Proper attic ventilation is necessary to remove moisture and heat from air in the attic. This protects the roof from problems such as rot and ice damming. Instead of blocking attic air vents to reduce heat loss from the house, use adequate insulation between the living space and the attic.
During the winter, warm, moist air from a house's living space rises into the attic. If the attic becomes too warm, snow can melt on the roof and refreeze in the gutters, creating ice dams that may damage the roof. Moisture can also condense on rafters or roof sheathing, encouraging rot. In the summer, poor ventilation can result in superheated air being trapped in the attic, overheating the shingles and raising cooling bills.
Attics need about 1 square foot of vent opening for every 150 square feet of attic space. A combination of rafter vents and soffit vents ensure good passive air flow. Powered attic ventilators are usually unnecessary and can actually do harm by creating negative pressure in the attic. This has two unwanted consequences: the negative pressure can draw up air-conditioned air from the living space, forcing the air conditioner to work harder, and it can cause back-drafting from combustion gas appliances, possibly introducing toxic carbon monoxide gas into the home.