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Hip roof vent


kurt
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From what I've read, adding gable end vents to a roof that already has continuous soffit & ridge vents can cause a ventilation interruption in the attic. I can't name the source right now, but I recently read that in 2 separate articles. Continuous hip venting seems like it might create the same phenomenon.

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Never occurred to me that there might be the same conflict with this setup. How might that happen?

For the same reason a mid-height gable end vent stops the full air movement in the attic. Air comes in low through the soffit then washes out through the gable (or cont. hip) instead of reaching the cont. ridge.

But then again Jim is probably right.

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The entirety of insulation and ventilation practices is a mess of confusion based on mis-perceptions going back to the 1930's when insulation first started being installed in houses. Bill Rose provides a pretty good history of the mess in his book. I'm not particularly embarrassed to have been making such stupid calls about it for the last 20 years because all the codes were providing the wrong information. Now that we have a base of competently performed inspection information, we can make some educated calls regarding the (mostly non) issue.

That said, getting insulation and ventilation right in new structures is hyper critical. I've been seeing some absolutely amazing mold piles lately, all traceable to changes in building methods and materials starting in the 90's.

Inasmuch as I seem to be unable to get any of my customers to foam their houses so we could move beyond the problems, I want to get the ventilation thing right.

So, back to the question.......Could the hip thing mess up ventilation or make it better?

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I was under the impression that with a ridge/ soffit set up , the hotter air pressing against the ridge and escaping "sucked" the air through the soffit kinda like stack effect. So when you add gable vents it doesn't get "suction" at the bottom and pulls air through the gable ends. Does this make any sense?

Kurt, what is the name of Mr. Rose's book?

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Given a standard little 1200sf home with a straight hip roof, I think cont. soffit and cont. ridge (and/or mushrooms if the ridge is too short) would provide complete air movement through the attic. If cont. hip were added as well, you might not get upper attic air movement because the cont. hip would become exhaust vents.

Other roof designs may be more suitable for the cont. hip so I guess it's good that they are out there as an option, but I'm having a difficult time imagining what design that would be.

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The issue w/ the hip vent is that it acts both as an intake and an exhaust. It's very likely to provide ventilation at the ridge only.

Thats what I was thinking. The convection effect of the soffit and ridge combination works well with the intake at the lowest point and the exhaust at the highest. The stuff in the middle might interrupt that flow.

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I was under the impression that with a ridge/ soffit set up , the hotter air pressing against the ridge and escaping "sucked" the air through the soffit kinda like stack effect. So when you add gable vents it doesn't get "suction" at the bottom and pulls air through the gable ends. Does this make any sense?

Kurt, what is the name of Mr. Rose's book?

"Water in Buildings" by William B. Rose.

It is an absolutely wonderful dissection of all the ways water gets into buildings, and what it does after it gets there.

It should be required reading material for this job.

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The issue with hips is that the commons on the perpendicular slopes at each corner of the hip roof all dead end on the hip rafters and there is no clear path for air to flow from the eaves to a ridge vent at the upper roof. The hip vent opens the ends of those rafter bays so that air can flow from from the eaves to the hip ridges.

Think of the hip vents as a continuation of the ridge vent down the hips. The same thing that makes the ridge vent work makes the hip vent work - wind hits the dam, is diverted up and across the hip and bernoulli's principle takes over and starts sucking air up under both slopes.

Jimmy, I don't have any experience in Mass. but good attic ventilation is absolutely critical here. There's a lot of spore in the air here. If it gets into an attic and mixes with moisture trapped on the underside of a roof, and the roof deck is plywood or OSB, the roof is going to inevitably rot. Plank decks rot here too; just much slower.

ONE TEAM - ONE FIGHT!!!

Mike

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My brother's house would be a perfect candidate for hip vents, 1962 ranch, 900 SF, nearly square, 5/12 full hip, the ridge is literally only a few feet long.

John, the convection current concept is simply wrong. If all we were dealing with were temperature differentials it might work, but when you consider even just a few of the other factors at play the air flow becomes too chaotic to predict. The only way to ensure that the intakes remain intakes, and the exhaust remains exhaust is to apply a more reliable engine than convection currents, and no I'm not advocating power vents.

Why did we ever get away from the vent scheme that actually worked? Attic windows or gable vents have had a proven track record for over a century, and continue to function when people dink around addding insulation to buildings that were never intended to be insulated.

The only way the current vent theory is going to work is if the code requires the 'system' be engineered just like the structure is, and not left to three trades (roofers, siders, insulators) to dink up trying to work around each other with the current fast and loose guidelines. Given the current methodology it's a wonder any of it works at all.

Tom

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Why did we ever get away from the vent scheme that actually worked? Attic windows or gable vents have had a proven track record for over a century, and continue to function when people dink around addding insulation to buildings that were never intended to be insulated.

Tom

Because, again I'm speaking for this area, gable end vents don't work well unless the cover is cedar shingles, shake or one-by planking - all of which are fairly drafty.

There are lots of houses built with gable end vents here from the 60's, 70's and early 80's that were built with with plywood decks that have have had severe problems due to having just gable end vents. Many builders still haven't figured it out though; and they routinely build new homes with eave vents, upper vents near the ridge and then, like idiots, they cut in gable end vents.

They look at it as a belt-and-suspenders approach but they fail to understand why/how it doesn't work.

Lstiburek has tried to show how to moisture proof and vent buildings in various parts of the country. His methods work to a certain extent, but if we really wanted to eliminate ventilation issues we'd have as many methods as there are regional issues that affect ventilation and then we'd have no consistency whatsoever.

Of course, if we really, really trained our builders and gave them a good grounding in building science, so that they could understand how this stuff is supposed to work, maybe the non-consistency of builders figuring out their own areas and designing stuff to work in their areas, instead of begin slave to a code that was written by someone thousands of miles removed, would be the perfect solution.

ONE TEAM - ONE FIGHT!!!

Mike

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After Kurt suggested "Water in Buildings", I placed it in my Amazon shopping cart. I'm guessing that quite a few people bought this book after that, because the price went from seventy something dollars up to $97.02! Nice going Kurt! Of course this may have been a coincidence, but I think Amazon owes you some cabbage my friend.

I'll wait for the price to decline again before I hit the checkout button[:-monkeyd

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After Kurt suggested "Water in Buildings", I placed it in my Amazon shopping cart. I'm guessing that quite a few people bought this book after that, because the price went from seventy something dollars up to $97.02! Nice going Kurt! Of course this may have been a coincidence, but I think Amazon owes you some cabbage my friend.

I'll wait for the price to decline again before I hit the checkout button[:-monkeyd

You can borrow my copy anytime.

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My rancher has a hip roof. I have a ridge vent and full length soffit vents on all sides. The corner bedrooms (under the hip on the east side) have higher humidity in the summer. You can feel it pushing down from the ceiling when you walk into the rooms. The insulation is sufficient and pretty much even so I ruled that out. The opposite hip is over a carport so that's why we don't notice the difference at that end of the house.

I should try these vents on the east end and see what happens.

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