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Old 02-11-2010, 08:02 PM   #1
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Attic Ventilation Question


I have a house with a footprint of about 44' x 45'.

The roof pitch is very steep, and there are 4 sides to the roof. Two sides are 10/12, the other two are 7/12.
Does any have a handy equation to calculate the cubic volume in my attic?
On top, of course, is a ridge vent, and because the roof is steep, this ridge vent isn't that long.

I've been considering adding an attic fan to be used year round, but am also considering adding static roof vents also, but I've read that static roof vents will throw off the intention of the ridge vent.

Would the ridge vent be rendered useless if the attic fan is on also?

Also, what do people think about this solar powered attic fan?

Since this one only handles about 1,250 ft2, I guess I would need two.

http://www.costco.com/Browse/Product...=1&topnav=&s=1


Last edited by handy man88; 02-11-2010 at 08:23 PM.
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Old 02-11-2010, 09:29 PM   #2
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Attic Ventilation Question


44x45=1980 square feet. 1980 / 150 = 13.2 Net Free Area (NFA) of ventilation. That is the most common rule of thumb used by various manufacturers and building codes. That 150 is preferrably split evenly between the intake and exhaust, therefore you may divide by 300 to determine exhaust and intake. www.airvent.com www.rollevent.com has more information.

Ridge vent is 18" NFA per linear foot, typically although it may vary by product. Therefore if you have basically a 2000 square foot attic space you'll need approximately 55-65 linear feet of ridge vent and 100-130 linear feet of 2" continuous soffit vents. Or alternatively 30-40 4"x16" soffit vents. Sounds like your ridge is probably 10' too short. Unless you have a hip roof, then maybe it's even shorter than that

An attic fan and ridge vent are mortal enemies and should never be mixed. It creates what airvent calls a "short circuit". Ridge vent is a good ventilation system but is very finicky and very ez to disturb. Having a ridge vent and fan in combination is worse than useless, it's counter productive. An exhaust vent such as an attic fan knows I am to suck air. It doesn't know it is supposed to take the air from your soffit vents so that it cools the entire attic, no it's lazy. The fan when on will suck air from the path of least resistance which is your nearest opening which is likely your ridge vent, therefore your ridge vent becomes an intake when the fan is on and you only vent the upper few feet of the attic. Ouch.

However when the fan is off, it becomes an intake for the ridge vent, again only venting the upper few feet of the attic. Again I said ridge vent is finicky. Ridge vent will NOT work without at least equal or greater intake ventilation and no competing sources, like mushroom vents, fans or gable vents.

I have installed alot of solar powered attic fans. I don't buy my building products at the same place where I buy my food. Home depot sells the GAF model which moves 550 Cubic Foot Per Minute (CFM) therefore I prefer the Air vent Model which moves 880 CFM. It does cost about $300 where as I think the home depot model costs like $250 or something. Sometimes the airvent solar cool attic fan goes on sale, and I stock up and keep them at my shop.

If you were to get the air vent solar cool attic fan, for an attic your size you would need to remove the ridge vent and install no less than 2 fans on your house. My house is roughly 20'x60' plus attached garage and due to the shape, I installed 3 solar fans. One is dedicated to the attached garage.

What people don't understand and I felt I needed to point out because of your comment "has ridge vent of course" is that attic ventilation is a science that is unique to the architecture of each building. Therefore there are no one size fits all ventilation solutions. In other words when it comes to attic ventilation there is no such thing as "of course"

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Old 02-11-2010, 09:36 PM   #3
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Handyman88,

Just thinking about your question makes my head hurt, so I wont answer it.

Instead I will just add that my code calculates roof ventilation off the area, not the volume, which is easier.

If I have enough pitch, and there is no doubt that you do, and I can install vents at least 3' above the eave (not a problem for you), I can size my total minimum venting at 1:300, providing more than 50% but less than 80% is "high," and the remaining 50% to 20% is at the eaves.

If I had a 1,980 sq ft attic, I would need 6.6 sq ft of total vent minimum, and I would want between 50% and 80% of the total vent area as close to the ridge as I can install it.

I have also heard that adding vents between the existing vents can somehow interrupt the flow of air from the eaves up and out closer to the ridge, but I never understood why that would be so. I have always thought that warm moist air is going to rise, regardless, and if it's not rising, it's because it's cold and dry and doesn't need to. Maybe someone can offer some experience with occasions when added vents have been a problem.

I would be more inclined to add run of the mill vents to get the system working without adding fans, but again, maybe there are roofers or builders on the forum who know better.

I am curious to know if you are addressing this because of moisture problems, excessive heat, or something else?

Cheers,

Rory

RDG Read Development LLC
Portland, OR
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Old 02-11-2010, 10:31 PM   #4
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Quote:
Originally Posted by Grumpy View Post
44x45=1980 square feet. 1980 / 150 = 13.2 Net Free Area (NFA) of ventilation. That is the most common rule of thumb used by various manufacturers and building codes. That 150 is preferrably split evenly between the intake and exhaust, therefore you may divide by 300 to determine exhaust and intake. www.airvent.com www.rollevent.com has more information.

Ridge vent is 18" NFA per linear foot, typically although it may vary by product. Therefore if you have basically a 2000 square foot attic space you'll need approximately 55-65 linear feet of ridge vent and 100-130 linear feet of 2" continuous soffit vents. Or alternatively 30-40 4"x16" soffit vents. Sounds like your ridge is probably 10' too short. Unless you have a hip roof, then maybe it's even shorter than that

An attic fan and ridge vent are mortal enemies and should never be mixed. It creates what airvent calls a "short circuit". Ridge vent is a good ventilation system but is very finicky and very ez to disturb. Having a ridge vent and fan in combination is worse than useless, it's counter productive. An exhaust vent such as an attic fan knows I am to suck air. It doesn't know it is supposed to take the air from your soffit vents so that it cools the entire attic, no it's lazy. The fan when on will suck air from the path of least resistance which is your nearest opening which is likely your ridge vent, therefore your ridge vent becomes an intake when the fan is on and you only vent the upper few feet of the attic. Ouch.

However when the fan is off, it becomes an intake for the ridge vent, again only venting the upper few feet of the attic. Again I said ridge vent is finicky. Ridge vent will NOT work without at least equal or greater intake ventilation and no competing sources, like mushroom vents, fans or gable vents.

I have installed alot of solar powered attic fans. I don't buy my building products at the same place where I buy my food. Home depot sells the GAF model which moves 550 Cubic Foot Per Minute (CFM) therefore I prefer the Air vent Model which moves 880 CFM. It does cost about $300 where as I think the home depot model costs like $250 or something. Sometimes the airvent solar cool attic fan goes on sale, and I stock up and keep them at my shop.

If you were to get the air vent solar cool attic fan, for an attic your size you would need to remove the ridge vent and install no less than 2 fans on your house. My house is roughly 20'x60' plus attached garage and due to the shape, I installed 3 solar fans. One is dedicated to the attached garage.

What people don't understand and I felt I needed to point out because of your comment "has ridge vent of course" is that attic ventilation is a science that is unique to the architecture of each building. Therefore there are no one size fits all ventilation solutions. In other words when it comes to attic ventilation there is no such thing as "of course"
I agree with most of your statements. I will though, add a comment to the statement below:
Quote:
Originally Posted by Grumpy View Post
An attic fan and ridge vent are mortal enemies and should never be mixed. It creates what airvent calls a "short circuit". Ridge vent is a good ventilation system but is very finicky and very ez to disturb. Having a ridge vent and fan in combination is worse than useless, it's counter productive. An exhaust vent such as an attic fan knows I am to suck air. It doesn't know it is supposed to take the air from your soffit vents so that it cools the entire attic, no it's lazy. The fan when on will suck air from the path of least resistance which is your nearest opening which is likely your ridge vent, therefore your ridge vent becomes an intake when the fan is on and you only vent the upper few feet of the attic. Ouch.

However when the fan is off, it becomes an intake for the ridge vent, again only venting the upper few feet of the attic. Again I said ridge vent is finicky. Ridge vent will NOT work without at least equal or greater intake ventilation and no competing sources, like mushroom vents, fans or gable vents.
In theory, this statement is correct, but given the "small size" of the ridge vent (flow restricted), it could also say that:

In the heating or cooling season, because the ridge vent is size challenged, it alone may not be the sole intake for the the attic fan, especially with the hot air in the attic rising to the roof peak from the soffits as another intake.

The attic fan can not be just pulling air from the ridge vent unless the ridge vent is oversized

My neighbor across the street actually had two attic fans installed and he noted a considerable difference in how cool the 2nd floor is in his house during the summer. He actually remarked that before, he could never get his AC down to 76F, but now he can.

In conclusion, I think what you are telling me is that if I decide to get an attic fan, I should close off the ridge vent. Right?

What happens during the summer if the attic fan fails and there is no ridge vent? That would be scary.
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Old 02-11-2010, 10:36 PM   #5
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Attic Ventilation Question


Quote:
Originally Posted by Rory Read View Post
Handyman88,

Just thinking about your question makes my head hurt, so I wont answer it.

Instead I will just add that my code calculates roof ventilation off the area, not the volume, which is easier.

If I have enough pitch, and there is no doubt that you do, and I can install vents at least 3' above the eave (not a problem for you), I can size my total minimum venting at 1:300, providing more than 50% but less than 80% is "high," and the remaining 50% to 20% is at the eaves.

If I had a 1,980 sq ft attic, I would need 6.6 sq ft of total vent minimum, and I would want between 50% and 80% of the total vent area as close to the ridge as I can install it.

I have also heard that adding vents between the existing vents can somehow interrupt the flow of air from the eaves up and out closer to the ridge, but I never understood why that would be so. I have always thought that warm moist air is going to rise, regardless, and if it's not rising, it's because it's cold and dry and doesn't need to. Maybe someone can offer some experience with occasions when added vents have been a problem.

I would be more inclined to add run of the mill vents to get the system working without adding fans, but again, maybe there are roofers or builders on the forum who know better.

I am curious to know if you are addressing this because of moisture problems, excessive heat, or something else?

Cheers,

Rory

RDG Read Development LLC
Portland, OR
So what you are saying is to add more static vents near the top of the roof to allow more openings for the hot air to escape?

It does not make sense to add more static vents if the ridge vent is oversized, but since my ridge vent is NOT oversized which would slow the exit flow rate of hot air, then adding static roof vents near the ridge vent seems to make more sense than not.

The reason why I'm asking about this is for the summer, really, but on a side note, I do plan to add more insulation to my attic as I think warm dry air is escaping to the attic during this winter.

Last edited by handy man88; 02-11-2010 at 10:40 PM.
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Old 02-12-2010, 05:36 AM   #6
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Attic Ventilation Question


Adding more venting near the top will hinder the draw of air up the eave venting.
The ventilation will take the path of least resistance, so the ridge venting and the added vents will draw from each other and fail to draw air all the way from the eave vents.
In my experience, you would be better off with closing/removing the ridge vent and adding Whirlygigs near the top of the roof on the back side. They are more effective, IMHO.
What is the existing soffit venting like? Might it be plugged up and not allowing air to draw???
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Old 02-12-2010, 06:04 AM   #7
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Here is a guide to help you.

http://www.lomanco.com/VentilationGuide/ventguide.html
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Old 02-12-2010, 09:22 AM   #8
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Handyman88,

My 1cent, most likely worth less than everyone else's 2cents, is that you figure how much venting you do have, including the ratio between the eave and ridge venting.

If you have less than code, add more venting. And, as others have noted, you need to make sure that the vents are not blocked up.

If you have more than code, and the vents are not blocked, and the venting is properly distributed between the high and low vents, then maybe you do have an insulation problem instead.

If you are contemplating adding more loose insulation to the attic, make sure the eave baffles are long enough to rise above the new level of insulation. Take a take a close look at ceiling fixtures and other penetrations and seal up any cracks and openings. Ideally, and possibly necessarily, all the fixtures would be air tight and rated for direct contact with insulation. Consider a fresh coat of latex paint as an added vapor retarder.

To some of the roof experts, a question: If I have a pyramid shaped hip roof, and it's steep, do I bring all the peak or high side vents down low enough so that they are in a line? On the one hand, this addresses the concern about drafting from the eaves, but, on the other hand, it creates a pocket at the peak, above the high side vents, where moisture would build up.

Cheers, thanks for all the advice,

Rory

RDG Read Development LLC
Portland, OR
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Old 02-12-2010, 09:35 AM   #9
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you have an irregular hip, I'm thinking 8' ridge?
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Old 02-12-2010, 11:51 AM   #10
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Quote:
Originally Posted by Rory Read View Post
Handyman88,

My 1cent, most likely worth less than everyone else's 2cents, is that you figure how much venting you do have, including the ratio between the eave and ridge venting.

If you have less than code, add more venting. And, as others have noted, you need to make sure that the vents are not blocked up.

If you have more than code, and the vents are not blocked, and the venting is properly distributed between the high and low vents, then maybe you do have an insulation problem instead.

If you are contemplating adding more loose insulation to the attic, make sure the eave baffles are long enough to rise above the new level of insulation. Take a take a close look at ceiling fixtures and other penetrations and seal up any cracks and openings. Ideally, and possibly necessarily, all the fixtures would be air tight and rated for direct contact with insulation. Consider a fresh coat of latex paint as an added vapor retarder.

To some of the roof experts, a question: If I have a pyramid shaped hip roof, and it's steep, do I bring all the peak or high side vents down low enough so that they are in a line? On the one hand, this addresses the concern about drafting from the eaves, but, on the other hand, it creates a pocket at the peak, above the high side vents, where moisture would build up.

Cheers, thanks for all the advice,

Rory

RDG Read Development LLC
Portland, OR
I think everything should be per code. It's a 10 year old house in a builder built community...nothing custom or add on by the homeowner.

I do know that certain areas of my attic have little insulation and I'm working on remedying that problem now.
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Old 02-12-2010, 10:50 PM   #11
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http://books.google.com/books?id=Z8a...20NFVA&f=false

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