Why is there frost build up in my attic?
Moisture is ever-present, yet invisible in the air. Added moisture is generated by daily activities like cooking and bathing. In the winter here in the Midwest, additional moisture is often purposefully generated by a humidifier to prevent dry skin, sore throats, dry nostrils, chapped lips, static electricity, and so forth.
Warm indoor air, now moist, rises into your attic space where it collides with cooler air. When the air gets cold, it loses the ability to carry the moisture resulting in small droplets of water called condensation. This condensation will collect on hard surfaces. As temperatures drop, the droplets freeze.
A few seasonal cycles of this freezing moisture can damage wood sheathing, joists, and generate attic mold.