Roof Vents


Old 04-21-05, 06:56 AM
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Roof Vents

I have about 1700 sq ft of attic space (based on 2nd floor sq footage). How many roof vents should I have? I had an air handler installed in the attic and it wasn't cooling properly. The HVAC guy said my attic was getting too hot and needed more ventilation. There are currently 5 vents plus multiple soffit vents. Any thoughts or opinions?
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Old 04-21-05, 11:43 AM
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1 sq ft of vent for 150 sq ft of attic when there is no intake...
1 sq ft of vent for 300 sq ft of attic when there is soffit intake.

That's what I've heard, anyway. The following link seems to agree.

According to that, you need 5 2/3 sq ft of vent, be it can vents or ridge vent. Ridge vent is more effective because it draw air up each rafter bay, while can vents usually will only draw the hottest air out the rafter bay they are located in.
Old 04-21-05, 04:11 PM
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Whether it's 1/150 or 1/300 depends on whether your attic insulation has a vapor barrier or not (1/300 if you have a vapor barrier and 1/150 if you don't, but check your local codes to be sure in case they might want 1/150 regardless)

And the way I've heard it explained is that the amount of square feet of ventilation you have should be split 50/50 between the top half of your roof and the bottom. A balanced system would have an equal amount of intake ventilation to exhaust ventilation. Ridge vents are exhaust, soffit vents are intake, roof vents could be either depending on where they are placed.

Gable vents can also be either. You could have a gable vent on both ends of the roof, use one end for exhaust and the opposite end can be used for intake by connecting it to the bottom of your roof with ducting.

I was also told to remember that the formula gives you square feet, not number of vents... Typical roof vents, I've been told, are around .75 square feet.

Also, make sure your soffit vents aren't obstructed. You might also have the option of putting in forced ventilation

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