Better roof vents ?


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Old 07-18-16, 08:09 PM
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Better roof vents ?

I hope this is the correct forum. I was checking my attic this week and found it super hot and humid. It was clear air is not moving.

I've been reading so many contradictory articles on this. I'm hoping for some real world wisdom.

I've been in my house for three years. Have a pyramid shaped cottage roof. There are 4 of those mushroom vents at the top of each slope

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OK. there's about 16" of cellulose blown up there and a baffle every second rafter. I can see daylight through the baffles.(at least the ones I could get to)

So I want to replace the upper vents. I'm looking at turbines, solar and even AC powered...I really don't want to start wiring and also don't want something that will break down

Is there any practical way of improving air movement? Are there better vents? Maybe something that extends above the peak like a chimney? I don't care too much about exterior appearance if it protects my house.

Thoughts?
 
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Old 07-18-16, 08:36 PM
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That roof style is properly called a "hip" roof.

How do you define "super hot"? It is perfectly normal for the temperature in the attic to be 20 to 40 degrees Fahrenheit higher than the outside temperature. It is the difference in temperature that drives the air movement so the closer the inside (attic) temperature is to the outside the slower the air will move.
 
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Old 07-18-16, 09:27 PM
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It was clear air is not moving.
I agree with Furd that what you have found sounds perfectly normal. You say it's "clear" that air is NOT moving, but I would disagree with that. Get on the roof and put your face next to one of those vents and see how you like it. You aren't going to get a nice cool breeze in your attic if that's what you're thinking. Even well ventilated attics are too hot to be inside on sunny summer afternoons. If the moisture was a problem you would have mold, sheathing buckling and shingles cracking.

Keep in mind that limiting heat loss in the winter is probably a bigger concern than the normal conditions you are seeing, so adding more ventilation may be great now but might come back to bite you in the winter if it increases heat loss.
 
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Old 07-19-16, 05:21 AM
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We're talking over 60 C (140 F). On a day that's 26 C (79 F). That's normal?
 
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Old 07-19-16, 06:31 AM
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Hi Joe,
First "Maybe something that extends above the peak like a chimney?" Actually that is very common. Commercial buildings will often build a shaft above a flat roof to provide a high point to enhance the air flow. Someone recently posted this link for getting above the snow, but offers some height benefits as well:
Product information for the Slope Roof Model #301 Roof Ventilator

But, Furd and X are correct, hot is normal for a naturally drafted attic. Power venting can move more air, but most existing building have many leakage paths from inside the home into the attic so you can end up cooling off the attic with your expensive conditioned air from inside. I'm assuming you are running ac in the house.

16" of cellulose is your best defense against that heat. The ventilation is more for removing moist air that seeps in during the winter.

Another comment on power venting. Technology and building techniques will eventually manage the attic exhaust fans to minimize their negative side and benefit from moving that heat in a rapid fashion. Long before I got into energy auditing I installed an attic fan to allow the house to cool down after the sun set and it worked nicely. I have also encountered many homes that use a whole house fan exhausted into the attic to draw fresh air into the house while purging the attic heat, also works well but also exhausts that cool dry inside air.

Bud
 
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Old 07-19-16, 07:11 AM
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Thanks Bud. The taller stack is starting to appeal to me. Thinking of how a chimney draft works. Thank you
 
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Old 07-19-16, 07:45 AM
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The equation for natural air pressure (stack effect, simplified version) is:
pressure (Pascals) = .007 x height (ft) x delta temp (F). If high and low vent areas are equal then that number is divided half for high vents and half for low vents. The result is a very small number and thus the reason we need so much vent area.

But, double the height and you double the resulting air flow.

Wind is also a major contributor so you are not just relying on the above calculation.

Bud
 
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Old 07-19-16, 08:05 AM
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"double the height and you double the resulting air flow." That's the part I understood :-)

Thanks again
 
 

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