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Old 02-01-2012, 05:35 AM   #1
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Attic ventilation


I recently had soffit vents installed. I currently have 3 roof vents and a gable vent on each end of the house. I have a gable vent fan in one of the gables that has a louver that opens and closes when the fan is on/off. Should I cover up the other gable now that I have soffit vents, or leave it open?

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Old 02-01-2012, 07:36 AM   #2
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Attic ventilation


You will need to supply more information as far as the sizes of the gable vents, roof vents, and placement/layout of the roof vents and attic spaces.

When it comes to attic ventilation, you always want to err on the side of more intake air vs. exhaust air.

This helps eliminate the potential for stack/chimney effect in the attic.

I would say that it is very likely that you can un-hook the gable fan at this point an just open it up (i.e. remove the louvers).

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Old 02-01-2012, 07:40 AM   #3
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Attic ventilation


If you add a ridge vent you can get rid of all your other vents on the roof and it will look and work much better.
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Old 02-01-2012, 07:51 AM   #4
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Attic ventilation


Depends. For most situations, more ventilation is best.

Where are you located?
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Old 02-01-2012, 07:56 AM   #5
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Attic ventilation


Im located in Pittsburgh PA. I had an issued with mold in the past and had installed a gable vent fan, that did not really seem to help much and have since realized that the soffits were just covered over with a soffit vent (no holes were ever made into the soffit). Hence having the vents installed. I have also added an extra layer of R30 insulation. Wasn't sure if leaving the one gable vent open would effect air flow from soffit vents.
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Old 02-01-2012, 08:04 AM   #6
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Attic ventilation


Ideal ventilation would be as joe mentioned, open up the ridge and vent through the ridge to balance the soffit incoming air.
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Old 02-01-2012, 08:22 AM   #7
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Attic ventilation


thanks, plan is to have ridgevent installed this summer. One last question, I only have a bout a 2-3 inch overhang soffit, the contractor was original going to use a sawzall and make a continues cut, however, that was not possible because the soffit consisted of a double layer of wood and was too thck (believe consisted of 2x4 and other piece of wood not sure of type or size). Contractor used flat drill bit believe 1.5" - 2" in diam. and drilled a hole every foot. Ideally I know I should have more vents. Will these vents provide any ventilation? Attic is 750sq ft, and has 3 12" in diam. roof vents.
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Old 02-01-2012, 06:41 PM   #8
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Attic ventilation


You should focus first on preventing moisture and warm air from entering the attic from the living space. Sealing attic hatches, duct work, recessed lighting etc. That, along with proper insulation, should take care of the mold problem.
“The major causes of moisture problems in attics and roofs are holes in the ceiling and paths for unwanted airflow from basements and crawlspaces. People should focus first on preventing air and moisture from leaking into the attic. “ (Rose, The JLC Guide to Moisture Control)

If you think you need more ventilation, a small hole every foot or so in the soffit won't help much. If you are redoing the shingles, that would be the time to re do the soffit to open it up as much as possible. Soffit ventilation is somewhat more important than higher roof exhaust vents.
"If a roof had only one type of vent device, I would choose soffit vents, because they work well as inlets and outlets."(Rose)

For your situation, if unable to remedy the attic moisture intrusion, a combination of ridge vent and soffit vents, along with gable end vents and/or powered attic ventilator would be a your best solution.
“A minimum effective airflow of 0.8 cfm_ per sq ft of floor area, at winter design conditions, is necessary for attic moisture control in areas where severe moisture condensation occurs.” (Hinrichs, ASHRAE 1962)
According to this study, a combination of ridge and soffit vents in a test structure had an effective airflow of just 0.166 cfm per sq ft of floor area. And that was a ridge vent without a filter.

There is no evidence that leaving the gable vents open will interfere with normal ventilation.
There is some evidence, my own testing with scale models and on site testing of my own and customers attics, which shows how all roof vents work together to ventilate an attic space. http://www.roofingcontractorreview.c...c-Ventilation/

As for the 1/300 – 1/150 rule;
“The minimum free ventilating area presently specified is inadequate to provide proper attic ventilation.”
(Hinrichs, ASHRAE 1962)

“The attic ventilation ratio “1/300” is an arbitrary number selected by the writers of FHA (1942) with no citations or references.” (Rose)

Also, if your roof is prone to ice dams, and your roof and ridge vents are covered with snow, your gable end and soffit vents will continue to exhaust warm air from your attic.

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