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Old 03-23-2012, 10:28 PM   #16
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I talked with many of the ridge vent manufacturer's reps today about this same thing. Out of the five here at the show all five said the same thing. When adding a ridge vent, block off the gable vents. I first heard about this at a building science class given by John Straube. He is the leading authority on this stuff in Canada. Canada has spent millions researching building science long before the USA realized that our housing stock needs work. You can easily test this with a smoke pencil. leave the gable vents as is.... On a windy day where it blows into the gable vent smoke will come up through the soffit and twirl around aimlessly. Now close off the gable. The air moves quickly from soffit to ridge almost tightly against the sheathing. Which is what you want. Too many people think it is to cool the attic. Not so... just to remove moisture. Air seal the attic floor completely and insulated with blown insulation to R38 or more. Baffle the venting. Now utility bills are greatly reduced, moisture is not longer an issue and the whole house is happy.

I do this for a living. And not one of my last 2000 clients has reported anything but more comfort, less noise and lower utility bills. I offer a written statement that utility bills will be reduced by 25% I can do this because all details are addressed. A test in and test out shows results. IR camera helps show what cannot be seen. And bills are usually cut by 50%-60%

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Old 03-24-2012, 12:22 AM   #17
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Quote:
Originally Posted by dmc@RCR View Post
Bob,

Can you show me one study or scientific paper that explains the "short circuit" idea?
hear is one statement that was on the Fine Home Building forum

*IF** you have soffit vents, it's generally recommended to close off the gable end vents when installing ridge vents. The theory is that air will be drawin in the gable ends rather than up from the soffit. (I've never read of any actual testing of this, it's just "conventional wisdon".) If, on the other hand, you do not have soffit vents, I'd say you want as much venting as you can get -- ridge + gables. Only in relatively ideal conditions will you get "flow through" ventilation from one gable to the other, but the ridge vents provide some purely convective ventilation, and some "siphoning" when the wind is at right angles to the ridge.
Of course, the ideal thing is to add the soffit vents and eliminate the gable vents, but adding new soffit vents is usually (but not always) difficult and expensive (compared to ridge vents).
But a lot of variables here, in terms of location/climate, shape and structure of the roof, use of the building, etc. Generally a building like a church has relatively low occupancy and fewer problems with "moisture drive" from the living space into the attic area, reducing moisture as a reason for being concerned about attic ventilation.


Close those gable vents. You (post #205382, reply #3 of 5)

by florida in reply to suzypq [original] on Fri, 11/25/2011 - 11:58
Close those gable vents. You spent the money to get good ventilation in your attic by installing soffit and ridge vents. If you leave the gable vents open they short circuit the soffit to ridge system and you've thrown your money away. It is just like putting your A/C return right next to your supply, lots of air flow but your house won't get cooled. I'd put foam board over the inside of the gable vents and seal them well.

Last edited by Bob Mariani; 03-24-2012 at 12:23 AM. Reason: another post...same site
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Old 03-24-2012, 12:25 AM   #18
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Here are some of the sources I have found.

http://www.upea.com/winter/Influence%20of%20climate.pdf
http://irc.nrc-cnrc.gc.ca/pubs/fulltext/nrcc38715/
http://irc.nrc-cnrc.gc.ca/pubs/fulltext/nrcc38517/
http://www.fpl.fs.fed.us/documnts/pdf1999/tenwo99a.pdf
http://www.agresearchforum.com/publi...5/100_page.pdf

There are a lot more if you google “wind flow over a building“ and “computational fluid dynamics” etc.
Do you have any specific questions about my ideas or items on the webpages that I could maybe try to explain better? Is there something you see there that doesn’t look right?
If you know of any studies or sources to refute my ideas I would be happy to read them.
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Old 03-24-2012, 10:54 AM   #19
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Bob,

None of those sources mention " the short circuit effect".

Again, can you show me one scientific paper or study that explains the "short circuit" idea?

Others have tried, and failed.
Maybe you could ask all those manufacturers reps.

If you would like, I can show you a video of a powered attic vent running next to a ridge vent with no ill effects.
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Old 03-24-2012, 11:08 AM   #20
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To the OP, a 15 degree delta between exterior temperatures and attic temperature is completely normal...I would even call that low if you had direct sun on the roof for a good portion of the day. Certainly keeping the heat within a manageable range will help with cooling efficiency of the conditioned space below, but your main concern should be moisture control.

It's commonly accepted practice to use power vents with gable vents if you don't have soffit vents. That is my circumstance in our old house and given it's construction (craftsman style with open eaves meeting up with stucco sheathing) soffit vents would not even be possible. Ridge vents work by convection and gable vents are not conducive to even air circulation and evacuation. Sure it's possible to add a ridge vent in this circumstance, but you're better off adding the power vent between the gables so you get air moving through the entire unconditioned space.
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Old 03-24-2012, 09:28 PM   #21
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For written information about short circuited attic ventilation caused by combining roof vents and gable end vents, refer to the following free publication:

Certainteed Shingle Applicator's Manual, Tenth Edition, Chapter 7, Ventilation Standards and Systems".
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Old 03-24-2012, 11:25 PM   #22
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So, has anyone read this: http://digitalcommons.unl.edu/cgi/vi...ventilation%22

Gary

You better remember everything while at the conference, Bob....... Lol, we will quiz you later. I bet you're having fun!

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Old 03-25-2012, 09:14 AM   #23
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I kept answering all the questions so I came home with a years supply of shirts and mugs... but it was fun. Marvin Windows had to do one of their demos (their guy was sick) on home to install a window over foam. Adding foam on the exterior or using sip panels or studs with embedded foam will soon be required as cities and states adopt the new IRC code since this now will include the ECC Energy Conservation Code.

Without exception every manufacturer rep and presenter agreed that gable vents should be sealed off. After sealing the attic floor properly and adding sufficient insulation the attic becomes part of the outside environment and even venting becomes optional. As time goes by and we start learning more you will find this to be common place.
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Old 03-25-2012, 09:39 AM   #24
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I don't have any gables, because the shape of the roof is weird. The original roof shape was 4 ridges coming to one point. Through various apparent additions throughout the years I have about 7 different ridges. The roof material is also apparently tongue and groove.

In this case would I be better off just adding a couple more standard 1 square foot vents?

Also, is there any way to verify I do in fact have a continuous soffit? The roof overhangs the house about a foot and a half and it's perforated on the overhang. Inside the attic there's not enough room where the floor and roof meet to check.
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Old 03-25-2012, 10:36 AM   #25
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have someone hold something giving off smoke and you can watch the air flow from the attic. But if I were you I would do this.
1) Air seal the attic floor with foam. Can foam will work fine. Seal around all electric wires, plumbing and duct work penetrations.
2) move insulation to allow access to all interior wall partitions. You need to foam along both edges of the top plate.
3) Use baffles along the eave edges to prevent the existing soffit vents from being clogged
4) add blown insulation (cellulose is the best) over your existing insulation. I would add at least 8"
5) Be sure bath vents are vented through the roof
6) install an insulated air sealing cover to the access to the attic

periodically inspect the attic and monitor the humidity levels. It should match the outside air.

the above procedure if done carefully will save at least 25% in utility costs, stop ice damming and reduce the need for attic venting.
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Old 03-25-2012, 03:19 PM   #26
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Quote:
Without exception every manufacturer rep and presenter agreed that gable vents should be sealed off.
And if a manufacturer told you that their roofing withstood hurricane force winds would you take their word for it, or expect to see tests and ratings?

There was a time when everyone agreed the world was flat and the sun revolved around the Earth. Until someone took some measurements.

Matt,
Quote:
Also, is there any way to verify I do in fact have a continuous soffit?
Try going into the attic on a bright day. You should be able to see light at the eaves.
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Old 03-26-2012, 06:30 AM   #27
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In the case of closing off the gable vents, I have felt and seen the difference. I had one customer that insisted I leave them open. Attic was too hot. I closed them up and temps dropped. Been closing these for three years now. No more ice dams or moisture in any of my attics. so test or not, I agree with all the leading building scientist and accept the wisdom presented.
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Old 03-28-2012, 03:13 PM   #28
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According to Air Vent they say their research shows the best ventilation they get in their test labs are from ridge vents (they sell both) for proper attic ventilation. They say if you have equal amount of intake (soffit vents) as you do ridge vent then your attic is balanced and should be more than vented properly.

Hope this answers your question.

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