To start, frost shows up in the attic when moisture-laden air from the house gets into the attic. That’s about it, pretty simple. When the moisture gets into the attic, it condenses on the roof sheathing in the form of frost. The frost itself doesn’t do any damage, but once it melts things get wet, which is when the damage occurs:
Melting frost can lead to deteriorated roof sheathing.
Mold on the roof sheathing.
Water stains on the ceilings.
All bad stuff, you definitely don’t want frost is your attic.
Frost comes from air leaks
Frost gets into the attic from air leaks, or attic bypasses. check out my post on Attic bypasses, for some good examples of attic bypasses. Of course, any type of exhaust fan needs to be exhausted directly to the exterior, and never into the attic. Even if the exhaust fan is aimed at a roof vent, this isn’t good enough. A lot of moist air will still find it’s way back into the attic.
The best way to prevent frost from accumulating in an attic is to seal off attic air leaks. While seemingly small air leaks may not seem to be important, these can add up to a lot of frost accumulation in the attic. It’s important to seal all attic air leaks; not just the big ones. Once every little air leak has been perfectly sealed, the attic will be frost free. The only problem with doing all of this air sealing is that the air leaks are located underneath the attic insulation, and it can be very difficult to find every air leak without completely removing the attic insulation. For this reason, it’s nice to start with the easier stuff first.