XSleeper and I have posted on another thread about this and so as to not destroy that thread I'm continuing that discussion here.

@ Bud "First, ridge and gable vents work fine together along with soffit vents, the caution you hear is an old roofing myth that got traction and is now very hard to eliminate. The long explanation is available any time."

@ madmattz "I would like to hear more about how the gable vents and ridge vents can be done together."

@ Bud "As for ridge vents working in conjunction with the gable vents, the air coming in the soffits is pushed in by the weight of the colder outside air. Just like a boat is buoyed at the surface, the force comes from below, thus regardless as to where the warm air is pushed out, the fresh air will continue to come in. The only concern is where prevailing winds may blow snow or rain into a gable vent, but that is a concern all vents have. That's the short explanation, I have a worksheet that explains how house pressures develop and are calculated."

@ XSleeper "I sure wish I could find it, but there was a pretty detailed article regarding this in JLC years and years ago, and I don't recall that it was a myth. It seemed to be well illustrated and proven science that the mixture of multiple types of ventilation would result in a haphazard ventilation that had the potential to create swirling "dead zones" in the attic, along with cross flow, possible downdraft problems (snow) etc. plus the possibility of introducing straightline winds into the mix, possibly causing downdrafts from the ridge. Maybe, Bud, you have come across the article and know which one I am referring to.

At any rate, if there is new information and you have a link handy why don't you go ahead and share it with us. I'm all ears.

The closest thing I can find is an illustration in the June 2003 JLC, but that's not it, as these illustrations don't depict the downdraft, dead air zones, and cross winds that were studied. The specialist quoted in this short Q&A I can find was Mike Keogh who stated, "if you choose to add ridge vents (to an existing soffit and gable vent setup) you must either remove the gable vents or seal them up."

Bill Rose seems to take the complete opposite tack, "Gable venting and ridge venting are both fine. Soffit venting with baffles is fine. Combinations are fine."

The book "Complete Building Construction", on page 605 contains 7 specific criteria for an effective attic ventilation system, showing areas where gable venting really doesn't bring anything to the table, when compared to soffit-ridge ventilation.

Companies that specialize in ridge ventilation generally will say that gable ends should be closed off when used in conjunction with their products, so all things aside, end users who don't close of gable vents are really not following the mfg's guidelines. (of course, this could be driven by their desire to promote their own products)."

That will get this started. I will prepare a reply.
Thanks for the discussion X

Mods, if there is a better way to do this have at it, but this is a discussion that needs a good answer.

Bud

 

Thanks Bud,

I will be watching this thread !

 

Bud, clean up your PM folder. The only thing I would do is cross reference the original thread so others would have the option of reading it all. Wondering minds, you know.

 

Done >>>>>>>>>>>>>>>>>>>>>>>>> done

 

Note: The original thread is here: http://www.doityourself.com/forum/ro...ost-attic.html

X, I was unable to locate the JLC article you mention and all I found was a short forum response but I think it touches on some of the concerns you have mentioned:
Q&A: Can You Combine Ridge and Gable Vents? - JLC Online
Note, his emphasis is on issues related to wind and his mention of just soffit and gable vents fails to understand why air enters those soffits. It really doesn't care where the exit is, but we will get to that. But before we can consider the potential effects of the wind we must first understand how temperature drives our passive air flow.

All air has weight, warm air is just lighter. When it is cold out and warm inside, the cold air below our soffits provides a greater pressure than the warm air above them. This pressure difference is pushing air into our attic regardless or a ridge or soffit exit (or both).

Note, warm air does not rise by itself. When someone describes the warm air as exiting the upper vents and pulling cold air into the lower vents they are oversimplifying the explanation and overlooking the real driving force. Stack effect inside our homes is a larger scale example of what happens inside our attics. I'll add a link to my stack effect worksheet to help explain how air "stacks up" to crease the pressure differences that ventilate our homes and attics.
Hot Air

Bud

 

A further reply to Keogh's comments that appeared in the June 2003 JLC appears in a Letter, "Defending Gable Vents", to JLC in the Sept 2003 issue.

 

Interesting and demonstrates the varied opinions and claims to science that this seemingly simple topic is inundated with.

BTW, have you read the origin of the 1 in 150 sq ft of attic venting requirement? Bill Rose again, it is hilarious that this has become a requirement: http://www.structuretech1.com/wp-con...entilation.pdf

Bud

 

1:300... FHA, Jan 1942... Pretty interesting.

Also interesting are the Penn State findings that "ventilation in excess of 1/300 could lead to degraded performance" as far as moisture/frost is concerned.

Now that makes sense to me. To me... this is the biggest reason I would argue in favor of closing gable vents... based on the idea that more ventilation is not necessarily better.

If you have increased ventilation and a colder roof, it would increase the heat loss from the home via the ceiling, would it not? With increased heat loss, you would have more moisture in that warm air getting "wrung out" when it comes into contact with a cold surface- it would then condense/freeze.

Or am I off base to think that?