How does attic vent fan work

The hotter an attic is, the warmer the home will be during the summer. However, by expelling the hot air, an attic fan brings your attic closer to the outside temperature. Your air conditioner can work on cooling your house rather than working against the air coming from your attic. These fans also have some benefits in the winter.

Often during colder months, the warm air from your home meets the cold air just under your roof. This could lead to excessive moisture and possibly some damage to your roof from mold or ice damming. The fan will circulate the air, which could prevent any of this excessive moisture and potential damage.

Many people may get caught up in the promises of a fan for the top of the house. Therefore, your air conditioning will not need to work as hard. However, an attic fan will not cool an entire house. Now, what about the cost? Because attic fans take away some of the work that your air conditioning would be doing, you will spend less on running your air conditioning. However, running an electric fan in attics could mean the cost just goes towards the attics rather than the air conditioner.

So are attic fans good or bad? Well, there is thankfully an alternative to choosing between no attic fan or paying for an electric attic fan. Solar attic fans work just like electric fans and have all the same benefits. The difference is that after you pay to install them, you never pay for them again. They run on solar energy. So the only thing powering your attic fan is the sun rather than your electric bill. This means that with a solar fan in your attic, you can take the pressure of your air conditioning without paying for it in your attic.

Unlike electric attic fans, this can actually end up saving you money every month, especially in the hottest weeks of summer. But a solar attic fan will do everything that you would want your electric unit to do.

It will push out hot air in the summer. In extremely warm climates it has been scientifically proved to cool the attic by an average of 20 degrees. And it will keep your attic dry in the winter, preventing future costly roofing and maintenance issues.

Your fan will run 12 months of the year without costing you a dime after installation. Read our review on the best solar attic fans. Now you know everything there is to know about how attic fans work, and you may be getting a good idea of whether it may be right for your home. But are attic fans good or bad? Knowing the benefits as well as the disadvantages will help you decide if fans in your attic would work in your home. Based on our industry experience, the answer is, yes.

According to the Home Ventilating Institute HVI , powered attic ventilators need to move a minimum of cubic feet per minute cfm for 1, sq. Ideally, there should also be plenty of soffit intake space- their calculations recommend net square inches of open soffit ventilation to supply cfm. HVI recommends a ratio of 60 to 40 for soffit ventilation to gable or ridge ventilation.

Attic ventilation fans, whether hard-wired or powered by their own solar panels, seem like a low-cost and effective way to help keep your house cool. However, the utility of attic ventilation fans is actually quite a controversial topic. Good insulation reduces fan effectiveness.

Fans can cause increased loss of conditioned air. Another downside is that homes that do not have air-sealed attics can lose some amount of their conditioned air from the suction of the attic ventilation fan, depending on how much soffit vent space is available. Prove it to yourself either way. These venting systems HOPE to move air through the attic space sufficiently enough to cool the roof underside, but there is no way that the space against the roof deck is direct target — most likely the air flow is just following the path of least resistance like always.

I suspect that the reason some have seen cooler attics when a reflective barrier is placed under the roof is because it traps air between the roof deck and the barrier, causing the air space to heat up even higher than without the barrier and causing a greater thermosyphon action to occur against the roof deck underside.

To see if this might be so, I have experimented in my attic about , no soffit vents, ridge vent roofers… and 6 windows now partially open to supply outside air by adding conventional pink foam attic vent troughs along the entire under-deck area to accelerate the air flow against the roof deck and reduce radiation into the attic space.

I have experienced a lowering of the attic air temps and lower radiant ceiling temps even though there is no direct path of outside air to the bottoms of the troughs. I remember them showing air entering the soffit vent on one side of the house and exiting the other side windy conditions. To me, in combination with a reflective metal roof, this is the ultimate roofing assembly. BUT making statements like you have gives the product a bad reputation because the people professing its attributes are overselling it and that looks bad.

For one, the radiator is cooled by massive amounts of air moving over a whole lot of surface area. Take a look at all those cooling fins. Second, the air moving over the radiator is making direct contact with the surface of the radiator. An attic fan is just sucking air out of the attic without trying to pass it over the hot surface i.

Allison…your comments are Allison…your comments are infuriating and show and general lack of understanding. BUT there are times when the installation a unit can be beneficial but it depends on several things…1 proper installation 2 proper conditions environmental factors 3 on a lesser note…. Nor am I. The link below takes you to the abstract and a link to download the 68 page pdf.

Literature Review of the Impact and Need for Attic Ventilation in Florida Homes Yes, their focus is on Florida, but their literature review covers research in a number of locations. The main takeaways from the research are: 1. PAVs may help reduce cooling loads a little bit, but their energy use wipes out most or all of the savings.

PAVs can backdraft combustion appliances. So I follow and agree with So I follow and agree with all points, however, my point in installing an attic fan, and everyone I know who has, and does, and wants to, is exactly because we want 3 to happen.

The physics are not in question. What our goal is and what the intended goal of attic fans has always actually been is, to draw cool air up through the living space from the cellar and deep crawl spaces and out of the attic. You do this during the part of the year where the outside temperature is not so high as to overwhelm the natural cooling in the subterranean areas.

I have never heard of anyone 'ceptin' city folk who actually is trying to "cool" their attic to reduce the effect on the living area. The other benefit of doing this, is it keeps air moving through the cellar and crawl spaces helping to reduce certain build ups of naturally occurring gasses… I recently discovered that my use of my attic fan has been extended a bit in that I recently installed a GeoSpring water heater.

This unit uses a heat pump as the primary heating source for the water in the tank. It outputs cool air into the cellar as a result and dehumidifies… So far my results are fairly exciting.

I commented sometime last week. In either event, my post was in regards to me testing my attic fan with no insulation to see how much air leakage I had. I used a smoke puffer, and there was no air leakage. I'm trying to understand if this is the case, why would an attic fan be bad?

Gary, you're Gary, you're talking about a whole house attic fan. That's not what this article is about, as noted elsewhere in the comments. That's a whole 'nother topic. John, you didn't John, you didn't provide enough information but I'll make the following general observations…. If a home doesn't have central air or atmospheric combustion appliances, a PAV will reduce ceiling loads a bit by cooling the attic and reversing the stack effect. In summer, stack effect tends to draw hot attic air into the house.

Your puffer experiment doesn't prove no leakage. Smoke can be helpful at finding larger leaks but it can't be used to prove a lack of leakage. For that, you'd need to have a blower door test done along with some simple pressure diagnostics. In any case, I've never seen an air-tight ceiling. The only ones that come close are in purpose-built tight homes e. Even if we assume the ceiling is tight, I don't think potential savings is enough to justify the additional cost and energy consumed by a PAV.

I went through the math here, in another forum. If a home has serious defects that cause unusually high ceiling loads, a PAV might be cost effective, but in most cases, it would cost less in the long run to fix the defects. Thank you for your Thank you for your information. I would be happy to provide additional information, please let me know what information would be helpful. And this is a gable fan. I understand a blower door test would show the air leaks, and I've seen this 2x, but I'm trying to understand with an attic fan on in an unsealed attic, the attic fan should pull the air in the home up into the attic?

If this is so, I didn't really see any smoke being pulled whatsoever. With the attic fan on, I tested the outlets and light switches in two rooms below the attic fan, and the smoke puffer didn't get pulled. Please help me to understand, that although the attic fan is on, there is no insulation in the attic, the attic isn't sealed, why is the smoke puff test not reliable in this situation?

Thank you for your time. John, there are a couple of John, there are a couple of things about your comment that aren't clear. When you say your attic is not sealed, that goes without saying… you can't have a PAV in an encapsulated attic, so it's obvious that you must have a vented attic. Did you mean to say that your ceiling is not sealed? Also, why is there no insulation?

Presumably this is a temporary situation? As for the result you described… without being on site I can only speculate, but to reiterate, smoke pencils puffers are only useful for finding larger leaks. Leakage between your house and attic likely consists of many tiny leakage paths, too small to affect the smoke but collectively enough to impact the result. Or there may be larger leaks that you simply didn't find with the puffer. That's why pressure diagnostics is the only way to demonstrate the impact of the fan or lack thereof on house air changes.

I removed my Thanks again. I removed my old insulation, had the attic air sealed, and had blown-in done. Your answer on the puffer not being able to detect small leaks makes sense, I just thought being 20 feet away from my recessed light, below in my room, to my gable fan, in the attic, would be able to show air leakage with the puffer, but I guess it is leaking just not strong enough to pull the smoke but collectively as a whole it could be a problem, I can understand this.

Either way, regardless, I guess I don't have to worry since the attic is complete now, air sealed and re-insulated. I do have the gable fan running though, and now that the hot air isn't coming into my home the attic is super hot and the fan is running trying to remove it. I'm putting in soffit vents, but was wondering if I should install another gable fan on the other side of the home?

I just feel that if there isn't a significant pull of air from the other side of the home then it will just take much longer for the one gable fan to pull all the hot air out, even with soffit vents. These gable fans, being only at 40 watts and pulling CFM, I don't think my energy bills will be high, but having another installed to pull air in should help a lot.

I was thinking of just having them wired together, so when the one goes on to pull the air out, the other will be pulling the air in to help. I'd appreciate to hear what you're thoughts are on this? I installed 2 attic fans last I installed 2 attic fans last fall. Could it be that although the attic is a little cooler, it is, in fact, drawing in more humid air from outside and, because the temp in attic is lower, the dewpoint is also lower?

Larry, more likely your AC is Larry, more likely your AC is running more because the fans are pulling air through your house. Longer AC run times mean colder ducts.

You could also have duct leaks exacerbating the issue well you do have duct leaks because we all do, it's just a matter of how much. My recommendation is to turn the attic fans off. I would also recommend getting a BPI certified contractor to diagnose your house for you. They can tell you exactly what's going on and lead you to the correct strategies for improving your situation without wasting money like on attic fans.

Everything you say makes Everything you say makes great sense. For Georgia! But work with me here please? I'd love to hear your thoughts. I really respect where you are coming from but I am in Oregon, where it is very dry in the summer; and we run AC very little maybe 4 days last year. Setting those days aside for the moment…I don't care at all if air is being drawn through my ceiling see note below ….

I have the windows open in any case most of the time. An attic ventilation fan seems quite reasonable. I have pretty good soffit venting and turtles on the roof, but the attic heats up a great deal. Even if the heating is radiant and conductive, better airflow will still cool the surfaces somewhat—and I have no moisture problem or AC efficiency problem to worry about.

So why not? When it gets HOT I close the windows in the morning maybe 15 days a year. On those days, there will be more resistance to drawing air from the house and if some does, I'll get some outside air in. But I don't see this as being much of a problem—I should just experiment and see what keeps the house cooler. But I could also just turn off the exhaust fan on those days after doing the experiment—because it's so rare that cost isn't an issue, and there's zero likelihood of damage due to humidity, so really it's cooling that I care about.

So there's a monstrous "leak" that is intentional. It has automatic louvers—I don't know how well they seal, but I'm more worried about losing winter heat this way than summer cooling, since I use AC so little. Ok, I'm sold! I'm moving to Oregan!!! Ethan, I realize there must Ethan, I realize there must be at least comments posted here, but your situation has already been addressed more than once. However, you'll get much better results by using the whole house fan. John, holding a smoke pencil even a foot from a leaky fixture is unlikely be detect anything.

You mentioned you plan to install soffit vents. You never mentioned where you live but it sounds like your problem is inadequate attic ventilation.

And no, I do not recommend adding a second fan. You should turn off the fan you have, add some soffit vents and see what difference your new insulation and air sealing efforts make. Thank you David. I added the Thank you David. I added the soffit vents 16 x 4 vents , 10 on each side of the home, evenly spaced apart. It does make the attic cooler, but the temp on an 85 degree day still reaches for most of the day. The insulation and air sealing keeps out the radiant heat, my interior home temp is fine.

I am just worried if my roof and shingles will warp since the attic temp is high and the heat remains trapped in the attic. Should I install a ridge vent or leave the attic the way it is? Thanks Again! John, the attic temperature John, the attic temperature will vary from top to bottom, highest at ridge and lowest at ceiling level.

Regardless of where you took the measurement, F is not very hot for a vented attic in summer. Shingles are designed to withstand much hotter temps. I believe you said you have gable vents. As long as they have at least as much net free area as the soffit vents, you don't need to install ridge vents. My gable vents on each end are sq ft prob less since I need to minus the louvers , either way totals sq ft with both gables. Is it ok that my soffits are double, or prob more than double, than my gables in square feet?

Should I close some soffits to create more of a suction? David, thanks. I did read David, thanks. I did read all the comments took a while and came to the same conclusion. I also had two other thoughts as a I reflected on this, that may be worth considering and may actually drive the conversation forward a bit. First, the concerns are real—humidity, backdrafts, etc. I am grateful for the article and discussion which have been helpful and informative. Let's make it better…. First, airflow isn't on or off—it's relative.

With soffit vents in place, there will be some balance of house air versus outside air drawn in once the pressure in the attic is lowered by the fan. The precise balance will depend on the balance of resistance of the soffit-vent versus through-ceiling paths and the through-ceiling path also has resistance on entry to the house—from crawlspace etc. Not easy to measure but a good number of unblocked soffit vents would favor airflow from outside.

The point is that in some cases the cooling could be inefficient or even dangerous if drawing conditioned air or creating backdraft or humidity problems. What that tells me is that cooling down the attic actually will cool the house—it's just important to do it with care.

Third, Allison's thesis that the ceiling must be airtight before venting the attic ignores relative costs of the two approaches.

In an area like mine where it could be useful and safe, the question then becomes one of cost tradeoff. The louvers should seal well enough to not mess with the soffit-to-turtle passive airflow when the fan is off. John, John, If you are concerned with your attic temps which are really a symptom of the true problem — radiant energy from the roof deck , then you could run baffle panels the entire length of your roof deck underside.

Ethan wrote: "The Ethan wrote: "The precise balance will depend on the balance of resistance of the soffit-vent versus through-ceiling paths. True, but typical attic vent ratios don't provide NEARLY enough free area for a PAV to operate without creating a negative pressure zone in the attic, which is what sucks air out of the house.

Prevailing winds will also impact ratio between attic vent path vs. It doesn't take a lot of discussion to figure that out. One of the main points of the article is that PAV's have a lot less impact than most folks seem to think. Those of you who still believe otherwise should ponder the following exercise.

Make your own guesstimates and see what you end up with…. People will write anything these days. This is utter crap, and can be disproven by a 5 year old. You open the door, and the seats are hot. VERY hot. Well, according to Professor Bailes here, you're plumb out of luck.

And you'll just have to burn your buttocks and sit on a hot seat. Unfortunately for her, anyone that's ever been INSIDE OF A CAR knows that if you open the windows and "air it out," the temperature drops drastically and within a few minutes, your seats are no longer too hot to sit on. Jeff Moore, Jeff Moore, The point of this site is to explain how attic ventilators take the air inside your home out of your home and into your attic. What don't you agree about this?

Well, you seem to know as Well, you seem to know as much about physics as you know about my gender, Jeff. I've lived most of my life in Texas, Louisiana, Florida, and Georgia. I know a thing or two about hot cars that have been in the sun all day. I also know a thing or two about heat transfer in attics. You might want to learn that yourself before you come here with your drive-by attempt to flame me. Now, let's see if you can handle the heat when it's reflected back at you or if you fade back into the anonymity of the Internet.

John, no, you don't John, no, you don't want to close off any soffit vents. Your attic will get more ventilation with 2x soffit vent area than if you had fewer soffit vents. That said, you'd get even more ventilation if the vents are balanced. You could add one or two of those passive turbines. BTW, just as the louvers determine the net free area of a gable vent, soffit vents also have a net free area that's less than the total soffit vent size, depending on the type of vent.