Roof Vent Analogy


A simple, and wrong, roof venting analogy often used as an example when trying to support the reasoning for closing off roof vents when installing ridge vents

Think of your roof ventilation system as a drinking straw. The top of the straw being the ridge , or highest, roof vent that exhausts the air from an attic. The bottom is the soffit or eave vent that allows cool replacement air into the attic.

Now, cut a hole between the top and bottom of the straw and let that represent any other vent on the roof. It could be a powered attic ventilator, or a static roof vent, or maybe a gable end louvered vent.

Now place the straw in a glass of water and try to get a drink. Practically impossible. Air is sucked in through the hole in the straw and you go thirsty. The water stays in the glass.

This analogy would be useful if we were trying to suck water into the attic through the soffit vents. 

But most of us, maybe all, don't live in a house surrounded by water that we want to suck into our houses. We prefer to keep the water out.

So, instead of trying to suck water into the straw, try putting the end of the straw into "air".

Now,place a finger over the bottom of the straw and inhale. Feel the suction?

No, it won't be quite as strong as if there were no hole higher on the straw, but, if that middle hole were restricted in any way, say a cap or screen or even negative pressure from wind blowing over the outside of the straw, the difference in suction would not be significant.

Go ahead, try it for yourself. 

Leave your gable vents open.

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