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Old 12-28-2010, 11:38 AM   #1
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Question versus ridge/gable vents


We have a 1970s single-story ranch home. Early this year, we had the roof replaced and ridge vents were installed. I've heard differing opinions on what do to able the original gable vents (each end of the house is vented and one end has a thermostat-controlled fan). The roofing contractor told me that the gable vents should be left uncovered and the fan left on, but that guy was a hack (they did a good job with the roof, but his crew busted some of my lawn furniture and screwed up a soffit; after they came out three times to fix it and I still wasn't satisfied, I hired a GC to fix the soffit).

In turn, the GC told me to completely cover the gable vents and remove the fan. Another roofer told me to leave the gable vents open and simply unwire the fan.

Any thoughts?

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Old 12-28-2010, 11:46 AM   #2
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Question versus ridge/gable vents


do you have soffit vents to allow air into the attic space? You should but just need to be sure.

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Old 12-28-2010, 12:13 PM   #3
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Question versus ridge/gable vents


Quote:
Originally Posted by nap View Post
do you have soffit vents to allow air into the attic space? You should but just need to be sure.
Yes, we do have soffit vents. The roof approximately 1,900 square feet, with 34 square feet of soffits, and 19 square feet of ridge vent. The two gable vents add another 12 square feet.

Based on what I've read, this seems to be a very good ratio (1 square foot of ventilation for every 30 square foot of roof, evenly divided between the upper and lower edges of the roof), and I'm thinking that I can unwire the fan (I think the fan is undersized anyway; it's about 12" in diameter and runs constantly when the temp hits 85 or higher, which happens during most of the year in south Georgia). The attic seems to stay dry (there's no sign of condensation or mold).

My gut is that the fan is not needed since we have the ridge vent now, and to leave the gables open, but I'm hoping for an expert opinion.
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Old 12-28-2010, 04:50 PM   #4
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Question versus ridge/gable vents


The problem with your current system is that you are creating a short circuit so to speak. The intake air is entering in through your gable vents and exiting your ridge vent instead of coming in through the soffit vents and exiting the ridge vent.
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Old 12-28-2010, 05:10 PM   #5
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Question versus ridge/gable vents


Ridge vent is 'arguably' the best exhaust vent on the market, it is designed strictly for exhaust purposes.

The intake vents in your eaves are designed strictly for intake purposes.

Gable vents inserted into the gable ends have a two fold design, meaning either of them can act as intake/exhaust simply depending on which direction the wind/s are blowing.

Having the gable vents as a second source of intake would not be an issue,
but when one of them are acting as an exhaust source it could create an negative/short circuiting of airflow by drawing air in through the ridge vents.

My 30 plus years experience recommends you disconnect the fan and block the gable vents off.
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Old 12-28-2010, 05:41 PM   #6
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Question versus ridge/gable vents


if it were me.

I would block the gable without the fan. If there is no louver on the fan, I would install a louver to prevent inflow and leave the fan connected. If the ridge/soffit system is overwhelmed due to high ambient temps, is not actually adequate for normal ventilation, or is caused to have reduced efficacy due to some blockage, the fan would act as a back up.
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Old 12-28-2010, 06:22 PM   #7
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Question versus ridge/gable vents


I would leave it as is. Possibly raising the temp on the thermostat to run on the hottest days.
On hot days, the gable vents will act as additional exhaust, and on windy days they will be either exhaust or intake. Depending on the wind direction.
The soffit vents can also be either exhaust or intake.
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Old 12-29-2010, 09:36 AM   #8
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Question versus ridge/gable vents


Quote:
Originally Posted by dmc@RCR View Post
I would leave it as is. Possibly raising the temp on the thermostat to run on the hottest days.
On hot days, the gable vents will act as additional exhaust, and on windy days they will be either exhaust or intake. Depending on the wind direction.
The soffit vents can also be either exhaust or intake.
The only way your eave/drip edge/lower roof intakes would ever become exhaust would be if there is a second intake source, eliminate the second intake source an you eliminate that risk.
Proper roof ventilation "as described by the manufacturers of both ventilation systems & roofing systems is defined in three steps.
1. Intake
2. Unobstructed Flow.
3. Exhaust.


I can't find my old newspaper clip from 05' so you'll have to take/not take my word on this, but I roofed a home in 2005 "or in that time frame, maybe have been earlier/later" after it had burnt to the ground and was rebuilt on the same site.

The fire marshals report clearly stated the fire had been fueled by improper attic space venting.

Scenario.
The older home was re-sided at which time intake vents were installed. Existing siding had none.
Plus they left the gable vents open, roof had no exhaust vents in it.

Years later the home was re-roofed and roof vents 'turtle/box' were installed. Existing roof had none.
The gable vents were left intact.

Kitchen fire starts in early afternoon 'electrical I believe' and non one is home.
Shortly there after the home owners arrive to see smoke and call the fire department, the time frame from the start of the fire to some one seeing it was so short that the fire marshal's first thought was it had been started on purpose and had been fueled by something to make it burn quicker.
After investigating the situation his final report was, improper attic ventilation was the fuel source and it was accidental.
The winds were strong that day so both the eave & one gable vent was pulling fresh air in and caused a minny twister effect inside the attic that actually sucked the fire upward into the attic and sped up it's burning time.

I'm not suggesting every home with a double intake source is a fire hazard,
but the example above clearly shows it could be problematic in a fire situation.

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Last edited by Slyfox; 12-29-2010 at 09:39 AM.
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Old 12-29-2010, 11:40 AM   #9
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Question versus ridge/gable vents


Guys, I do appreciate everyone's insights. It seems that there's really not a consensus on this, lol. So let me ask this question (keeping in mind that my house is southern Georgia, and faces west, and has dark red architectural shingles):

On a summer day, what is a reasonable temperature variance between the ambient air and the attic? I naturally expect some heat gain, but I would assume that if the attic is ventilating properly, there should be some correlation between the temperature in the two spaces. The house has trees in the backyard, so the roof is shaded in the morning, but from about noon until sunset, the front of the house takes on the full sun. Will this sort of measurement be an indication of proper ventilation?

I've checked the soffit vents around the house to make sure they are clear by shining lights into them from outside and then going into the attic to ensure that I can the light shining through. Every vent has at least 3" of clearance between the roof decking and the top of the insulation pile. Is this sufficient?
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Old 12-29-2010, 07:34 PM   #10
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Question versus ridge/gable vents


Quote:
The only way your eave/drip edge/lower roof intakes would ever become exhaust would be if there is a second intake source, eliminate the second intake source an you eliminate that risk.
It all depends on the wind direction. And it's not as simple as that.
But I won't argue with you.
Most every website on the internet will re iterate the above boilerplate, but if you want some in depth information, check the link in my signature below.

I'm not sure about Georgia, but up here in Ohio, my attic runs about 15-20 degrees warmer than ambient in the summer. On a cloudy winter day it is the same temp as outside. Ridge, gable and soffit vents.
Quote:
Every vent has at least 3" of clearance between the roof decking and the top of the insulation pile. Is this sufficient?
Yes.
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Old 12-29-2010, 08:54 PM   #11
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Question versus ridge/gable vents


Quote:
Originally Posted by dmc@RCR View Post
It all depends on the wind direction. And it's not as simple as that.
But I won't argue with you.
Most every website on the internet will re iterate the above boilerplate, but if you want some in depth information, check the link in my signature below.

I'm not sure about Georgia, but up here in Ohio, my attic runs about 15-20 degrees warmer than ambient in the summer. On a cloudy winter day it is the same temp as outside. Ridge, gable and soffit vents.

Yes.
I did check out your link and purposely did not comment on what I read there because like you I do not wish to turn this into an argument between roofing contractors, but would rather try and stay on track of helping the home owner here by supplying him with as much info- as possible.
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Old 12-29-2010, 09:09 PM   #12
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Question versus ridge/gable vents


Quote:
Originally Posted by clashley View Post
Guys, I do appreciate everyone's insights. It seems that there's really not a consensus on this, lol. So let me ask this question (keeping in mind that my house is southern Georgia, and faces west, and has dark red architectural shingles):

On a summer day, what is a reasonable temperature variance between the ambient air and the attic? I naturally expect some heat gain, but I would assume that if the attic is ventilating properly, there should be some correlation between the temperature in the two spaces. The house has trees in the backyard, so the roof is shaded in the morning, but from about noon until sunset, the front of the house takes on the full sun. Will this sort of measurement be an indication of proper ventilation?

I've checked the soffit vents around the house to make sure they are clear by shining lights into them from outside and then going into the attic to ensure that I can the light shining through. Every vent has at least 3" of clearance between the roof decking and the top of the insulation pile. Is this sufficient?
In my area of northeast Ohio the interior attic temps would be similar to dmc@RCR's response and I would also agree with him when he said you have sufficient clearance on your intakes.
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Old 12-31-2010, 09:13 PM   #13
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Question versus ridge/gable vents


Are there baffles at the soffit vents to prevent wind-washing of your insulation? Especially important over the perimeter (exterior) walls because of the added heat leaving there from possible convective loops in the wall cavity insulation: http://www.diychatroom.com/f98/how-b...ulation-90438/
Is the attic air sealed from below? What size and how often are the soffit vents to figure the NFVA?
What is the ridge vent rated at? 18 NFVA? I ask these questions as you mention square feet of ridge vent rather than lineal feet and NFVA..... http://www.airvent.com/homeowner/pro...it-specs.shtml


Some good wind pressure info; http://www.nrc-cnrc.gc.ca/obj/irc/doc/pubs/bpn/57_e.pdf


Gary

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