Thursday, August 27, 2009

Up, up, and away!

Before I proceed to talk about air outflow, a small diversion into the theory of operation of a modern ventilated attic might be in order. Most of us are taught at a very early age that hot air rises. In fact, this basic principle is the foundation for the entire self-ventilating attic concept, an idea elegant in its simplicity yet apparently surprisingly hard to come by in the real world.
The idea is that the attic will naturally heat up during the day, from radiated heat from the sun hitting the roof as well as convected heat from the house below (cooking, lights, computers, hot showers, etc.) If this heat is given a vent to flow out of, and if there is a suitable vent for cooler air to flow in from, the basic principle of hot air rising will force the hot air out, and the cool air will be naturally drawn in by the reduced air pressure (the suction left behind, if you will). The elegance of this temperature-difference-driven cooling engine is exhibited by the fact that it has no moving parts to wear out and that it uses no electricity. In short, a properly ventilated attic puts the laws of physics to work for you!
So, back to my particular attic. Having satisfied the rule of thumb for the amount of air intake for my size attic, I needed to make sure that my air outflow was properly accounted for. As it turns out, the two small square vents that I had were a good bit short of the net free area that I needed. The net free area is a measure that can be found on all ventilation devices, and it is less than the actual measured area of the device since all of them have some sort of covering, grill, or mesh that impedes airflow. In addition, my vents were a good 30% below the ridgeline of the roof. In other words, the top 30% of my attic airspace would heat up all day, and never leave the house. This, once I thought about it, was intolerable. I was keeping the hottest portion of the air in, while venting the cooler portion out! I might as well have stood there all night, tearing up dollar bills, because that is effectively what my house was doing via my energy bills.
In short, dear readers, the situation with my attic outflow was intolerable. How could I best get more outflow area? How could I get it located higher up on the roof? I returned to my computer and began more web research...

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