Ok, I'm having a new roof installed. I just had my soffits repaired and soffit vents installed. I'm having a ridge vent installed on the roof which will replace two aging turbines. I also have two gable vents at each end of the roof.
Do I close off the gable vents so that the ridge vent will pull air from the soffit vents? I see lots of debate on this. My brother is a mechanical engineer, who says the gable vents should be closed, so that the ridge vent pulls from the soffit vents. The roofers who quoted me on the job said they had heard of this theory, but thought the more ventilation the better, so they voted for leaving the gable vents open.
Close the gable vents if putting on ridgevents..
Listen to your brother. It isn't theory, he is absolutely correct.
Think of it this way. What happens with a straw when it has a crack in the side? You can't pull any fluid through it can you? In other words, it is the vacuum that allows you to pull liquid through the straw.
While not exactly the same, similar situation with your ventilation. You want to completely exchange the air in your attic to have the most effective ventilation. You do this by pulling air from the lowest point and exhausting it at the highest point. Having gable vents would essentially be the "crack in the straw".
Dad, that is a very brilliant analogy. Did you come up with that yourself or does someone else get the credit?
I dunno the way I see it the as long as the gable vent is high up to the ridge it could only help,, as long as air is circulating I dont see a problem with it. If the gable vent was down too low from the ridge vent say 4 feet from the ridge then I might see some un circulated air there but very little.. My mans vacuum theory is correct there has to be a vacuum but I see the vacuum being created simply by the hot air moving up and out regardless, which is what you are trying to accomplish. Thats only opinion.. as far as for winter the more cold air in the attic the better as far as Im concerned, to keep away the infamous ice damming, but im sure someone will pipe in and contradict that theory as well. Personally if it were my house and I was really concerned, I would try it for a year with vent open, then a year with vent closed and monitor my house and my bills to see what the difference is,, as far as the longevity of the new roof I doubt there will be much difference (and if it were to be, it would show in your electric bills.) but what the heck do I know. Even with the CRACK IN THE STRAW theory hot air is still escaping the straw (your attic). mission accomplished the way I see it.
dstew, in this case, the analogy is my own creation. I will be the first to admit however, most of my best material is stolen. LOL
sleeve, intuitively, it would seem more is better when it comes to ventilation. Reality is different though. Were there a constant prevailing 15 mph wind flowing through the gable vents, your concept would probably work. If you think about it though, the majority of the time you need the ventilation to work the most is when you have the hottest days when there is little or no breeze.
Another way to think about it is with electricity or water. Both will seek the path of least resistance. Think of air as a fluid. When you have the gable vents, they present the path of least resistance. In other words, the air flow you will get is air flowing into the gable vents and out the ridge vent. Unfortunately, this leaves all the air volume below the gable vents essentially stagnant. So it would in fact be the opposite of what you theorized, i.e., the gable vents would be less of a handicap if they were lower rather than higher.
One other way to think about it. I've seen power vents located a foot below ridge vent. All that happens is the power vent pulls air down through the ridge vent.
Just get rid of gablevents if puttin in ridgevent,it will make a more Direct flow for air to pass thru..
same here. close them up
the only way is to close them as said before. it works off of the bernuli effect ridge vent that is. It works just like an airplane wing. As air passes above the wing (above on the roof) the air creates negitive pressure in the attic/under the wing making the roof pull directly from the closest air source. Hence using the soffit vents. Also make sure there is enough soffit vents so the ridge vent effective.
The idea is to have a 40% exhaust and 60 intake. Airvent.com can help you with this. hope it helps
Bernoulli's Principle and you almost got it right on the way it works.
I assume my post was deleted due to the link and the awesome competition my website poses.
Anyhow, the drinking straw analogy above is not quite accurate.
This analogy would be useful if we were trying to suck water into the attic through the soffit vents.
But most of us, maybe all, don't live in a house surrounded by water that we want to suck into our houses. We prefer to keep the water out.
So, instead of trying to suck water into the straw, try putting the end of the straw into "air".
Now,place a finger over the bottom of the straw and inhale. Feel the suction?
No, it won't be quite as strong as if there were no hole higher on the straw, but, if that middle hole were restricted in any way, say a cap or screen or even negative pressure from wind blowing over the outside of the straw, the difference in suction would not be significant.
Go ahead, try it for yourself.
Leave your gable vents open.
AD That would be a typical defensive response from someone who suspects they are wrong, but won't admit it.
Show him your stone roof repair.
Gable vent is a ridge vent.
They work together great.
except the ridge vent isnt as rain and moisture proof as the gable vent.
I will go with Authentic Dad,the explanation of the vacuum given by him is absolutely correct.
I guess we'll have to agree to disagree. For most engineering calculations, air is considered the same as a fluid. Bernoulli's principle is based upon fluids but in most cases, also applies to gases.
And BTW, I never said the drinking straw analogy was an exact comparison. It was an analogy which is simply meant to provide an easier way to picture something in your mind.
If you'd like, feel free to contact me and I will attempt to explain it in proven factual technical terms if you'd like to have that conversation.
Good. Now please explain how this Bernoulli effect only works on the ridge vent of a house.
Who is saying it only works on the ridge vent of a house? Do you struggle with reading comprehension?
Well, if the hole in the straw represents a roof or gable vent, and you say that is the path of least resistance, then there is no negative pressure (ie. Bernoulli effect)on those vents and other areas of the roof.
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