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People often think that they should block attic vents in the winter to keep the attic warmer. This is not true. You should leave all the attic vents open and consider the attic area as part of the unconditioned outdoors. The insulation on the attic floor will keep the heat inside your house.
Moisture from inside your house works its way through the attic floor and gets into the attic insulation. Even with a good vapor barrier, this still happens to some extent. Outdoor air flow through the attic is important to carry this moisture away. If you block the attic vents, the moisture stays in the insulation and most likely will get cold enough to condense into water. With mild condensation, the insulation gets wet and loses much of it insulation value. With heavy condensation, the water can soak the lumber and cause serious structural damage.