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What is the rule of thumb for spacing soffit vents for attic?

What is the rule of thumb for spacing soffit vents for attic?

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Old 03-10-09, 11:28 AM
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What is the rule of thumb for spacing soffit vents for attic?

An inspector years ago told me that soffit vents should be installed X number of feet between each other. I cant remember what he said.

Is there a general rule of thumb for spacing of soffit vents?
 
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Old 03-10-09, 11:38 AM
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The distance between vents will vary, depending on the volume of ventilation needed. The ratio for most homes is 1 sq ft of ventilation for 150 sq ft of attic space. The resulting figure would be divided in half, with half being exhaust, and the other half being intake (soffit vents).
 
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Old 03-10-09, 01:40 PM
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Originally Posted by XSleeper View Post
The distance between vents will vary, depending on the volume of ventilation needed. The ratio for most homes is 1 sq ft of ventilation for 150 sq ft of attic space. The resulting figure would be divided in half, with half being exhaust, and the other half being intake (soffit vents).
Is there any cons in having to many soffit vents?
 
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Old 03-10-09, 01:57 PM
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Too much soffit ventilation can lead to an excessive amount of air movement in an attic space, which can decrease the effectiveness of your insulation. Fiberglass, for instance, will only have it's posted R-value in a dead air space. Air movement actually carries the warm air trapped within the insulation and carries it right out the roof. So there is a reasonable limit on how much ventilation you want... and in most cases, that is the 1:150 ratio mentioned above. You can have that split as much as 40:60 between exhaust and intake, but probably not much more.

I've also seen the effects of crosswinds in some homes that actually blew insulation away from the walls. Or blew loose cellulose insulation clean off the ceiling and into the soffits on the leeward side of the house. In windy conditions, you don't necessarily have a bottom to top airflow... you'll often have air exiting out the leeward side soffit vents like it was an exhaust vent.
 
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Old 03-10-09, 02:22 PM
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[QUOTE=XSleeper;1535709] Air movement actually carries the warm air trapped within the insulation and carries it right out the roof. /QUOTE]

I see. It may benefit me though having more vents, since I live in Louisiana. It hardly ever gets cold here, sometimes it 70-80 degrees in the winter. Getting that hot air out of my attic would help my electricity bill.
 
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Old 03-10-09, 03:08 PM
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And area of soffit vents should about equal ridge vents area.
 
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Old 03-10-09, 03:54 PM
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Originally Posted by Just Bill View Post
And area of soffit vents should about equal ridge vents area.
I don't have ridge vents.
 
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Old 03-10-09, 04:13 PM
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Originally Posted by XSleeper View Post
The resulting figure would be divided in half, with half being exhaust, and the other half being intake (soffit vents).
Your exhaust can also be can style vents, or gable vents depending on your roof configuration. From what I've heard, ridge vents aren't usually recommended in hurricane prone areas.
 
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Old 03-10-09, 05:24 PM
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Yea, I have gable vents. They are about a triangle, about 3ft along the bottom and 15 degrees angles.
 
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Old 03-11-09, 06:18 AM
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A couple of comments and these are from memory so please correct if wrong. As I remember the ratio is 1 to 300. 1 to 150 applies when all of the venting is above the soffit i.e. no soffit vents.

If your gable vents are as described, they are too small, even with the 1 to 300 ratio and 40/60 split.
Real rough estimate says your attic needs to be 750 sq ft or smaller, or you need more gable venting.

Measure your attic area and measure those gable vents and we will give you some numbers.

Bud
 
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