We recently wrote about ice dams on roof overhangs, how they are created and how to prevent them. Our discussion included improving attic ventilation and adding ceiling insulation to keep the house warmer and the attic cooler. Often, there isn't enough room in our column to elaborate on all of the methods available to solve a particular problem. So, this week we will cover various attic ventilation methods. Keep in mind that our tips affect summer comfort too.

First we'll dispel an old wives tale that you should cover up your roof vents in the winter to keep the cold out of the attic and the warm air in. An attic must be well ventilated year round. Never cover a roof or eave vent, no matter how cold it gets. The trick to keeping your house warm is to have a thick layer of ceiling insulation. Covering the vents when it gets cold will cause reduced air flow in the attic which in turn will promote excessive condensation. This can result in mildew growth and other types of moisture damage. The condensation can get so bad as to trickle down the rafters into the house giving the illusion of a roof leak. Constant air flow in the attic will reduce condensation _ year round.

Insulation has its limits. Regardless of the R-value, the greater the difference between inside and outside temperatures, the greater the heat loss or gain will be. Hot air always travels toward cold air. So, in the winter warm air escapes to the outside. And in the summer warm air travels to the inside. Make sure that you are well insulated, and that the air cannot pass through holes between the ceiling and the attic, or vice versa.

Attic venting is always included as a standard part of construction when a new home is built. Codes require a specific amount of ventilation. But the building department doesn't necessarily specify what is best - only what minimums must be met. In the case of attic ventilation, the more the merrier.

Vents are positioned to allow air into the attic at its lowest point _ the underside of the roof overhang (the eave or soffit). And since warm air rises, exhaust vents are mounted high on the roof, at its ridge, above the ridge or near the peak of a gable wall. These are known respectively as roof vents, ridge vents, cupola vents and gable vents. The gable vent is the only type that is not actually mounted somewhere on the roof itself.

So why are gable vents used, you ask? With one there is less risk of leaks. This is because it is separate from the watershed configuration of the roof covering. And, installation is cheaper because it doesn't have to be woven into the shingles. But even with gable vents we like the idea of seeing them mounted on the roof as well. Also, the ridge-vent system is effective, reasonably easy to retrofit and less expensive than other alternatives. Adding a ridge vent to an existing home is more user friendly than other alternatives. By the way, the cupola is not an addition that will necessarily work well with every home's architecture. Study such an addition thoroughly. Even though the cupola is the highest point on the roof, costs could easily cause this good-looking alternative to be scrapped for a more practical retrofit.

The higher the exhaust venting is located on the roof the more effective it will be. Air currents are created between the upper and lower vents. As the hot air rises and leaves the attic, cooler air is drawn in from the vents below. The siphoning action that results can cause air to move in an unusually rapid cycle. This is called passive ventilation. After you provide the inlets and outlets, Mother Nature does all the work. Sometimes though, nature needs a jump start. This is when active methods should be employed, such as turbine vents and electric attic fans. Since these are sometimes difficult to use in snow country, it is a good idea to consider their use during non-snow months only.