# proper roof ventilation



## bluebird5 (Dec 13, 2010)

I am not asking this question about a job, but rather about my own house. I already tried a search.

My house is 60 feet long, gable on both ends 5:12 pitch. The house has both gable and ridge vents. The owens corning ridge vents recommend plugging the gable vents. Why is that? 

Do you get less flow with both sets of vents open or does is just not allow as much air to flow out of the ridge vents? :whistling


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## Mr Latone (Jan 8, 2011)

This topic gets absolutely beat to death.

Pretty much two camps on this one:

Camp 1
Block all of the extraneous ventilation that will "short circuit" the proper flow of air from the soffit vents to the ridge vent. Soffit and ridge only.

Camp 2

Warm air expands and moves upward as it is displaced by cooler air. Short circuiting may alter the air currents, but does not change the physical reality that warm air will move up and out if the path is available. Gable vents are fine with ridge and soffit vent,


Notwithstanding the arguments from the 2 camps, there are indications that some ventilation layouts may induce an undesirable draw into a vent due to wind patterns around a structure. This may result in the intake of blowing snow (possibly, but less likely rain).

Good luck getting everyone to agree on the answer you seek.


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## bluebird5 (Dec 13, 2010)

Well thanks for the reply. What you stated is exactly why I am hazy on the subject. On one hand im asking myself will it hurt anything to leave the gable vents? On the other hand im asking if I plug the gable vents, will my attic be any cooler?


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## Tinstaafl (Jan 6, 2008)

Depending on the exact layout, I doubt your attic would feel any cooler. However, you would have more uniform air flow under the entire roof from the soffits to the ridge. That's probably a good thing.


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## Tom Struble (Mar 2, 2007)

thank you Martha Stewart


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## CompleteW&D (May 28, 2011)

You didn't indicate whether you have adequate soffit ventilation to support the new ridge vent. Before the ridge vent was added, the two gable vents were likely more than adequate to handle any attic ventilation. By introducing the ridge vent, it upset the air balance in the attic.

Air is like water. It seeks a level and takes the path of least resistance to get there. With your gable vents being the most "open" vents now in your attic (more air can flow more easily through your gable vents than it can through soffit or ridge vents), they will become the sole source of intake ventilation as the hot air rises inside your attic. You will essentially, stop the ventilation process in the area of your attic below the gable vents. 

The theory that hot air rises naturally so you won't have to worry about this (camp 2 above).... is fatally flawed in this respect. Cold air is heavier than warm air. Even as the warmer air in the bottom of the attic gets hotter and hotter, the cooler air being drawn in through the gable vents will keep it down near the bottom of the attic.

The other thing that can happen as alluded to in the first response, is.... depending on the wind flow pattern, your ridge vents can now become your intake and your gable vents the exhaust. This primarily happens in winter months when the air is colder and heavier at the top of your attic. When you get that reverse draw phenomenon, I've actually seen attics vented like yours fill with snow. NOT a good thing.

So, bottom line, figure the exact free air space you need to support your ridge vent. Make certain you meet or exceed that slightly by your soffit vents and then close off your gable vents. You will have a more properly functioning attic ventilation "system" doing what it's supposed to do.


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## bluebird5 (Dec 13, 2010)

for soffit intake, I have a 2 inch continuous vent in the back and, in the front, I am going to cut in some about 10 4x6 inch rectangle vents. I think I am going to go ahead and close up the gable vents and see of it is cooler. I will take some pics


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## CompleteW&D (May 28, 2011)

Rather than just cutting in more soffit vents "willy-nilly" style.... I would strongly recommend determining what you need, what you have now, and if - how many - what size vents you need to add to bring everything into balance.

Here is a basic worksheet I found with a quick search. http://www.reesewholesale.com/articles/media/documents/attic-ventilation-sheet.pdf 
It's from a distributor and only has the brands they carry. With a bit of effort you can find all kinds of ventilation tables to help you balance your attic ventilation.

If you're going to do it, take the time to do it correctly and get it right.


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## bluebird5 (Dec 13, 2010)

ah man I like willy-nilly get er done style. Thanks for the link. I will see what I come up with.


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## seeyou (Dec 12, 2008)

bluebird5 said:


> for soffit intake, I have a 2 inch continuous vent in the back and, in the front, I am going to cut in some about 10 4x6 inch rectangle vents. *I think I am going to go ahead and close up the gable vents and see of it is cooler.* I will take some pics


Think about the highlighted statement above. There is no way closing some vents will make the attic cooler. You cannot have too much attic venting and it's only going to get so cool up there. On a hot sunny day, the attic is gonna be hot no matter how well you've vented it. If it's transferring to the living space, then you need more insulation.


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## CompleteW&D (May 28, 2011)

seeyou said:


> Think about the highlighted statement above. There is no way closing some vents will make the attic cooler. You cannot have too much attic venting and it's only going to get so cool up there. On a hot sunny day, the attic is gonna be hot no matter how well you've vented it. If it's transferring to the living space, then you need more insulation.


Semantics.... the goal of proper attic ventilation is air flow. Preferrably from bottom to top. By not closing off the gable vents (once a ridge vent has been introduced), you interrupt that bottom to top air flow and can in essence create hot spots in the attic. Most noteably, in the worst possible place.... right down by the attic floor.

Follow up: This problem isn't as critical in a hot summer attic with good insulation. But let this happen in a cold winter attic and watch all of the condensation form on top of the insulation, underside of the roof deck etc. You know you've got big time problems when you see the top of your insulation wet, snow piled in the thing or frost forming on the underside of the roof deck. I've seen it happen many times with an attic ventilated this way.


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## seeyou (Dec 12, 2008)

*By not closing off the gable vents (once a ridge vent has been introduced), you interrupt that bottom to top air flow and can in essence create hot spots in the attic.*

Nonsense. Let me give you a different scenario, but one that should produce the same results: You have a 50' long vented ridge, but the last 5' of it on either end does not have ridge vent. You add 5' of ridge vent on either end. What's the difference? 

You're picturing a situation where there are different temperature layers of air stacked in the attic. That doesn't exist in real life. The hottest air is the lightest and will move toward the top. If there's an opening, it will move out and heavier, cooler air will be pulled in to replace it. If there's adequate soffit or other inlet vent, the cooler air will enter there. If there's not, it will enter elsewhere, which could be the gable vents. If outside air is entering and being heated enough or mixing with heated air until it's hot enough to rise out of the ridge vent, heat and moisture are being carried along. 

*let this happen in a cold winter attic and watch all of the condensation form on top of the insulation, underside of the roof deck etc. You know you've got big time problems when you see the top of your insulation wet, snow piled in the thing or frost forming on the underside of the roof deck. I've seen it happen many times with an attic ventilated this way.*

If there's that much moisture in an attic in winter, then there's another problem. You may have seen frost forming in an attic vented like we're discussing, but the problem is too much moisture in the attic. Your statement is akin to saying black cars are more likely to have flat tires than other color cars because you saw two black cars along the road with flat tires, but you didn't notice the box of roofing nails that fell off of my truck right in front of them. Snow entering the attic is a symptom of another problem as well. It doesn't matter much how an attic is vented as long as it's air sealed and insulated properly and any moisture removal devices are vented outside the attic correctly. Where will all this moisture you speak of originate?

Will some air short circuit from a gable vent to the ridge vent nearby? Maybe, but it will pull heat and moisture as it does and that scenario is most likely to happen when wind driven. Go back and look for other problems in the attics you speak of. I bet there are other problems that need attention that blocking off a gable vent wont solve.


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## CompleteW&D (May 28, 2011)

seeyou.... that was a very eloquent post and if I were someone without many years of true field experience I could easily believe all of it. While I won't disput all of what you say, I do take issue with some of it. In years past I spent countless hours in attics with my Lomanco rep doing smoke tests to find where and why there was a particular air flow and/or moisture problem. So, I do know for a fact that what I've said is true. Simply put.... I've seen it happen in real time, not just after the fact guessing what may have happened. I've seen some awfully weird things happening in attics (especially in winter) when you have an unbalanced air situation.

Whether that's from adding a power vent to existing roof ventilation without making any adjustments in existing ventilation - just because the HO thought it was a good idea (talk about REALLY screwing up balanced air flow), ventilation improperly done on hip roofs with varying heights, vents on dormer roofs that then open into a vented main attic, or the situation the OP describes.... you name it, I've seen tons of different problems associated with improper roof/attic ventilation.

Does that make me an expert? No, absolutely not. But, I can tell you.... In my years of roofing, I solved a lot more problems than I ever created. So, to the OP and anyone else reading, take my views for what they are worth.... Not very much I suppose because each situation is different, YMMV, IMHO, FWIW, Just my $0.02 and all that.


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## seeyou (Dec 12, 2008)

W&D - 

And I've been through similar scenarios. I don't argue that you've seen what you've seen. And I never suspected that you caused any problems. Just maybe that you were treating the symptom rather than the disease. 

You don't say where you're located. That could also have large bearings on what does and doesn't work. What works and doesn't work in Central KY may differ in your location. I can only speak from my local experience, but the majority of so called "attic venting" problems I've encountered are the result of poor insulation, poor air sealing, and/or poor execution of kitchen and bath air venting. In most cases the attic ventilation was also lacking, but if you keep the moisture out of the attic in the 1st place you don't have to be so diligent about venting it out. 

I've performed and witnessed numerous smoke tests as well. The only conclusion I've come to from seeing them is that what's true on Tuesday in the attic may not be true on Thursday. I think that's the biggest reason we have disagreements about attic venting. None of these tests are actually very conclusive. We do a smoke test in an attic on 90F day with no wind. We see the results. We make an opinion. But for that test to be conclusive, we need to do the same test in the same attic under numerous different temperatures and atmospheric conditions. I've done just a couple on my own home (I prefer to avoid hot, spiderwebby attics) and what happens on a clear, windless 90F day is very different from what happens on an overcast, breezy 90F day. 

The only conclusion I've been able to draw from what I've seen and what others tell me is: Insulate your attic well, air seal it, and direct your moisture out of the building. If you have an attic above your insulation, vent it with some sort of inlet and some sort of outlet. The vent manufacturer's specs are a good starting point. You can't over vent. I've never seen snow enter the ridge vent on a 12/12 pitch roof. I have seen it enter on walkable roofs. 

I've had several snow entering the ridge vent issues. All were solved by changing the ridge vent. All ridge vent is not created equal. Some are better at keeping the snow out. Some are better at letting the air out. 

I've literally done hundreds of re-roofs where I've added ridge vent to gable roofs with gable vents. Most had inadequate soffit/inlet. Some HO's opted to add more inlet, some didn't. I've never had a callback due to anything remotely related to not closing off the gable vents. But, we have been called back due to the roof "leaking" when it's not raining. About half have been due to fart fans or kitchen vents pumping moisture into the attic. The other half have been from plugged condensation pans under the AC units in the attic or poorly insulated duct work. YMMV. Cheers, I'm buying..........


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## jmiller (May 14, 2010)

bluebird5 said:


> The owens corning ridge vents recommend plugging the gable vents. Why is that?
> 
> Do you get less flow with both sets of vents open or does is just not allow as much air to flow out of the ridge vents? :whistling


I believe you get more air flow out the ridge vents with gable vents, but consequently less air flow under the decking at the eave- above the top plate which is almost always the least insulated part of the attic. It's a short circuit and a recipe for ice dams.

Air sealing and venting out is important, but who actually follows the building science recommendation of caulking the drywall at the ceiling corners? That's _one_ way conditioned air gets up there.


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## jmiller (May 14, 2010)

seeyou said:


> I've never seen snow enter the ridge vent on a 12/12 pitch roof. I have seen it enter on walkable roofs.


Not your typical roof repair, but preventing the short circuit/ wind tunnel effect worked on this 12-14/12. Beyond the ridge vent (and its manufacturer), they had snow getting sucked in through any slight air gap in dormer walls:


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## bluebird5 (Dec 13, 2010)

well I put plastic over the inside of my gable vents and added 10 intake vents along the soffit. I will see if my attic feels less stagnet and cooler.


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## CompleteW&D (May 28, 2011)

I'll just make a final comment since it was brought up and I'd like to be clear. I've never seen where snow was sucked in through a ridge vent. But, I have on numerous occassions seen snow sucked through wide open gable vents when ridge vents were added and gable vents became the primary intake vent.

As for my location, I suppose I need to fix my profile. I'm in the same part of the country as you; central Indiana in Indianapolis.


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## CompleteW&D (May 28, 2011)

jmiller said:


> I believe you get more air flow out the ridge vents with gable vents, but consequently less air flow under the decking at the eave- above the top plate which is almost always the least insulated part of the attic. It's a short circuit and a recipe for ice dams.


And I said my last post was my final comment.... LOL This is EXACTLY right. When adding ridge vents where gable vents are already present, the gable vents become the primary intake vents and it slows down the intake near the bottom of the roof. Damn jmiller.... you said so clearly in a couple of lines what took me several posts. :wallbash:


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## BrandConst (May 9, 2011)

tomstruble said:


> thank you Martha Stewart


:laughing:


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