With the introduction of humidifying systems to our central HVAC systems have come issues with over humidifying our homes especially here in the Chicago area.
 During the winter months it is important to maintain your Indoor Relative Humidity between 30%-50%.Warm air moves from the inside of the home outward through unseen openings. If your humidity is elevated, let's say in the 60% to 80% range you may notice frost forming in attics or on interior walls and ceilings.
 Frost may form in your attic on the roof sheathing as a result of inadequate ventilation or inadequate insulation. Warm moist air will enter the attic through improperly sealed attic access doors,light fixture electrical boxes and any other unsealed openings into the attic. As the weather warms up, the sun warms the frosted areas and it begins to "rain" in the attic. I have done many home inspections where this is the norm.
 When your exterior walls are improperly insulated, the moist air will find its way through your interior walls and find the cold spots and frost will form behind the drywall and even on the drywall inside the home. Be careful not to place furniture over a supply vent near an exterior wall as this may divert heat directly onto the exterior wall and accelerate the problem.
 This then can become an indoor air quality issue as mold has a nice moist area to grow. Poeple with mold allergies will be affected by this. Bottom line is maintain your Indoor Relative Humidity around 30%-50% and these problems should not occur.