Attic Ventilation: Gable Vents and Ridge Vents

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Old 12-01-12, 08:26 PM
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Attic Ventilation: Gable Vents and Ridge Vents

My house has a ridge vent and a gable vent - one on each end of the house. It has a low slope roof. I understand this is not the best arrangement for acceptable attic ventilation. It was recommended by an Energy Contractor that I block off approximately one-third of the ridge vent on each end of the house and leave the middle one-third open for ventilation. It seems to make sense in that air will be forced to move toward the center before exiting the ridge vent. As it is now air is entering the gable vents and exiting the attic at the section of the ridge vent located in the vicinity of the gable vent. This results in poor air flow near the center of the attic. Seems to make sense. Any comments? Are there other relatively inexpensive methods to improve attic ventilation. Thank you.
 
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Old 12-02-12, 12:09 PM
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Welcome to the forums! More importantly, do you have soffit vents? If so, I would be more inclined to occlude the gable vents and let the soffit/ridge combination do their intended job. If all you have is gable vents, then you only have half the venting you need. Check out the soffit vents and let us know.
 
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Old 12-02-12, 12:35 PM
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Hi aimfire and welcome to the forum,

Static (no wind) attic venting is powered by a temperature difference between inside and outside and the height difference between the low and high vents. With just gable and ridge vents, you will have minimal height difference, thus almost no natural driving pressure.

As Chandler said, check for soffit vents as they would work much better than what you currently are describing. More on that if you have them.

Assuming you don't, then you can create a height difference by ducting your gable vents to near the attic floor. They can even be ducted out to and across the soffit area. Since the gable vents are slightly lower than the ridge, cold air will begin the process by flowing in and down the new ducts. Those ducts should be insulated so that the cold incoming air remains cold. With the column of incoming air now extended to the attic floor you will have established the full available height of the attic to power your venting.

As for sealing off parts of the ridge, I would not, as vent area is more important than location. Any incoming cold air will head to the attic floor and force the warmer attic up and out. If your current gable vents are your source of incoming cold air, it is the mixing of that cold air with attic warm air that reduces the effectiveness near the middle of the attic, and not the exit point.

But static venting is only part of the process, as wind plays a bigger role when it is blowing and that is another reason to leave all of the ridge vent open.

Let us know if you have soffit vents or not and then we can continue.

There are also ways to add edge venting, but it is easier to do when a new roof is going on.

Bud
 
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Old 12-02-12, 05:52 PM
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Do not have soffit vents (should have included that important piece of information). The ducting of the gables to the floor is a good idea (actually, very clever). However, I have a very, very low slope roof. When inside the attic I can only crawl on my belly down the centerline with very little overhead clearance. The Gable Window is both at the floor level AND just below the ridge vent. There is very little height difference of which to take advantage - perhaps 2 feet at the centerline. The attic is blown with celluose insulation over existing R-9 insulation. Thanks again.
 
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Old 12-02-12, 06:21 PM
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If your gable vents are that low, there's not much you can to to increase the flow of air in the attic. Hopefully there is a good deal of blown in insulation on top of the R9 you stated you had. THAT is where you will gain or lose in heating and cooling. Your attic space probably isn't large enough to make a difference.
 
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Old 12-02-12, 06:46 PM
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One concern would be, did they install baffles that hold the insulation back from the bottom of the roof deck? Even without soffit ventilation, leaving an inch or so above the insulation will allow any moist air leaking into the attic to find its way out.

Moisture is the problem and air sealing should have been done before all of the insulation was installed. Too late this year, but perhaps a spring project.

You can seal all penetrations from below if you have access through a basement or crawl. Plumbing and electrical are the big ones.

You said Gable Windows? Are those louvered vents as opposed to actual windows? If so, then you are probably all right, at least as good as possible. Since the wind is rarely zero, gables plus ridge will still move out some heat and moisture.

If you have condensation problems and find you must add venting at the soffits, there are vents that fit under the bottom row of shingles. Again, difficult this time of year.

Bud
 
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Old 12-09-12, 09:13 AM
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Most attics with little or no ventilation, ridge vent will help a lot or even solve the problem. The ridge vents need some intake from either soffit vents or lower gable vents. The ridge vent will start venting around 120 degrees (depending on you attic). I never recommend an attic get over 130 - 140. At 130, the roof decking is heated from the top and bottom and cooking your shingles from both sides. A 30 year shingle will only last about 15 years if it has no ventilation. Winter ventilation is important to remove moisture from the attic. This moisture will make the attic cooler and harder to heat while causing mold and mildew.
 
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