What are common signs of moisture problems inside an attic?

Here is an overview of the common signs of moisture trouble inside the attic.

The first and most obvious are the wet spots on the ceiling. Usually these are signs of a very severe moisture problem because it means that there is enough water present to wet all the way through the attic insulation. The next sign, in levels of obviousness, is the actual wet insulation, usually accompanied by mold and mildew. Mold and mildew exist where there is a persistently moist area, which allows it to grow. If the moisture is not so severe to show through to the interior, the insulation might be able to dry between periods of condensation and not build up enough moisture. The next sign of a moisture problem is condensation on nails, the underside of the roof deck and the rafters. Condensation can occur with very small amounts of moisture present in an attic because it is a function of both humidity in the attic and temperature of the surfaces. In fact, the point of condensation is so precise that you may see condensation on the underside of roof deck on the north side, but not on the south side. This is because the sun raises the temperature of the roof deck on the south side, but the cooler north facing deck is just cold enough to condense the moisture. The first thing to check for is what are the sources of the moisture and how is it getting into the attic. There are many moisture sources, but here are some of the most common ways for moisture to move into the attic. Attic bypasses (recessed can lights, attic access door, electrical chases, etc.) can create moisture problems. Bathroom exhaust fans or kitchen range hoods being exhausted directly into the attic dumps too much moisture into the attic. These fans should be ducted straight up and out through the roof. Do not run the duct horizontally to the soffits for two reasons: 1) There will be condensation in the duct, it will be cold in the winter, and if there is any sagging in the duct the water will collect in the those low spots and potentially result in the molds and mildew; 2) depending on the wind dynamics the moist air may be pulled right back into the attic through intake vents ? Wet crawlspaces can allow moisture to migrate up through the walls and into the attic. Moisture problems in the attic may also be attributed to a poorly installed attic ventilation system. For example, a power vent or ridge vent without enough intake ventilation, no holes cut for the intake vents or exhaust louvers, no slot cut for a ridge vent, the felt covering the slot of a ridge vent, or a bag covering the wind turbine.