By Kenton Shepard

Most of us agree that keeping the attic cool is a good thing. Keeping the attic cool helps keep the roof-covering material cool which extends its lifespan. During the summer, ventilating the attic helps lower the cost of keeping the home cool. During the winter it helps prevent ice dams from forming and damaging roofing material.

In the past, popular thought held that the best way to vent an attic was to provide inlet vents along the lower edges of the roof, usually in the soffits, and an outlet vent, usually a continuous ridge vent, at the roof peak. The theory was that since hot air rises, this would provide for optimum flow as the roof and attic area became hotter during the day. Hot air rising is called "thermal boyancy".

The newest thinking on attic space ventilation is that air pressure caused by wind moving past the home has a much greater effect in moving air through the attic space than thermal boyancy. Here's why...

As wind blows past the home, a vacuum (an area of low air pressure) is created on the downwind side of the home. As we all know, nature abhors a vacuum and tries to fill it. Anything that can move into that area of low pressure will move into it, including air from the attic. So basically the theory is that air will be pulled out of the soffit vents before it has a chance to rise and exit through the countinuous ridge vent.

On a day when there is no wind, or when the wind blows parallel to the ridge line this this system will obviously not work as I have described it.

Using the same principles (air moves into areas of low pressure) a continuouse soffit vent which has baffles which route the wind up and over the vent cover will be more effective in vetilating the attic than using a vent with no baffles. As the wind move up and over the vent cover, an area of low pressure is created just above the vent cover which pulls air out of the attic space.

This all works on the same principle which moves a sailboat through the water. It's not the wind pushing on the sails, but the area of low pressure in front of the sail, created when the wind blows past the sail, into which the boat is pulled by the sail. 

One last note on keeping roof-covering materials cool. Ventilation has maybe a 5% effect on roof material temperature. The greates effect by far is the color of the roofing material. Black roofs absorb around 95% of the light stiking them and turn the light into heat. White roofs will reflect around 95% of the light striking them and if the light is reflected, it's not turned into heat.