the ceiling, to reduce moisture transmission and prevent the build-up of moist air. It may be important.
But not enough. You should seal all the air leaks involving the chimney and the attic floor, such as those surrounding plumbing and wire penetrations, or attic hatches, or ceiling fixtures leading to the attic.
And you should also solve moisture problems in the lower parts of the house, or gaps and openings around the windows and doors. See: Moisture, Drainage and Foundations.
Moisture & Attic ventilation
Pay also attention to the attic ventilation system. Attic ventilation can be useful to prevent overheating during the summer, but may also cause moisture damage.
Wind-driven rain and snow can leak into the attic through poorly located vents, or with inadequate design...
Make sure that you have a good attic ventilation, or that the bathroom fans and clothes dryers are vented to the outdoors, and not to the attic.