Read this document from Canada’s housing agency CMHC:
Here is an excerpt that may shock you:
"What To Do About a Wet Attic
There are many signs that an attic is wet. Prolonged wetness will rot out the roof sheathing. Often this is first noticed when re-shingling. If you have ceiling leaks only in the spring, it may be that ice has been forming on the sheathing all winter and it suddenly melts when a warm spell arrives. You may see water stains or evidence of mold on the sheathing, rafters, or trusses when you are inspecting the attic. You may find the insulation has been packed down or stained by water or ice. The smell of a moldy attic will enter the house under certain weather conditions, usually in summer.
The usual response is to increase attic ventilation. This is the wrong approach. In some cases, adding ventilation will actually pull more moist house air up into the attic and make the problem worse. (I have worked on houses like this)* The best way to fix a wet attic is to stop air movement, or leaks, from the house. Once this is done, the existing ventilation is usually more than enough to keep the attic dry."*
My own instructions are:
(1) clean up moisture sources such as damp basements, don’t store damp firewood indoors, don’t dry clothes indoors, etc
(2) Vent all areas/appliances that produce moisture to the exterior (kitchens, baths, clothes dryers)
(3) Fully airseal at the attic floor level. (this is an energy saver also!!)
(4) You now probably won’t have to add more attic ventilation.
A small bit of frost on nail points/shanks is not a large problem and won’t cause problems…perfection is not needed but management of the moisture. I don’t like adding attic ventilation if there is already some there, I have seen too much snow blow into attics…something that was mentioned in the last week by another poster.