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Old 05-18-2011, 01:26 PM   #1
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Attic venting questions


I have a house that has an attic space of around 2300 sq feet. Ventilation is currently soffit vents for intake and then ridge vents and two powered vents of exhaust. The picture attached shows the location of the two powered vents. There are several small ridge vents and one large one (where the red line is painted). By my calculations I should have 552sq inches intake and 552 sq inches exhaust (2300/300 = 767 x 144 = 1104 / 2 = 552). So according to that 30 feet of ridge vent should be enough. But most of that is over where the red line is drawn. The rest of the house has very short runs of ridge vents.

I am getting ready to put a new roof on my house and I am wondering what is the best way to vent. I would prefer not to have the powered fans, but I just don't think the ridge vents are enough. The roof is 17 years old and was in ok shape before the hail storm so I am inclined to leave well enough alone and just make sure I seal up the recessed lighting and top caps in the attic to minimize the fans sucking out my cooled air. I was worried about the short circuit effect, but seeing some real world examples in videos has made me believe that may be a myth.

Thanks for any suggestions.
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Old 05-23-2011, 05:06 PM   #2
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Attic venting questions


On roofs like this I almost never spec ridge vent. Look into solar powered attic fans. I have used air vent, which are 880 cfm, 55% more powerful than the GAF solar fans.


It is not a myth. Where do yout hink the fan will pull air? The path of least resistance. That often means the closest opening, that closest opening is likely the ridge vent 2' away from the fan which leaves 98% of your roof unvented.

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Old 05-23-2011, 10:38 PM   #3
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Attic venting questions


I saw a video of an attic filled with smoke and it had a ridge vent and an attic fan. The smoke really wasn't moving much at all until the fan kicked on. When it did all of the smoke in the attic started moving toward the fan. It seems that if the fan was pulling from the ridge vent, the smoke wouldn't have moved at all. This and other posts I have read from people way more knowledgeable on the subject than I am is why I referred to it as a possible myth. Also, regardless of how much is being pulled in from the ridge vent, hot air will rise. If the air in the attic is not being ventilated, it will be getting hot and sooner or later it will rise up to the top and mix with the air that is being pulled out by the fan.

It seems that many people recommend solar powered fans. Is that because of the energy they save, or because they will run pretty much all day?
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Old 05-24-2011, 08:21 AM   #4
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Attic venting questions


I don't know what dis-information you got from proclaimed experts. I have been a roofer for 13 year and have really been studying attic ventilation for the past 10. I have been arguing with guys about ventilation on these forums since I began studying it. The ammount of "I do it my way" and "I know more than the manufacturer" that is out on the internet is staggering. You may find "guys" saying it's ok, but show me just one fan or ridge vent manufacturer who approves or encouraged the use of fans in conjunction with other vents (excluding intake) and I will forever denounce the short circuit theory. I, or anyone, can do and say what ever I want, that doesn't make it true. As a professional roofing contractor it is my obligation and duty to install the products that I do install to the instructions and methods as prescribed by the manufacturer of said products. Doing otherwise I assume risk and liability and most of all risk wallowing with the trunk slammers and hacks that do what ever they feel like.

I admit I am not an engineer. I have not spent thousands of dollars building test houses to perform an independant study to prove nor disprove the theory. But I do what the product engineers who have done the calculations and studies tell me to do. Simply taking the time to read installation instructions would eliminate this argument all together.



I recommend solar fans in retrofit applications because they require no wiring. I am not a licensed electricin and my insurance exlcuds electrical related activities. Therefore running a circuit up from your breaker is not something we are qualified to do. Getting an electrician involved will cost a few hundred bucks to a few thousand bucks. From a pure economical point of view, solar fans are much cheaper to install when no fans already exist.

The running all day thing is also an added benefit, especially in winter. Most attic fans unless they have a humidistat will not operate at all in winter, but attic ventilation is equally as important in winter as it is in summer. Most traditional hard wired attic fans don't have humidistats because they cost extra money. People are often inherintly cheap.

The money a solar fan saves on electricity is also a bonus. And the money you get tax back is certainly an incentive.

The cons to solar fans is that a solar fan costs more than a traditional hard wired fan, but not that much more, maybe $100 more than a similiar powered wired fan. The other con is that no solar fan is as powerful as the average 1100CFM wired attic fan. 1100 CFM or very close to it is pretty much the standard, although I have installed larger and smaller wired fans. This means you often need more fans. The cost thing really isn't a factor on retrofit applications because of the electrical scenario described above.
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Old 05-24-2011, 09:40 AM   #5
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Attic venting questions


I hope I'm not hi-jacking the thread with this question - in a hot climate such as S. Texas/Arizona/Nevada, using solar-powered roof fans, wouldn't there be an issue with residual heat once the sun goes down and the fans stop?
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Old 05-24-2011, 09:50 AM   #6
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Attic venting questions


The solar fans will still allow air to ventilatewhen the sun goes down. Though they would then become passive instead of active, very much like a large mushroom/breather vent.

There is a product that I know of that turns a solar fan into a hybrid solar/electric fan. However it then requires wiring.
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Old 05-24-2011, 09:50 AM   #7
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Attic venting questions


Quote:
Originally Posted by lemmy View Post
I saw a video of an attic filled with smoke and it had a ridge vent and an attic fan. The smoke really wasn't moving much at all until the fan kicked on. When it did all of the smoke in the attic started moving toward the fan. It seems that if the fan was pulling from the ridge vent, the smoke wouldn't have moved at all. This and other posts I have read from people way more knowledgeable on the subject than I am is why I referred to it as a possible myth. Also, regardless of how much is being pulled in from the ridge vent, hot air will rise. If the air in the attic is not being ventilated, it will be getting hot and sooner or later it will rise up to the top and mix with the air that is being pulled out by the fan.

It seems that many people recommend solar powered fans. Is that because of the energy they save, or because they will run pretty much all day?

You are correct about the "short circuiting". If it does occur, heated/moist air is still being exhausted, just maybe not quite as efficiently as it would be if short circuiting did not occur.

My primary concern with power vents is that if there are any breaches in the envelope between the conditioned space and the attic space, conditioned air can be pulled from the living space into the attic, which is a very undesirable side effect.

So, before installing a power vent, air sealing is necessary as is making sure there is adequate intake. If there is plenty of intake to satisfy the power vents pull, then short circuiting through gable or ridge vents already in place should be minimal.

The problem with solar powered fans, IMHO, is the initial expense and they quit running at sundown. There's usually still a lot of stored heat in the structure that needs to be exhausted after sundown.

Power vents can be very effective at attic cooling, but a properly designed static system can as well. The smoke test video offers up some real information, but every instance is not equal to the situation shown. I use power vents only as a last resort.

If you have a problem with heat transferring through the ceiling, then you have inadequate or improperly installed insulation. Trying to cool the inside of the house down by sucking hot air out of the attic is treating the symptom, not the disease.

If you have a moisture problem in the attic, then you certainly need more/better venting, but you are only going to get the attic temperature down so far on a hot, sunny day, no matter how or how well the attic is vented.
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Old 05-24-2011, 10:10 AM   #8
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Attic venting questions


Quote:
Originally Posted by Grumpy View Post
I don't know what dis-information you got from proclaimed experts. I have been a roofer for 13 year and have really been studying attic ventilation for the past 10. I have been arguing with guys about ventilation on these forums since I began studying it. The ammount of "I do it my way" and "I know more than the manufacturer" that is out on the internet is staggering. You may find "guys" saying it's ok, but show me just one fan or ridge vent manufacturer who approves or encouraged the use of fans in conjunction with other vents (excluding intake) and I will forever denounce the short circuit theory. I, or anyone, can do and say what ever I want, that doesn't make it true. As a professional roofing contractor it is my obligation and duty to install the products that I do install to the instructions and methods as prescribed by the manufacturer of said products. Doing otherwise I assume risk and liability and most of all risk wallowing with the trunk slammers and hacks that do what ever they feel like.

I admit I am not an engineer. I have not spent thousands of dollars building test houses to perform an independant study to prove nor disprove the theory. But I do what the product engineers who have done the calculations and studies tell me to do. Simply taking the time to read installation instructions would eliminate this argument all together.



I recommend solar fans in retrofit applications because they require no wiring. I am not a licensed electricin and my insurance exlcuds electrical related activities. Therefore running a circuit up from your breaker is not something we are qualified to do. Getting an electrician involved will cost a few hundred bucks to a few thousand bucks. From a pure economical point of view, solar fans are much cheaper to install when no fans already exist.

The running all day thing is also an added benefit, especially in winter. Most attic fans unless they have a humidistat will not operate at all in winter, but attic ventilation is equally as important in winter as it is in summer. Most traditional hard wired attic fans don't have humidistats because they cost extra money. People are often inherintly cheap.

The money a solar fan saves on electricity is also a bonus. And the money you get tax back is certainly an incentive.

The cons to solar fans is that a solar fan costs more than a traditional hard wired fan, but not that much more, maybe $100 more than a similiar powered wired fan. The other con is that no solar fan is as powerful as the average 1100CFM wired attic fan. 1100 CFM or very close to it is pretty much the standard, although I have installed larger and smaller wired fans. This means you often need more fans. The cost thing really isn't a factor on retrofit applications because of the electrical scenario described above.

Those are some excellent points about the solar vents.

I prefer not to install power vents except as a last resort, but I have not been able to find any product literature that says NOT to combine passive and powered exhaust. I can find a lot of opinion about the subject on about every forum that exists, but no manufacturer's recommendations. You are correct that there are no recommendations to use the two together, but I'd like to see some product specs/literature/instructions, that says specifically not to combine the two if you have it.
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Old 05-24-2011, 03:11 PM   #9
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Attic venting questions


Quote:
Originally Posted by seeyou View Post
but I have not been able to find any product literature that says NOT to combine passive and powered exhaust. I can find a lot of opinion about the subject on about every forum that exists, but no manufacturer's recommendations. You are correct that there are no recommendations to use the two together, but I'd like to see some product specs/literature/instructions, that says specifically not to combine the two if you have it.
Here you go:
Quote:
Originally Posted by AirVentInc.
Can I install a power fan if I have a ridge vent already on my roof?
Mixing a power vent with a ridge vent can short-circuit the attic ventilation system just as a gable vent can as discussed above. This happens because air follows the path of least resistance. When the power vent turns on, it can pull air from the ridge vent, which could lead to weather infiltration and unbalanced airflow along the underside of the roof deck. When the power vent turns off, it acts like a roof louver — an opening on the roof without a motor. In this scenario, the ridge vent pulls its intake air from the power fan leading to possible weather infiltration and less than optimal ventilation along the underside of the roof deck.

Basically, mixing two different types of exhaust vents on the same roof that has a common attic is not recommended because it can lead to short-circuiting of the attic ventilation system.
taken from: http://www.airvent.com/professional/...shooting.shtml

It's also found in their ventilation education certification found here: http://www.airvent.com/professional/vip.shtml

Not only have I gone to their seminars every other year, but I have also completed both certifications for specifiers as well as installers. I have also required my employees to get certified with Air Vent as well.



I am currently negotiating with another solar fan manufacturer to become one of their dealers as well as their exclusive certified installer for the Northern IL area.
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Old 05-24-2011, 04:30 PM   #10
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Attic venting questions


Thanks.

I notice that they're very careful to say "CAN" vs "WILL" short circuit.

I still think that if there's proper inlet, in most cases, short circuiting will be fairly inconsequential. Not saying it doesn't happen, but I think the real myth is that air will enter a gable or ridge vent and exit through a power vent without picking up some heat or moisture. Will that path be as efficient as entering at a lower point? Absolutely not, but it doesn't seem to me to be the problem some think it is.

Now, FWIW, I've installed maybe 10 power vents total in about 30 years of roofing, so I'm not advocating mixing power vents with other types of outlet and I don't have encyclopedic knowledge of them.

I've seen smoke tests done inside the living space when a power vent is running with "assumed" adequate intake. Lots of smoke can get sucked through the penetrations in a ceiling. So, given the options, I'd rather see some air pulled in a ridge vent than pulled from around a can light.
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Old 05-24-2011, 06:19 PM   #11
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Attic venting questions


What so bad about "short circuiting?" It's not like the hot air in the attic is not escaping just because the ridge vent is being used as a soffit vent.

The draw from this exit flow will pull the hot air out also, as it rises, and faster than natural convection.
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Old 05-25-2011, 07:37 AM   #12
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Attic venting questions


Copper, the answer to fans pulling from light fixtures is not to install a ridge vent as an intake but to install some form of intake as an intake and better seal the fixtures.

Mixing a roof fan with gable vents doesn't bother me so much when there is no intake. However mixing let's say a gable fan with a ridge vent is just dumb because it'll pull air strictly from the ridge vent and not the corresponding gable vent.

What's so bad handy man is two things. 1 if I do it according to specification I can bever be blamed should there be a mold, condensation, premature shingle failure or other ventilation related problem. The problems that result from poor ventilation are numerous. I have substantial and documented proof that I did my work according to specification which is my job and was what I was hired to do. This is the reason I try to always do everything that I do to specification. If there is a problem and I did something the manufacturer said not to do, guess who is now at fault? Me. When customers insist on me going agains spec, I always make them sign a disclaimer. This would be one of those instances.

Liability aside. You're right if you want to split hairs. The attic ventilation will be more effecient than passive vents or as you put it natural convection, even if the fan pulls some air from the ridge vent. But let's be honest the fan would be much much more effecient if the ridge vent were not there for the fan to pull air from, and the fan were pulling air from proper intake at or near the overhangs. So what you are saying is that the ventilation will still work, and I agree. What I am saying is the ventilation will work BETTER, and for some reason people refuse to agree.
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Old 05-25-2011, 07:50 AM   #13
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Attic venting questions


Any recommendations on a solar fan? I have found an attic breeze model that is 1350-1550 CFM. Do the solar fans last as long as the electric ones?

my house is already wired for a powered fan so electrician cost really isn't an issue.

I haven't lived in this house very long. Only a couple of months. The roof was 17 years old and was recently damaged by hail. So it is getting replaced and I really don't know if there were problems or not. The roof was in ok shape but needed replacing fairly soon before the hail damage. So I assume whatever ventilation system that was in place was working ok.

I plan on sealing my house (top cap and canned lights) as much as I can. But I can't get to the top of some of the lights because it is a cathedral ceiling in areas. however my one concern with a solar powered fan is that it would run even more than an electric one so it seems it would suck even more air conditioned air from the house than an electric one (given the same CFM).
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Old 05-25-2011, 09:09 AM   #14
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Attic venting questions


the answer to fans pulling from light fixtures is not to install a ridge vent as an intake but to install some form of intake as an intake and better seal the fixtures.

Mixing a roof fan with gable vents doesn't bother me so much when there is no intake. However mixing let's say a gable fan with a ridge vent is just dumb because it'll pull air strictly from the ridge vent and not the corresponding gable vent.


Grumpy, we're pretty much in agreement here. The point I'm trying to make above all, is that there is no one answer to attic ventilation. My concern is those who would install a power vent without adding adequate intake (and that's advice frequently given on this forum) and air sealing the living space. Adding a power vent to counter inadequate insulation is also foolishness, IMHO.

A seasoned pro such as yourself is able to quickly and accurately assess a situation in person and arrive at the best solution. Since attic venting questions are asked so often on this forum, it seems like we ought to come up with a check list of questions for the person with a hot attic to ask themselves. Anyone please feel free to add or subtract and maybe we'll start a new thread to refer to when we get the check list finished.

1) type or roof - hip, gable, flat
2) slope
3) roof material
4) attic sq feet
5) type/thickness of insulation
6) has the living envelope been air sealed?
7) does the house have soffits and are they currently vented? If so, describe vents, quantity and size.
8) existing roof venting - describe - quantity and size
9) what am i forgetting?



Last edited by seeyou; 05-25-2011 at 10:15 PM.
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Old 05-25-2011, 09:24 AM   #15
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Quote:
Originally Posted by lemmy View Post
Any recommendations on a solar fan? I have found an attic breeze model that is 1350-1550 CFM. Do the solar fans last as long as the electric ones?

my house is already wired for a powered fan so electrician cost really isn't an issue.

I haven't lived in this house very long. Only a couple of months. The roof was 17 years old and was recently damaged by hail. So it is getting replaced and I really don't know if there were problems or not. The roof was in ok shape but needed replacing fairly soon before the hail damage. So I assume whatever ventilation system that was in place was working ok.

I plan on sealing my house (top cap and canned lights) as much as I can. But I can't get to the top of some of the lights because it is a cathedral ceiling in areas. however my one concern with a solar powered fan is that it would run even more than an electric one so it seems it would suck even more air conditioned air from the house than an electric one (given the same CFM).
There are several available caulks and foams to seal around the perimeter of can lights as well as bulb baffles that will greatly reduce the infiltration through the cans for situations like you describe. The top plate where wires and pipes penetrate is also an area of concern.

What I've read on several solar fan sites is that since the solar units are less powerful, they would in theory create less depressurization of the attic than a hardwired unit. So, in theory, the solar fan is running constantly at a lower rate, creating less interior suck. The hardwired unit is running hard, cutting off, running hard, cutting off. However, real life experience tells me that on a fairly warm, sunny day, the hardwired unit is not going to cut off very often.

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