Fans in the Attic: Do They Help or Do They Hurt?

Night Breeze doesn't need windows opened
by Kris Knutson

Another fantastic article Martin!

Davis Energy Group of Davis, CA developed the Night Breeze system which does not require window opening to cool the house. This would seem to mitigate the concerns about security - it also supports a filter for incoming air. I have never used one, but it seems like a good system.

http://www.esource.com/esource/getpub/public/pdf/cec/CEC-TB-4_NightBreez...
http://new.advancedenergyproducts.com/wp-content/uploads/2011/11/NightBr...

The attic "coffin" you mention, which is used to air seal and insulate pull-down stairs, scuttle hole, or a whole house fan, is good in theory, but in practice, is cumbersome to install. Even though a manufactured cover costs more, it is far easier to install. It also requires no assembly.

Response to Kris Knutson
by Martin Holladay, GBA Advisor

Kris,
The NightBreeze has been around for a while; I first wrote an article about the product 8 years ago, for an article that appeared in the September 2004 issue of Energy Design Update. In that article, I wrote:

"The NightBreeze includes a variable-speed air handler with an ECM blower, a hydronic heating coil, a large motorized damper to control the intake of exterior air, and a wall-mounted control unit. The thermostat-like control unit also regulates the operation of the air conditioner, if any. In addition to providing nighttime ventilation cooling, the NightBreeze system provides year-‘round whole-house ventilation.

"In order to pull in enough outside air to cool the house at night, the NightBreeze requires a large exterior intake grille — usually about 3’x3’ (assuming the grille has 50% free area). If the air handler is located in the attic, the air intake grille can be mounted in a gable, in a doghouse dormer, or at the top of a false chimney. The air-intake duct is connected to a damper box containing a large hinged motorized damper blade. Damper leakage is minimal — at 25 Pascals, it’s about 1.4% of total fan flow. According to David Springer, the president of Davis Energy Group, one of the main consulting groups that developed the NightBreeze, ‘It is possible to achieve 6 percent duct leakage in a system that includes the damper.’ The wall-mounted NightBreeze control replaces a conventional thermostat. ‘At my house, the control is set for a 5-degree delta T,’ says Springer. ‘The upstairs thermostat is set to 80°, so it’ll start ventilating when the outdoor temperature drops to 75°. By morning, the outdoor temperature might be in the fifties or low sixties, and the indoor temperature might be 68°.’

"The control unit uses historical monitored temperature data to predict next-day outdoor temperatures. When the control predicts relatively mild weather, the ventilating blower will run at a lower speed than when hotter weather is predicted. ...

"Like a whole-house fan, the NightBreeze system will not provide effective ventilation cooling in all climates. According to Springer, ventilation cooling works best in areas of the country with a mean daily temperature range of at least 30°F. According to the ASHRAE Fundamentals book, such areas include Idaho, Nevada, Utah, and Wyoming, as well as most of Arizona, California, Colorado, Montana, New Mexico, and Oregon. In most of the rest of the country, however, the mean daily temperature ranges are narrower than 30°F. An investment in ventilation cooling equipment makes more sense in a dry Western climate than in a humid Eastern climate. ‘I was convinced it would never work in Florida, but Danny Parker [from the Florida Solar Energy Center] convinced me otherwise,’ said Springer. ‘He showed me that even in Florida you can do some good with ventilation cooling, at least during a few weeks of dry weather in the fall. Whether the investment in the equipment can be justified is another question.’ ..."

Good stuff!
by curt kinder

I'm constantly up against folks who sell or have bought powered attic ventilators for use in Florida. Most who have them "get it" when I explain that conditioned air is pulled up out of every ceiling penetration.

whole house 'attic fan'
by David Goldman

Grew up with one in the 60s in NY. worked pretty well and was an obvious choice before the family had central AC. Another thing to remember is to cover the large attic exhaust vent in the off season. In our case it was a louvered opening about 30in square that we covered with a plywood piece every September or the attic would get really cold.

thanks
by kelly Jones

Solar Attic Fans are easily managed and fixed and a great product for proper attic ventilation. [Brand name deleted by editor] solar attic fans are no. 1 by builders and are the highest attribute made in the USA!
[Website link deleted by editor]

Response to Kelly Jones
by Martin Holladay, GBA Advisor

Kelly,
It's amazing that your urge to post spam is stronger than your willingness to read.

Attic Temperatures and Roof Lifetimes
by Lee Dodge

Standard asphalt shingle roofs is northern Vermont, where Martin Holladay lives, last 30 or more years, while roofs in south Texas last about 15 to 18 years. I think that the difference is due to the hotter attic temperatures in the south, which increase the shingle temperatures. The cost of a new roof for a modest size house is roughly $8000, or $500 per year for a roof that lasts 16 years. If an owner could get an increase in roof lifetime by 4 years by keeping the attic cooler, that would cover the cost of about 20,000 kWh at $0.10/kWh!

That 20,000 kWh would go a long way toward operating a pair of thermostatically controlled, powered attic ventilators (operated off the same thermostat) that had fans both blowing air into and out of the attic, through gable vents, with a balanced pressure so that air was not sucked through the ceiling of the house. It would also reduce heat gain into A/C ducts in the attic, a standard practice in older homes.

I get the idea that people that suggest a hot attic with a well-insulated ceiling have never crawled around an attic in south Texas on a summer day trying to string a TV cable or electrical wiring. It is not a matter of comfort, but rather, a matter of survival.

Flat roofs & whole house fans
by kim shanahan

Martin,

Since the desert southwest is prime territory for whole house fans and a common building style is flat roofs with either no attic space or very minimal, non-accessible attic space, what is an appropriate detail for installing a whole house fan? This is relevant as a remodeling question also.

How important is insulating the fan cover?
by Derek Roff

While I agree that more insulation is generally better, I wonder how important it is to attain large R-values in the cover for a whole house fan installed in the floor of the attic/ceiling of the hallway. The article says of R-5, "Clearly, that’s not enough insulation." I wonder what the energy consumption difference would be, between an R-5 cover, and the R-26 advocated as the low end by Erik North. If we are talking about less than 10 sq ft of opening for the fan, then the difference in energy consumption between R-5 and R-26 for the cover will add up to how many pennies per year?

Obviously, it depends on the average ambient temperature, humidity, and the nature of the attic. Still, an example calculation for an average home in New Jersey, and another in Vermont, would be enlightening.

My guess is that way more attention should be given to creating a flawless air seal around the cover than to adding a few more Rs to the R-value. Of course, doing both is nice.

Response to Lee Dodge
by Martin Holladay, GBA Advisor

Lee,
Your argument rests on three legs. You assert:

1. Asphalt shingle roofs last 30 or more years in Vermont, and only 15 to 18 years in Texas.

2. The best way to lower the temperature of asphalt shingles is with a powered attic ventilator.

3. Cooler asphalt shingles last longer than warmer asphalt shingles.

To determine whether your argument has any merit, we need to look at each of these three assertions in turn.

1. In fact it is quite rare for an asphalt shingle roof to last 30 years in Vermont. Your estimate of 15 to 18 years is much closer to the average here (as it appears to be in Texas). Many factors affect shingle longevity; shingle temperature is only one factor.

2. The best way to lower asphalt shingle temperatures is to choose white-colored shingles over black shingles. No other factor, including attic ventilation, plays as much of a role as do color and orientation. This fact holds true in Vermont as well as Texas. While the correlation between shingle temperature and longevity has not yet been definitively shown, it remains true that anyone concerned about elevated shingle temperatures should simply choose white shingles.

More information on this topic can be found here:

http://www.jlconline.com/Images/Letters_tcm96-1126954.pdf
"In his recent book, Water in Buildings, Rose summarizes what he has learned from years of careful research: “Does ventilation significantly reduce shingle temperatures? The short answer is no, not significantly. Shingle color is a very strong determinant of shingle, sheathing, and attic temperature. ... In a vented cathedral ceiling, venting can cool shingles in the lower part of the roof; it cannot cool shingles high in the roof. ... As long as shingle manufacturers ignore the effects of shingle color as a determinant of temperature, they may be admonished for asserting so strongly the importance of venting to control temperature. ... In short, on the basis of currently available information, attic ventilation is only marginally beneficial to shingle durability. Attic ventilation does not deserve the attention it has received in relation to shingle durability.” If your clients prefer cool shingles to warm shingles, the amount of ventilation behind the sheathing is far less important than the shingles’ color; so advise them to choose white shingles."

http://www.buildingscience.com/documents/reports/rr-0302-roof-and-attic-...
"Our simulations and measurements have shown that asphalt shingles applied over vented roofs in hot-dry climates operate warmer than the same asphalt shingles applied over unvented roofs in hot-humid climates. To our knowledge, there have been no shingle warranty adjustments for Las Vegas versus Orlando, and that difference in location is more significant in regards to shingle temperature than vented versus unvented."

How Hot Is Your Roof? - Insulated Rooflines and Shingle Temperature.

3. On the third point, the jury is still out. However, if you really care about shingle temperatures, the best solution is clear: choose white shingles! Roof ventilation won't help much.

Response to Kim Shanahan
by Martin Holladay, GBA Advisor

Kim,
Q. "Since the desert southwest is prime territory for whole house fans and a common building style is flat roofs with either no attic space or very minimal, non-accessible attic space, what is an appropriate detail for installing a whole house fan?"

A. Several manufacturers make whole-house fans for homes without attics. Click here to see a website with many inexpensive models of whole-house exhaust fans for flat roofs.

A better option would be the TC1000-H fan from Tamarack -- a model with an automatic insulated damper that prevents heat flow and air flow when the unit is not in use.

Response to Derek Roff
by Martin Holladay, GBA Advisor

Derek,
Q. "I wonder what the energy consumption difference would be, between an R-5 cover, and the R-26 advocated as the low end by Erik North?"

A. Let's do the calculation. As I explained in my article, How to Perform a Heat-Loss Calculation — Part 2, "The heat loss formula for determining transmission losses through floors, roofs, and walls is Q = A • U • ΔT. In other words, the rate of heat flow through a building assembly (in Btu/h) is equal to the area of the assembly (in ft²) times the U-factor (in Btu/ft² • hr • F°) of the assembly times the ΔT (in F°)."

To perform the calculation you requested, we'll have to make some assumptions. You can adjust the assumptions to fit your own case and run your own calculations if you want.

Let's assume that the insulated box in the attic measures 40 inches by 40 inches, and it is 24 inches high. The exposed area of this box is therefore 26.6 square feet for the sides, and 11.1 square feet for the top. The total area is 37.7 square feet.

Let's assume that the attic temperature is 32°F, and the indoor temperature is 72°F. That means that the ΔT is 40 F°.

First we'll do the calculation assuming that the box has an R-value of R-5:
Q = A • U • ΔT = 37.7 • 0.2 • 40 = 302 Btu/h

Next we'll do the calculation assuming that the box has an R-value of R-26:
Q = A • U • ΔT = 37.7 • 0.038 • 40 = 58 Btu/h

The difference in the rate of heat flow is 244 Btu/h. Over 24 hours, that would be 5,856 Btu. Over 5 months, that would total 878,400 Btu, or 257 kWh.

If you were supplying heat to your house with electric resistance heat at a cost of $0.14/kWh, that would cost you $36 over the winter.

If your space heating costs were only 1/2 the cost of electric resistance heat -- a more likely scenario -- the difference in R-values would cost you $18 over the winter.

Cold climates will have a higher ΔT than I assumed, and warm climates will have a lower ΔT than I assumed. My calculation is only an example to show you how to figure this out for yourself.

Thanks for the help with the calculations
by Derek Roff

Thank you, Martin, for showing me how to do the heat-loss calculation, and providing an example. It certainly may be a good, average example, but I'd like to take other information from your article, to suggest a significantly different example. Let me know if I have misunderstood or miscalculated.

Choosing the Tamarack HV1600, the larger of the two Tamarack whole house fans that you recommend, the product literature says that the rough opening in the ceiling should be 14 1/2” x 22 1/2". This gives an area of ~2.25 sq ft. If we keep everything else equal, this is 1/17th of the insulated area in your calculation, so the rough cost differential over a season drops from $18 to just over $1, for the cheaper heating source. Of course, that isn't entirely fair, since the needed insulating "coffin" in the attic would be smaller for this fan, and the fan comes with insulating doors. But let me finish explaining the difference I am contemplating in insulation location.

We could get R-5 out of an inch of a high-quality foam. Even adding the necessary cosmetic and structural elements, it would be reasonable to build this 1" plus thickness into a flat insulating cover mounted below the fan opening, on the ceiling, inside the living space. In that scenario, calculating the insulation area based on the rough opening seems reasonable to me. Higher insulation levels would require an insulation coffin in the attic, which leads to greater surface area and greater heat loss area. So I'm thinking that there is even less final energy loss/cost difference between an R-5 insulated cover in the living space and an R-26 coffin in the attic, if we consider the advantageous placement of the insulation possible with R-5.

We could make a similar comparison with your example 40"x40" fan. The inside-the-house flat R-5 cover that I am suggesting would have an area of 11 sq ft. This is less than a third of the surface area of the R-26 coffin in your example. Again, keeping other things equal to your example, this would give us a seasonal cost difference of roughly $6.

Considering lower materials cost, easier fabrication, and simpler attachment and removal, I'm thinking that I would be very happy paying an extra $1-$6 per year to use an R-5 flat cover on the ceiling of the hallway. Have I missed something critical?

Response to Derek Roff
by Martin Holladay, GBA Advisor

Derek,
Most whole-house fans require a rectangular opening that is at least 32" x 32", and often larger; some whole-house fans are 48 in. in diameter. A ceiling grille measuring 32"x32" or 36"x36" isn't unusual.

Of course, the size of the Tamarack fans is smaller, as is the required ceiling grille.

Your plan to install an interior cover should work fine. But as long as you're making a cover, why not use at least 2-inch-thick rigid foam instead of 1-inch-thick foam?

Asphalt Shingles
by Tom Barrett

Asphalt shingles and other asphalt-based materials are the reason attics get so hot. I've recorded temperatures up to 180 deg F on back side of asphalt shingles in the California summer sunshine and attic temperatures of 160 deg F. White shingles are marginally cooler. The key is solar reflectivity and thermal emittance of the roofing material. Asphalt is a great material for absorbing solar radiation and has poor thermal emittance. Walk barefoot on an asphalt road and see how that works.

It is interesting that asphalt single warranties are based on having attic ventilation to code or the warranty is voided. I've measured attic temperatures with various types and amounts of attic venting and there is really little effect that can be detected with venting on attic temperature. Cooling ends up being localized around the vent openings, while the attic stays hot.

So the biggest cause hot attics is our roofing materials, not the lack of ventilation. Of course if you want to make lemonade out of that lemon, use the attic heat to preheat water and call your roof a solar collector.

Response to Tom Barrett
by Martin Holladay, GBA Advisor

Tom,
Thanks for your comments.

I agree with your conclusion that "there is really little effect that can be detected with venting on attic temperature."

Thanks for the follow-up response
by Derek Roff

Thank you, Martin, for your further comments on my point. I picked one inch of foam in my example to match the R-5 mentioned in your article. However, I agree that choosing two inches of foam for a cover would be twice as good.

Ridge vent versus powered attic ventilator
by Chris Grabowy

So one part of my roof/attic has a ridge vent but does not have a gable vent. And then the main part of my roof/attic has NO ridge vent but does have gable vents on both ends. I was thinking about adding some sort of powered attic ventilator. But I was wondering if I would just be better off installing a ridge vent along that main roof/attic. According to this article the powered option is not an option. So are the gable vents enough or should I install ridge vents? Also we have debated about putting in a whole house fan, since there are some weeks when its cool outside but hot inside the house with no wind outside to help. BTW, I live west of Philadelphia, PA. Anyway, great article!!

Response to Chris Grabowy
by Martin Holladay, GBA Advisor

Chris,
If your attic doesn't have any problems, I wouldn't worry about adding more attic ventilation.

What problem are you trying to solve?

Attic fan reduced my electricity bill
by j wing

I've been an energy engineer for over twenty years. My experience has taught me to think beyond articles like this one. In the real world, comfort, low first cost, and immediate gratification take precedence over theory and lab results. To wit, I have intalled two attic fans; both installations were resounding successes:

1) My 1960-vintage home in northern Alabama runs 5-tons of central air conditioning plus one small window A/C unit. In the summer, the upstairs bedrooms are unbearably hot, due to the ceiling radiating attic heat. The bedrooms can be filled with cool air, yet the hot ceiling makes them feel like being inside a broiler. This past summer,a major outdoor renovation forced me to disconnect the condensing units from the central A/C, leaving me with just the small window unit. So, I put in an attic fan to relieve the hot ceiling. The A/C is off during the day when I'm at work; but the fan runs. By the time I go to bed, the attic is cooler than the inside of my house, so it is drawing heat out. The small window unit is plenty to keep the house comfortably cool.

2) My mother-in-law stays in her hot home all day and was considering replacing her central A/C system with one of higher capacity. However, the cost was prohibitive, so I installed an attic fan. With a cooler attic, the A/C system had lower heat gain, therby reducing the supply air temperaure at the diffusers. Additionally, the hot ceiling effect was alleviated. MIL was able to turn her thermostat up a few degrees, thereby saving energy and increasing comfort.

It's very easy to write that better insulation, radiant barriers, and light-colored roofs are components of a superior solution. And they are; I can't argue otherwise. But in the real world, we look for practical solutions that people will actually implement. In the two cases, I've described, the most practical (by far) solution was contrary to the conventional wisdom.

Response to J Wing
by Martin Holladay, GBA Advisor

J Wing,
You have installed powered attic ventilators in two homes in recent years, so you obviously like them. According to your anecdotes, the electricity use decreased in both of these buildings.

Even if your anecdotes are accurate in all details, and even if you have no financial interest in promoting powered attic ventilators, two anecdotes aren't convincing. Needless to say, there are all kinds of reasons that electric bills fluctuate from one month to the next; one of the biggest reasons (when it comes to air conditioning) is variations in weather.

You have described two buildings with "hot ceilings." I have no idea why anyone with a hot ceiling would think it was easier to install powered attic ventilators than a layer of cellulose insulation. After all, the cellulose insulation doesn't require any electricity.

Here at GBA, we strive to advise readers of the simplest and best solutions to common building problems. I'd like to repeat my advice: if you have a hot ceiling, install more insulation on your attic floor. Don't install a powered attic ventilator, since these devices, on average, use more electricity than they save.

Thanks
by John Bigler

Excellent article. Thank you Martin.

Well, to me two anecdotes are
by j wing

Well, to me two anecdotes are more compelling than one article, no matter how many times it's been re-written. I have been a professional energy engineer for 2 decades; I understand that there are many variables that account for enrgy use.

The most important variable, however, is the human beings involved and the particular situation they are dealing with. In both of my anecdotes, the human beings felt that the cost in money, time, work, and hassle would not have sufficient return in comfort. In the case of my house, it is impossible to lay down another layer of insulation.

On the other hand, each attic fan took about $100 and hour to install. As a curious professional, I was willing to risk the investment to see for myself what effects are. If the attic fans didn't do the trick, then I'd move on to the next solution and be out only $100. The results of my experiments, so far, differ from this article and all the other ones that say the same thing. I report my findings not to argue or to be off-handedly accused of having financial interest, but to advance our understanding and knowledge.

Building a new house? Definitely insulate the roof and add a radiant barrier. Is your fan-coil unit in a hot attic? Consider an attic fan, especially if you turn the A/C off during the day. Are you at home with the A/C on all day? Consider a different roof heat-gain management system, if you can afford a long-tem investment. Not staying in the house long enough to make the investment worthwhile? Consider a less-expensive solution that may have some drawbacks. Does your attic have insufficient ventilation openings that will cause an attic fan to draw through your house? Increase your ventilation area, or consider a different solution.

Energy engineering is way more complicated than telling people that there is but one solution to a problem.

Response to J Wing
by Martin Holladay, GBA Advisor

J,
Thanks for sharing your anecdotes. I certainly appreciate the information, as I appreciate all information provided by GBA readers. And I accept your assertion that you have no financial interest in promoting powered attic ventilators.

However, after having read the reports of researchers who have studied the issue and measured energy use in buildings with powered attic ventilators, I'm going to stick with the conclusions and advice of the researchers, even in the face of two anecdotes that buck the trend.

What about a garage attic??
by Robert Williams

I'll be building a large garage with attic storage in NE Texas. I will use a radiant barrier decking, but does a powered attic ventilator make sense here ??

Response to Robert Williams
by Martin Holladay, GBA Advisor

Robert,
Running a fan always requires electricity. Of course, a fan cannot cool your garage unless the outdoor air temperature is lower than the indoor air temperature. If you have radiant barrier sheathing, the temperature difference between the two spaces won't be very great, and probably won't be enough to justify the use of the fan.

The other question: what are you storing in your attic? Does it matter what the temperature is? If you have an old lawnmower and some Christmas ornaments up there, it probably doesn't matter if the garage is 80 degrees or 110 degrees.

Whole House Fans
by Kurt Shafer

Martin,

Your research into attic fans is of great interest. It is valuable to have the data from those who have studied them recently.

Your comments about Tamarack are surprising. Tamarack is a fine company with leading edge insulation technology but you do your readers a disservice by inserting your opinion that they "make the best whole-house fans available".

I had hoped you would have done a little more research into other suppliers in order to offer a more unbiased view.

I would be very happy to assist you in a thorough market overview which I do regularly for the http://www.WholeHouseFanGuy.com

Like so many other "plug in"
by David Jones

Like so many other "plug in" solutions attic fans certainly have the potential to make things worse rather than better. Your article does a great job of outlining how this happens. As a general rule unless a homeowner has a highly qualified person to evaluate their house, I think its safe to say that attic fans should be avoided.

I do think that under CERTAIN circumstances an attic fan can be beneficial. The problem is that in most cases, there is no one available to accurately make that determination.

The primary reason attic fans are not helpful is that they can depresureize the attic and draw air out of the house. Many houses have significant leaks in the air barrier(attic floor) and it would be easy for the attic fan to draw a significant amount of air from the house. If a house has been air sealed by someone with an understanding of the air leakage, then it would seem that the depressurization would be minimal and the attic fan could then be beneficial.

The amount of air being drawn out of the house is a function of the pressure difference and size of the air leaks in the attic floor. In an attic with gable vents, soffit vents, and ridge venting, I suspect that the attic fan would not create a significant pressure difference and the air leakage from the house is probably small. (assuming even a rudimentary level of air sealing).

Lee Dodge suggests a "balanced" system where there is an incoming and exhaust fan. This seems like an approach that could also limit depressurization.

If air leakage is minimized, and depressurization is controlled with either a balanced fan or adequate openings for incoming air then it seems reasonable that the amount of depressurization would minimal and the attic fan could be beneficial.

Arnie Katz's study concluded that (an undefined amount of?) depressurization was enough to increase cooling costs. This assumes that there is a cooling system in place and operating. In a house not running an ac system any air being drawn from the house is being replaced by outdoor air. If the windows are open in the house there is no "cooling penality" to having some air exhausted from through the attic. if late in the afternoon the house temps rise above the outdoor air temps there is actually a cooling benefit. (we hope the amount of air being exhausted through the attic floor is minimal because the house is air sealed).

If the house without AC running was closing the windows during the day to keep the house cooler, then any air being exhausted to the attic would be replaced by warmer outdoor air, but that warming of the house is tempered by a cooler ceiling.

You propose the question "who cares how hot the attic is if there is enough insulation in the attic floor?" Insulation can only slow and not stop the energy transfer. Regardless of what the insulation level is, there will be some level of energy transfer from the warmer attic to the cooler house. So everything else being equal, any house would benefit from a cooler attic. In many houses it is impractical to increase the insulation in the attic. (At least when compared to the difficulty/cost of installing an attic fan. Of course the insulation is the better option if possible.).

My own experience points to anecdotes showing the attic fan is beneficial. I know of two houses using them and the owners are certain that they make a large difference in comfort. Both houses do not use AC. Both have a large amount of venting for incoming air.

Response to David Jones
by Martin Holladay, GBA Advisor

David,
If you install a powered attic ventilator, there is one thing you can be sure of: your electricity bill will go up. That's because running a fan requires electricity. This increase in your electric bill is especially guaranteed in homes without AC (because there is no possibility that running the fan might decrease electricity devoted to AC). But research has shown that, even in a house with AC, powered attic ventilators increase homeowners' electricity bills -- they don't lower them.

You wrote, "Insulation can only slow and not stop the energy transfer. Regardless of what the insulation level is, there will be some level of energy transfer from the warmer attic to the cooler house. So everything else being equal, any house would benefit from a cooler attic."

Just because insulation slows down heat transfer rather than stopping it, doesn't mean that insulation isn't the best (and most cost-effective) way to prevent hot attics from making homeowners uncomfortable. Insulation is still the best approach. It's cheap and it doesn't require any electricity to run. If you are worried that your ceiling is still warm, the solution is simple: pile on a little more insulation.

You wrote, "Everything else being equal, any house would benefit from a cooler attic." That statement is only true during the summer. Believe it or not, many homeowners with powered attic ventilators leave them running all year long, because they forget to turn them off or the thermostat breaks. As a result, the fans increase the homeowners' energy bills during the winter.

From start to finish, powered attic ventilators are nothing but trouble. Homeowners should avoid them like the plague.

Yes, of course more
by David Jones

Yes, of course more insulation with no moving parts and year round benefit is the better option. I should have made that clear.

Yes, as the insulation level increases the potential benefit from the fan decreases.

In some cases additional insulation on the attic floor is not practical. In those cases the options are to "sweat it out", cool the poorly insulated house with a large AC system, or try to limit the heat gain to reduce the temperature in the house. If a $100 attic fan (which consumes little electricity compared with an AC unit and contains far less embodied energy) can be part of a plan to reduce heat gain there may be a significant energy and environmental benefit compared to the AC option.

Can an attic fan offer some benefit? I think so under some very specific circumstances.

In a house with no ac running and the windows open there has to be some level of cooling benefit from lowering the attic temp with an attic fan. (You can certainly argue that the cost of electricity to power the fan outweighs the comfort benefit).

Yes, installing an attic fan raises the electric bill. If the attic fan increases the comfort level of the house, it may be worth it. If the attic fan keeps the house cool enough so that the owners leave the AC off, there is almost certainly an energy benefit.

Of course if the attic fan isn't working correctly and is running all year long that would be a significant problem. The same can be said for any mechanical equipment in the house that is not operating correctly.