Why is Moisture and Frost Forming in the Attic

On a cold winter day go up into your attic and look around. In particular take a look at the roof sheathing. If you see frost on the roof sheathing, water dripping from the sheathing and roof rafters, and/or damp attic insulation then you’ve got a serious problem that needs to be taken care of quickly. If you don’t deal with these issues promptly you’re risking a major mold and mildew problem that could lead to health issues with your family in very short order. Once mold has taken up residence in your attic it can be very expensive to remove.
The main root cause of frost forming in the attic is associated with improper or the complete lack of insulation and ventilation in the attic. The first step in resolving this type of problem is to install the proper amount of insulation in the attic floor area. Visit the Department of Energy’s website to learn what the proper insulation R-value is recommended for your attic relative to your geographical area.  
Besides installing the appropriate amount of insulation in your attic, it is also critical for it to be installed properly. When installing attic insulation it is imperative that there is a complete tight seal so that warm moist air from the living spaces below doesn’t drift upwards and into the attic. 
Besides installing batt insulation between the roof rafters I would highly recommend using expanding foam insulation around any small hole areas, e.g. where electrical wiring feeds down to the lower levels of the home. These small holes can act like chimneys if they are otherwise not sealed.
Also check carefully around vent stacks and masonry chimneys running up through the attic. Often batt insulation is not tightly placed around these areas, thus creating channels for warm moist area to work its way up from the living spaces of the home and into the attic. 
The second major issue that needs to be resolved for preventing frost forming in the attic is to ensure there is proper and adequate attic ventilation installed. To prevent condensation and frost forming in the attic the attic temperature should be as close as possible to the outside ambient air temperature. 
An attic should include both soffit vents along the roof eaves and a ridge vent near the roof peak. The ridge vent should run nearly the full length of the roof peak. If ridge vent is not used then gable vents should be installed. If your attic already has soffit and ridge vents or gable vents make sure that they are not blocked by insulation.
Another culprit for causing warm moist air to infiltrate the attic is bathroom exhaust fans. Unfortunately builders often do not properly vent them to the outside of the home. Instead they simply let the bathroom exhaust fans blow out warm moist air from the bathrooms up into the attic. As a result moisture winds up collecting on the roof sheathing, roof rafters and attic insulation. With poor ventilation inevitably this moisture provides the perfect environment for mold to grow.
So eliminate condensation and frost in your attic as soon as you detect it. First make sure you stop the conduits for moisture to enter the space, and then make sure the attic is properly ventilated and insulated.