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Old 12-27-2011, 01:59 PM   #1
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Attic Ventilation


I recently added a gable vent fan to generate more attic ventilation. The attic has 2 gable vents and 3 roof vents, there are no soffit vents. Should I cover the 3 roof vents and let the fan pull the air only from the other gable vent or should I leave them open? Thanks

Last edited by Mucluk; 12-27-2011 at 04:10 PM.
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Old 12-27-2011, 02:06 PM   #2
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Attic Ventilation


Leave them open. You will not only prevent negative air pressure from building up in your attic, you will also ventilate better the upper (hotter) part of your attic.
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Old 12-27-2011, 02:06 PM   #3
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Attic Ventilation


Your defeating the whole venting system if you have no soffit vents. All your doing is drawing warm air in from the rooms blow from the negative pressure your creating.
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Old 12-27-2011, 02:41 PM   #4
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Originally Posted by joecaption View Post
Your defeating the whole venting system if you have no soffit vents. All your doing is drawing warm air in from the rooms blow from the negative pressure your creating.
Agreed. You need soffit vents; but if you have them and the ridge vent...
then you shouldn't need the gable vents as well.

Let the natural convection do the air flow... it's all (most) attics ever need.

hth
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Old 12-27-2011, 02:41 PM   #5
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Leave them open. You will not only prevent negative air pressure from building up in your attic, you will also ventilate better the upper (hotter) part of your attic.
Don't be so certain. You'd be amazed at the power in hot air convection. I learned, the hard way, it's unwise to try to fight it.

I had this bit of electronics. Power supply on the bottom, exhaust fan about mid-way up the back side. I wanted to filter the cooling air, so I reversed the fan and upgraded it to compensate for the filter. Power supply overheated and shut down. After much measuring and experimentation, long story short: As soon as the power supply heated up, all airflow out the bottom of the device, under the power supply, would stop, as would most of the airflow in, via the fan. (There were minor case leaks that allowed a small amount of air exchange.) It was really quite remarkable.

It would be interesting to measure temperature and airflow, but I would not be overly surprised if the same effect might not occur in this instance.

Clarification: I'm not suggesting he block them. I'm just saying that, with the gable fan exhausting, I'd be surprised if there's much airflow through them in either direction.

Jim

Last edited by More Power!; 12-27-2011 at 03:40 PM.
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Old 12-27-2011, 02:53 PM   #6
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Attic Ventilation


Mucluk's attic doesn't have any soffit vents, that's a given. It would be preferable to have some. That said, do not cover your three roof vents.
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Old 12-27-2011, 02:56 PM   #7
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not sure I understand. I have approx 18in of R30 insulation on attic floor. How would it be pulling air from the house? Wouldn't more vents produce more air circulation?
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Old 12-27-2011, 03:40 PM   #8
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If all the fans are blowing out then where are they getting there make up air?
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Old 12-27-2011, 03:47 PM   #9
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I only have 1 fan. It is a gable vent fan. The other gable vent is open as are the 3 roof vents, which is where is air is being pulled from
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Old 12-27-2011, 03:47 PM   #10
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If all the fans are blowing out then where are they getting there make up air?
without soffit vents to supply air...
the gable fans will suck air up through every gap the structure has...
and from the ridge vent openings as well.

this attic job has had too many cooks...
at least one of which (the original builder) who can't cook at all.
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Old 12-27-2011, 03:52 PM   #11
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Attic Ventilation


isn't the other gable vent and the three roof vents supplying the air?
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Old 12-27-2011, 04:05 PM   #12
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isn't the other gable vent and the three roof vents supplying the air?
Yeah they are... but THAT is the problem.
They're supposed to be taking air OUT not bringing it IN.
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Old 12-27-2011, 04:50 PM   #13
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Here's how it's suppose to work, cooler air is sucked in through the soffit vents, it flows along the rafter bars and flows out the ridge vent, since hot air rises.
No energy used just natural convection.
You have a fan blowing out that needs air, it's sucking in anyplace it can for make up air, so it's sucking in hot air from outside and the air near the roof coming in from the roof vent. No air at all is flowing across any of the lower sections of the roof sheathing on the under side where most of the moisture is. So it stays wet and molds up.
It's also trying to suck air from every ceiling fixture, holes in the top plates were wires are run and vent pipes run through the ceiling.
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Old 12-27-2011, 05:11 PM   #14
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Attic Ventilation


Joe and the others are right my friend. All you really need is vents for air to come in at the eves of the roof, and vents to allow it to escape at or near the top.

The gable vents will help bring in more air if needed, but you won't need an exhaust fan on them. To do so is likely just a waste of your time and money.
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Old 12-27-2011, 05:18 PM   #15
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Attic Ventilation


BTW ......... I don't know about where you live, but here in Michigan, the biggest reason I make sure my roof is well vented is not so much to rid my roof of heat in summer, but to keep the roof cold in winter.

When you have heat escaping through your house into the attic, it melts the snow on the roof which drains down and refreezes on the eves creating ice dams. Then you have a bigger problem than just a hot roof.

If the roof vents well then the heat escapes and doesn't melt the snow as bad keeping the ice dams at bay
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