A properly installed soffit and ridge ventilation system, creating what's called "a cold roof", is one of your best defenses against ice dams and other moisture problems in the home. Such a system also increases your home's energy efficiency and helps to prevent premature deterioration of roofing materials, insulation, and other structural components. If you're having roof work done, this ventilation installation is a natural compliment to the project as a preventive against future problems. If you're experiencing problems already, no treatment of the symptoms is complete without the ventilation to keep them from happening again. Here's how the system works in one of its more classic forms.

To the right we see a stylized illustration of a home without any sort of proper ventilation for the roof. This is, in effect, like putting a plastic bag over your home. The heat generated within the home will eventually rise; this is a law of nature. As the heat rises, it carries along with it excess moisture (generated by all sorts of factors stemming from everyday life - breathing, cooking, showers, etc.) and other indoor air pollutants (mold, mildew, germs; even the cleanest house has these in the air). This heat keeps going up until it reaches the highest point in the house - the roof. With no ventilation, this heat has no place to go; it becomes trapped in the attic. Along with that trapped excess heat, the excess moisture and indoor air pollutants also remain. This is the start of major moisture problems and also what is known as "sick home syndrome".

As the excess moisture remains trapped beneath the unventilated roof, more heat (and along with it yet more moisture and more indoor air pollutants) continues to rise. Very soon, the air becomes stagnant and reaches the point of saturation. The excess moisture can no longer be held entirely as water vapor, and it begins to condense back into liquid water form. This is the start of a number of problems. The structural members around which this condensation occurs themselves become saturated and begin to rot prematurely. Any nearby insulation which becomes saturated loses its R-Value. Also, the heavier water tends to sink back down, saturating the ceilings, perhaps dripping into the walls... leaks, stains, and more structural rot can and will occur wherever this excess moisture finds a way back down to the living area. Some of this moisture will take those indoor air pollutants with it, which can cause or aggravate health problems for the people living in the home. Also, any excess heat which does manage to escape will do so straight through the roofing materials, causing shingles to cup and curl and further damaging structural members.

In order to solve the problems presented by excessive trapped heat, moisture, and indoor air pollutants, a means must be provided for these things to escape. To do this, the roof must be properly ventilated. One of the most common and effective means of this is to create a "cold roof", which is generally done by installing a combined soffit and ridge vent system. The portion under the overhang, indicated by the green "1", is the soffit. Here, at regular intervals, holes are drilled to allow outside air to come in. These ventilation holes are covered with perforated panels to assure that nothing other than air can enter. Meanwhile, a section of the peak of the roof, or the ridge, as indicated by the green "2", is cut away and covered with a ridge vent, which is designed as a passive home exhaust system, so to speak, and put together in such a fashion as to again not allow anything foreign to enter the home, while still allowing a means for the air to move. Often, ventilation chutes will be placed in the rafter cavities between the soffit and ridge to help direct the air flow.

The ventilation system is completely passive, requiring no external power source to function, powered by basic physics. As the outside wind (indicated in the drawing by the purple arrows) blows over the ridge vent, it creates a suction, drawing air from below up through the vent. Fresh, clean external air enters through the soffit vents and rises up to be expelled back out again via the ridge vent. This fresh air also takes along with it the excessive trapped heat, moisture, and indoor air pollutants which were previously stagnating in the attic. Air pressure and thermal effect keep the whole thing working. With the ventilation system in place, there is no opportunity for excessive moisture and pollutants to become trapped, thus greatly reducing the risk of interior moisture damage and "sick home syndrome". Such a system also increases the life span of your roof and insulation, and also serves to benefit your home's energy efficiency, helping the home stay cooler in summer and warmer in winter.

A proper venting system does more than just reduce excessive amounts of trapped heat, moisture, and indoor air pollutants. It also creates a go-between shield, if you will, between your home and the outside atmosphere. By allowing a fresh air flow directly beneath the roof, the roof temperature is effectively made the same as that of the outside air, as illustrated by the little thermometers in the picture. Among other things, this is an essential preventive measure against ice damming, since the roof's temperature equilibrium and controlled expulsion of trapped heat prevents premature melting of the snow pack. It should also be noted that the heat being expelled is excess heat; proper ventilation does not drive up heating costs; just the reverse. By increasing your home's energy efficiency and preventing the premature rot of roofing materials and degradation of insulation, a proper venting system can help make your home's heating system more effective in winter and air conditioning more effective in summer. If the attic area contains a living space, the ventilation system is designed to do its job between that space and the roof, allowing the attic rooms to enjoy the same level of comfort as the rest of the home.