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Old 07-05-2007, 05:21 PM   #1
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Soffit Vent Sizing - Really Hot Attic


I live in Dallas and my attic gets really hot. I decided to investigate why. I have 6 passive, dome vents on the roof. Each has an opening of 144 sq. inches. 144 x 6 = 864 sq. inches. Base on the standards that I see, I should have a minimum of 672 sq. inches of exhaust for a 2800 sq. foot attic. So I have enough exhaust. Is this correct?

Then I decided to check the soffit vents. I have 16 soffit vents. The covers are 6" x 16", so I figured the holes in the soffit were 4" x 14" or so. If that was the case, 4" x 14" = 56 sq. inches x 16 vents = 896 sq. inches. That should be more than adequate to provide enough intake air for the exhaust. Is this correct?

Here's the problem: When I removed three of the soffit vents, I found that 2 of them had a 2" x 4" hole and 1 of them had a 2" x 2" hole. Assuming that all 16 had a 2" x 4" hole, that's 8 sq. inches x 16 = 128 sq. inches. That's only about 15% of what I think I need. Is this correct?

So If I cut all the vent holes to "4 x "14 I should solve the problem. Is this correct? My attic can be 160 degrees when it's 105 degrees outside. If I fix the ventilation problem, what can I expect in a reduction in attic temperature. How significant a savings will I see in cooling costs?

Thanks in advance for any help.

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Old 07-05-2007, 10:55 PM   #2
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I can't give you hard numbers, but you'll notice the most change as the inlet equals the outlet.

Another thing to look into is whether or not you have insulation depressors. If not then the insulation in the attic may be blocking the air flow.

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Old 07-05-2007, 11:17 PM   #3
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Originally Posted by ldanielrosa View Post
I can't give you hard numbers, but you'll notice the most change as the inlet equals the outlet.

Another thing to look into is whether or not you have insulation depressors. If not then the insulation in the attic may be blocking the air flow.
I thought as long as the inlet was equal to or greater than the outlet you will be fine. Why would they have to be equal?

There are air baffles for every vent except for over the garage area where there isn't any insulation. I haven't seen any insulation filling up the soffit area.
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Old 07-05-2007, 11:24 PM   #4
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Soffit Vent Sizing - Really Hot Attic


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Originally Posted by dmo069 View Post
My attic can be 160 degrees when it's 105 degrees outside.
Anyone know what is considered an "acceptable" temperature rise?
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Old 07-06-2007, 12:42 AM   #5
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Originally Posted by tima2381 View Post
Anyone know what is considered an "acceptable" temperature rise?
I have read that 10 Degrees more than the outside temperature is what you should aim for.
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Old 07-17-2007, 11:59 AM   #6
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Soffit Vent Sizing - Really Hot Attic


If you have that much of a difference in temperature then there is something wrong. The fan is the wrong size, not working properly or the intakes are inadequate for the fan size. If you have the instructions for the fan it will state how many sq inches the fan will need. If you don't have the paperwork, you can get the info off the fan itself and get the needed info from the Web.
Ron
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Old 12-04-2008, 03:06 PM   #7
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Soffit Vent Sizing - Really Hot Attic


i heard the rule is 60% intake to 40% outtake...i agree equal or more is better.
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Old 12-04-2008, 09:33 PM   #8
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Quote:
Originally Posted by dmo069 View Post
I live in Dallas and my attic gets really hot. I decided to investigate why. I have 6 passive, dome vents on the roof. Each has an opening of 144 sq. inches. 144 x 6 = 864 sq. inches. Base on the standards that I see, I should have a minimum of 672 sq. inches of exhaust for a 2800 sq. foot attic. So I have enough exhaust. Is this correct?

Then I decided to check the soffit vents. I have 16 soffit vents. The covers are 6" x 16", so I figured the holes in the soffit were 4" x 14" or so. If that was the case, 4" x 14" = 56 sq. inches x 16 vents = 896 sq. inches. That should be more than adequate to provide enough intake air for the exhaust. Is this correct?

Here's the problem: When I removed three of the soffit vents, I found that 2 of them had a 2" x 4" hole and 1 of them had a 2" x 2" hole. Assuming that all 16 had a 2" x 4" hole, that's 8 sq. inches x 16 = 128 sq. inches. That's only about 15% of what I think I need. Is this correct?

So If I cut all the vent holes to "4 x "14 I should solve the problem. Is this correct? My attic can be 160 degrees when it's 105 degrees outside. If I fix the ventilation problem, what can I expect in a reduction in attic temperature. How significant a savings will I see in cooling costs?

Thanks in advance for any help.
Your calculations are incorrect.

When you do the multiplication, and determine a result, you have to decrease it by approximately 50% due to the reduction of air flow from the screening that is included.

40* temperature differential is common.

The temperature variance between a properly ventilated black roof and a properly vented white roof is about 10* higher on the black roof inside of the attic, recording the internal ambient temperature readings.

Unventilated, or improperly ventilated attics will have a temperature reading between 30* to 40* higher than the properly ventilated readings.

50% to 50% is the accepted standard, but a 60% Intake versus 40% Exhaust performs to near optimum capacity.

Ed

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