Frost gets into the attic from air leaks, or attic bypasses. A few of the most common places you'll find attic bypasses are, such as plumbing vents, furnace vent, space around masonry chimneys, recessed lights, attic fans , 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 its way back into the attic. Finally, the space below the knee walls in old one-and-one-half story houses can be a major source of air leakage.
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.
More Indoor Humidity = More Frost in the Attic
The more humid a house is, the more frost you'll find in the attic. The houses with the worst frost problems always have a whole-house humidifier running. Turn your humidifier off , If you have a frost problem in your attic, be sure to take care of all the easy, obvious stuff before crawling around in your attic.
Train everyone in the house to run the bathroom fan for 30 - 60 minutes after every shower or bath; this is how long it takes to get indoor humidity levels back to normal. Just running a fan while taking a shower won't do much.
If you don't have exhaust fans installed in bathrooms that are used for showers or bathing, fix that. I don't care what the building code says;you need a fan in these bathrooms.
If you have a kitchen exhaust fan, use it while cooking. Ovens generate a lot of moisture.
Consider installing an HRV (Heat Recovery Ventilator) if you don't have one. HRVs replace damp indoor air with dry outdoor air, and recapture a fair amount of heat at the same time. This will certainly lower humidity levels in the home. If you already have an HRV, make sure it's, properly installed, properly maintained and operating.
If you have too many plants in your home, get rid of 'em. If you have a damp basement or a crawl space with no vapor barrier, fix it. These are both major contributors to indoor humidity and attic problems.
House pressure affects frost
With all other factors being equal, the air in your house sees your house as a very wide chimney, because warm air rises. The trend is to have air leaving the house at the top, and entering the house at the bottom. The taller the house, the greater this effect. Split level homes with more than one attic space will always have the worst attic problems at the uppermost attic.
When a home has a combustion air duct connected to the return plenum, the house gets pressurized when the furnace runs, which increases the effects of attic air leaks. Combustion air ducts should not be connected to return plenums; they should just be dropped down into the room.
What about more Roof Vents?
Focus on all the other stuff listed above first. Proper ventilation in the attic may reduce frost accumulation, but if done wrong, simply adding more roof vents might actually make for morefrost.