A properly ventilated attic space is so very important to not only the shingles above the attic, but also the living space below it. In essence, all ventilation is about exchanging and circulating air to keep it fresh and to reduce moisture levels. About 90 percent (yes, you read that number correctly) of homes in the US are improperly ventilated and as a result have unreasonably high levels of moisture. Here are some important facts to consider when thinking about your attic space.
To properly ventilate an attic space you need two things: a source of cool air entering from a low point in the roof line, and point where that cool air can leave the attic space from a high point in the roof line. While there are many ways to achieve this, the most common method is through the cool air entering through vents in the low soffit line, rising up through baffles that run inside the roof framing, and leaving through one of many vents as close to the roof's peak as possible.
One common misconception we see is thinking that because heat rises, ventilating an attic space during the winter means you're releasing warm air and creating a drag on your heating efficiency. Poor insulation, not ventilation, is usually the culprit, although if you enter the attic on sunny, winter day, your attic space can be warmed by the sun more than the heat escaping from your ceilings and into your attic.
Unless your roofing system has insulation on the roofing deck and is designed without ventilation, your furnace should not be heating your attic directly or indirectly. Worse yet, inadequate insulation is almost surely allowing moisture-laden air into your attic. When this warm, moist air hits your roof, it's likely to form condensation that will lead to further deterioration of your insulation and/or wood rot. If you think this might be a concern in your attic, you can test your theory by waiting until the sun goes down and measure the temperature in your attic. It should be pretty close to the outdoor temperature.