Natural methods won't keep attic cool

Ask The Builder

September 23, 2007|By Tim Carter | Tim Carter,Tribune Media Services

My attic has natural ventilation. It has triangular vents at the top side walls of my home. How do I know whether I have enough attic ventilation? My attic gets so hot in the summer, you can barely breathe up there. It is impossible to stay up there more than a few minutes. How much soffit ventilation should I have? What is the best way to cool my attic naturally without any electric-powered fans?

I applaud you for trying to ventilate your attic the old-fashioned way. But without moving large amounts of air in a short amount of time through your attic space, I doubt you will ever get its temperature cool enough to make it tolerable to spend much time there. I can't think of one natural method of ventilation that will move vast amounts of air from sunrise to several hours past sunset.

Visit a tropical island where the outdoor air temperature gets nearly as hot as where you live, and you will see how islanders have kept cool for generations without electricity. They build wide-open structures that often have no windows. They want the natural breezes to blow through the houses and buildings. This air movement activates natural air conditioning when it blows across the skin of any creature that perspires.

When we sweat, the moisture on our skin picks up the heat from our bodies. As the perspiration evaporates into water vapor, it takes this heat up into the air. This is why you feel cooler almost immediately when a fan blows across your body when you are sweating.

But lumber and insulation do not have perspiration glands. When the sun beats down on your roof, it absorbs the infrared energy from the sunlight. This heat is quickly transferred to the wood roof sheathing, the framing lumber or trusses, any objects in the attic and the insulation. All of these objects radiate the heat just like the glowing coals in a campfire.

The only way to lower the attic temperature is to pass air over the lumber and insulation, lots and lots of air. You need to circulate large quantities of air over anything that is hot, because there is no water available to increase the rate of heat transfer.

Traditional static air vents in a roof don't provide enough air movement, and they work solely by convection. This means that as the air inside your attic heats up, it tries to float through the static vents. Floating air does not move quickly.

Turbine vents are a great way to move quite a bit of air through your attic, but they only work well when there is a strong, sustained wind blowing outdoors. You can't count on this in all areas of the country. Some places do have lots of wind, like the sea coast and land by other large bodies of water, but even then your attic may get so hot five or more turbine vents might not keep up with the heat being generated.

If you want your attic to be as cool as possible, you will need electric-powered ventilation. You need to move thousands and thousands of cubic feet per minute of air through your attic to pull away heat from the roof sheathing, framing timbers and the insulation. Even with the best fans operating at full speed, my guess is you would have a very hard time keeping the attic air temperature much below 115 degrees.

Expert home builder and remodeling contractor Tim Carter has 20 years of hands-on experience in the home industry. He is a licensed master plumber, master carpenter, master roof cutter and real estate broker. If you have a question, go to askthebuilder.com and click on "Ask Tim."

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