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Are Solar-Powered Attic Ventilators Green?

Fans that pull hot air out of attics can cause more problems than they solve

Posted on Feb 26 2009 by Peter Yost

At face value, attic exhaust fans make a lot of sense: if your attic is too hot, you force more air through it to cool it down. To be efficient, you use a solar-powered attic exhaust fan. When the sun is shining and heating up your attic, that’s when the photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. panel wired to the exhaust fan powers the fan. Pretty slick.

But there is a catch: why is your attic “too hot?” It is probably because living space under the attic is uncomfortable…from the less-than-well insulated and air sealed ceiling that separates the attic from those rooms. If you don’t have a continuous air seal at the ceiling plane, then your solar-powered attic exhaust fan can pull conditioned air into the attic—now that will cool it down! It can be worse than that: if that attic fan is depressurizing living space that has atmospherically-vented gas appliances or a problem with radonColorless, odorless, short-lived radioactive gas that can seep into homes and result in lung cancer risk. Radon and its decay products emit cancer-causing alpha, beta, and gamma particles., you may have moved from wasting energy to indoor air quality problems.


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Martin’s Useless Products List

Georgia Pulls the Attic-Ventilator Plug (Sort of)


Field research supports these scenarios. Work done in 1995 by John Tooley and Bruce Davis of Advanced Energy Corp (as reported in ”) revealed depressurizationSituation that occurs within a house when the indoor air pressure is lower than that outdoors. Exhaust fans, including bath and kitchen fans, or a clothes dryer can cause depressurization, and it may in turn cause back drafting as well as increased levels of radon within the home. issues and associated energy, moisture and combustion safety problems. And field research done by the Florida Solar Energy Center (FSEC) specifically on solar-powered attic ventilation (“”) concluded that the approximately $850 installed cost of the system yielded relatively modest cooling energy savings and an unfavorable payback over more than twenty years.

The bottom line? Proceed with caution. If you have a well-sealed and insulated attic floor and either no HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. equipment in the attic or all of the HVAC equipment and ducts in the attic are well sealed, then you could install solar-powered attic ventilation with some cooling energy benefit and little potential for safety or indoor air quality problems. For far too many homes, this is a mighty big “if.”

Joe Lstiburek of Building Science Corp. puts it this way:

“In order for the fan to work the air needs to come from the outside and not be pulled from the house so this means that the attic ceiling needs to be airtight. If the attic ceiling is airtight you don’t need the fan. Your money is better spent on something else.”

For most homes, it will be “greener” to take the money you would have spent on the PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow.-powered attic ventilation and upgrade the air sealing and insulation in your attic and on your HVAC system.

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Image Credits:

  1. Florida Solar Energy Center

Jul 18, 2009 10:47 AM ET

Solar attic fan and radiation
by solar fan

Why is the attic too hot? Not because of heat from below, but because of solar heating of the uninsulated roof! Too bad none of the authors are physicists. They seem to discount the fact that heat travels by radiation as well as by convection. An attic is heated to a very high temperature by solar heating. This high temperature will result in heat radiation from the attic into the home below. Ventilators cool the attic convectively so that the temperature is lower and the heat transfer into the home by radiation is much lower.

Jul 19, 2009 4:21 AM ET

Dear Solar Fan,
by Martin Holladay

I'm not a physicist, but I know that powered attic ventilators cause more problems than they solve. I'm well aware that the sun can make some attics quite hot. Here's the basic solution: Be sure your ceiling is carefully air sealed, and include enough insulation to provide thermal separation between your living space and your attic.

If your hot attic is making you uncomfortable during the summer, I guarantee that either (a) someone forgot to include an adequate air barrier at the ceiling plane, or — more likely — (b) the attic floor is inadequately insulated.

I've said it before, and I'll say it again: as long as you keep HVAC equipment and ductwork out of your attic, it really doesn't matter how hot your attic gets.

Depressurizing your attic with an attic fan is asking for trouble. In most homes, a powered attic ventilator will suck conditioned air from your home, raising your energy bills.

Jul 20, 2009 5:31 AM ET

On not being a physicist
by Peter Yost

Actually, the quote near the end of the article is from a physicist. We know our thermodynamics a bit. It's not that these fans are universally a bad idea; it's just that too often they are used without a thorough understanding of building physics.

Jul 31, 2009 1:07 AM ET

Solar Energy
by John

Broan Solar Powered Attic Ventilators operate without fuel, waste or pollution. And because they are a totally solar-powered solution, they can count as points towards Green building. All NGOs trying to make this world green should involve in educating people to utilize products like this.

Here also we can check the updates of solar energy,

Aug 2, 2009 11:23 PM ET

Attic Fans
by H. Kaplan, M.S. P.E. LEED AP

Actually you are all right. Air takes the path of least resistance. Which means, unless there is an intake into the attic (e.g. a louver) that is sufficiently large enough (static pressure say less than 0.05"w.g.) then the attic could become negative enough to draw air from the house. And yes it is it is good to run air through the attic that will come from the outside to lower the temperature in the attic. The temp. difference (delta T) between the inside of the house and the attic will then be less, therefore less heat transfer from conduction. Increasing the insulation would be helpful also as this is part of the heat transfer equation (q=UxAxdelta T)where U is 1 over the insulation R value). For those in a high humid area a vapor barrier on the warm side is highly suggested. Don't place another vapor barrier on top of your attic floor insulation. Hope this helps. Regards.

Aug 3, 2009 9:34 PM ET

Positive pressure?
by William Clarke

I am not a physisist, but I would like a cooler attic, less because of heat transfer to the living space, than because I store stuff up there! I'm glad to know about the dangers of sucking air up from the living space (although no AC, so maybe not a big deal). What about drawing outside air in with a fan at one of the gable vents? Obviously you don't want to push heated air into the living space, but wouldn't a ridge vent and the 2nd gable vent, couple with a reasonably tight and well insulated ceiling, make that unlikely? I've also considered stapleing relective insulation to the rafters, but leaving it open at the top and bottom, to reduce radiant heat gain. Would that be pointless? Thanks!

Aug 4, 2009 8:54 AM ET

THings to do besides fans to cool attics in hot climates
by Peter Yost

Any fan in the attic can pull air from the conditioned space, depending on the quality and continuity of the air barrier at the ceiling plane. If you knew that you had a good air barrier then pv-powered attic exhaust fans work. BUt without a good air barrier, you just don't know.

A radiant barrier on the underside of the rafters is a good idea in a hot climate. Remember that in order for any radiant barrier to work it must face an air space and it must stay clean (since radiation and emissivity are surface properties).

Sep 24, 2009 11:44 AM ET

Solar Attic Fans
by Travis Hipp

I find this to be a very misleading article. Begin both an engineer and attic ventilation expert, I think you owe it to your readers to present a non-biased treatment of the topic. While solar attic fans are not the “magic bullet” for every situation, the technology in general is both soundly based in physics and does offer real benefits for many applications. The key for success is in the proper installation and application of the product.

You reference a couple of articles as the basis of your claims (actually the same two articles that are always referenced). The first article by Advanced Energy talks about the potential for attic depressurization and is actually a good study on what can happen if you improperly install an AC powered fan. However if proper installation guidelines are followed and the installer allows for the proper amount of intake ventilation to support the fan, attic depressurization is both unlikely and improbable. Air flow will always follow the path of least resistance, so just allow for enough attic air intake ventilation and there is no issue. Also, most solar attic fans are designed in such a way that they cannot put up much pressure head. In layman terms, this means that a given solar attic fan is far less likely to present a negative pressure problem than the similar AC powered attic fans discussed in the study.

The second article you referenced was the Florida study on solar attic fan economics. The study found that the solar attic fan only saved an average 6% electrical usage on the home. However if you read the study, you’ll find that the home they used not only had an attic radiant barrier installed (this blocks on average at least 50% of attic heat gain), but also the home did not have HVAC equipment installed in the attic. Even with the radiant barrier installed, the solar attic fan still dropped the attic temperature over 20F and in the words of the home owner “made the house feel more comfortable”. Also for noting, the study assumed an installed cost of $850 and did not account for any rebates or tax incentives when they calculated the potential savings. Bottom line, a home without an existing radiant barrier would realize at least twice the savings (12%) of the case study home and potentially much more if their HVAC equipment is located in the attic (which is the situation for millions of homes in the south and southwest). Throw in the existing tax incentives of solar attic fans, and what you find is a completely different picture than what was presented.

Instead of assuming that solar attic fans will cause problems, why not educate the pubic on how to properly install them? This is simply another tool that can be used to reduce energy consumption and when compared to alternative methods of reducing attic temperature, the economics actually favor solar attic fans.

Sep 24, 2009 12:13 PM ET

This article is misleading
by Peter Yost

Hi Travis - thanks for your comments and perspective. You made me go back and re-read what I had written. Sorry, but I don't think it is misleading, inaccurate, or biased. I did not say don't use solar-powered attic exhaust fans, but to "proceed with caution." And I did say to carefully compare this investment to improvements to the building envelope.

If I could be convinced that everyone understood building physics as well as you do, and that the installer of the PV-powered attic exhaust system ensured that there was no potential for depressurizing living space, I would be more generally supportive of the technology's use. But some manufacturers of these systems do not even mention the issue of air sealing the ceiling plane or depressurization potential. That is my concern and why I would like GBA folks to understand the issues as they consider this technology.

Oct 2, 2009 12:35 PM ET

Path of Least Resistance
by Stephen Colley

Attic airflow is another consideration to be considered. Improperly placed, an attic exhaust fan (PV or AC) could suck air from an adjacent ridge vent or gable vent and not be as effective as desired in removing hot air from remote areas in the attic. We depend so much on soffit vents, but it doesn't take long for these to become blocked by dust, cobwebs, and other attic "schmutz" seriously lessening their designed effectiveness. Great discussion, folks.

Oct 6, 2009 12:06 PM ET

Hot air !
by Steven Leighton

Dear Mr Yost,
I can't argue with "Fans that pull hot air out of attics can cause more problems than they solve" but you didn't do a good enough job of defining the "can".Perhaps you ought to have swapped "can" for "might" and we would have all been happier.
Still, you generated great input from the people commenting above ... so overall a good job.

Oct 31, 2009 3:59 AM ET

Solar Powered Attic Fan
by Anonymous

Some of the problems that are envisioned by these so called "experts" are laughable, but could possiby occur in rare, isolated cases. If your attic has properly sized and properly installed ventilation openings, then an exhaust fan will do quite well in pulling out the air heated up by the solar radiation hitting the roof. The attic space will be much cooler throughout the day, putting less load on the living space AC system. If the AC system ducting runs through the attic and leaks to the extent that a ventilation fan will pull a significant amount of the AC air from the living space, the attic is probably being cooled by so much leakage and doesn't get warm. This is a loss of efficiency in the home AC system. But if the attic gets hot and remains hot throughout the day and evening, a solar powered fan is just the ticket. It will keep the attic much cooler throughout the day and cool off the attic with the late afternoon, early evening lower air temperature. This reduces the load on the AC system by reducing the amount of heat that radiates from the attic into the living space. Even if your attic is insulated well, a significant amount of heat from the hot air in the attic will still enter the living space. Being able to lower signigicantly the attic temperature, without any expenditure for the power required to drive the ventilation fan, is a good idea and will save you money.

Mar 18, 2010 12:01 PM ET

Solar powered fan for concrete roof
by Bosque

I think that solar attic fans are not the solution for every situation, but the technology can offer benefits for many of then. I am living in Puerto Rico where 90% (+/-) of the houses are made of concrete, including the roof, and where the temperature is between 80 and 95 all the year . Most of the household uses air conditioning only during the night, then, a solar fan could help to maintain fresh the inside of the home during the day. A technology that people start to adopted is to paint the roof with white color sealer's , that prevent water leakage and the increase of temperature of the concrete by the sun light.

Mar 29, 2010 9:10 AM ET

Attic heat turbines that produce energy ?
by Ricardo Avila

I am looking for alternative energy generating solutions. Partly to have multiple systems in order to be able to reduce my energy bills. I see Solar Panels and wind turbines are quite common. Is there anything out there which can use the hot air movement to generate useable energy for home use?

Mar 29, 2010 10:17 AM ET

Hot attic air
by Martin Holladay

Every few years, some inventor comes out with a new gadget to harvest hot attic air. There's one on the market now that tries to use hot attic air to preheat domestic hot water.

Take my word for it: all of these gadgets cost far more to install than the value of the energy could ever justify.

Jun 3, 2010 5:58 AM ET

You saved 20% on electric bill
by Peter Yost

Good to hear. I don't doubt that solar-powered attic exhaust fans can significantly lower vented attic temperatures in hot climates. My concerns are two fold: one, is the fan the best bang for the buck and two, do you have a good enough air seal at the ceiling attic plane that your attic exhaust fan is not depressurizing your house. Danny Parker at the Florida Solar Energy Center has done research to show that these units can save money. I just wish that all of the manufacturers made it clear under what conditions these units are the most cost-effective option and the safe and healthy choice. My support of solar-powered attic exhaust fans is "heavily qualified" at best. If you have any combustion appliances in your home that are atmospherically-vented, just make sure they are functioning properly when your attic unit is operating.

Jul 2, 2010 4:01 PM ET

what do you think about the
by Anonymous

what do you think about the solar attic fans that have high cfm output like "attic breeze" and "snap-fan"? are these viable products, or do you lump them all in the same category?

Jul 2, 2010 4:20 PM ET

Attic Breeze and Snap-Fan
by Peter Yost

The fans you ask about are attic exhaust fans, the subject of the blog. We do not recommend their use unless there is a continuous, comprehensive air barrier at the ceiling plane that would keep the attic exhaust fan from depressurizing living space. Further, we recommend that the total cost of the attic fan exhaust system be compared to other approaches to reduce heat gain in the conditioned space from the vented attic, as discussed above.

Jul 2, 2010 5:34 PM ET

comprehensive air barrier
by Anonymous

how do i know whether or not i have a comprehensive air barrier?

Jul 2, 2010 6:42 PM ET

Comprehensive air barrier
by Peter Yost

You have a energy technician do a blower door test on your home, a good investment all around. Look around the GBA website abit (type air barrier in the search box) or start here:

or here:

Jul 2, 2010 8:43 PM ET

Thanks Peter. Good Info. So
by Anonymous

Thanks Peter. Good Info. So under the assumption that a home has a good air barrier, are high CFM solar vents viable in your opinion? The reason I say high CFM is that most of what I've seen don't move much air yet make bold claims.

Jul 3, 2010 5:35 AM ET

Still an option
by Peter Yost

Have you looked at the FSEC report cited in the blog? I would base my decision after looking over that report and comparing it to your situation.

Jul 3, 2010 7:31 PM ET

FSEC Report
by Anonymous

interesting. although it's hard to tell whether the 800 cfm was for each fan or both combined. if it was 800 cfm per unit, that seems a bit over sized for 1,000 sq. ft house. nonetheless it did drop the attic temperature significantley given the existing radiant barrier. 6% doesn't have me cutting back-flips, but it would have been good if the study didn't have the radiant barrier so we'd get a more accurate assessment of pv vents.

Jul 7, 2010 11:52 AM ET

what about other bennies?
by seth

Solar exhaust fans also claim to extend roof shingle life, and remove moisture that builds in the attic? Any truth to those claims?

Jul 8, 2010 2:34 AM ET

hot summer
by Anonymous

I am thinking of installing a solar attic fan, but with my windows open on summer evenings, I don't think I would have any negative pressure potentials. Just cool my attic, which would cool the top floor of my house. I have not AC, HVAC, or gas fired appliances.

Jul 8, 2010 5:52 AM ET

No worries with negative pressure
by Peter Yost

Again, do a comparison of the level of effort and expense involved for the attic fan and improving the air tightness and thermal insulation in your attic. Depending on your climate, you would need to factor in the benefit of the attic fan compared to the air sealing and insulation during the winter when the fan won't help the reverse temperature difference but the air sealing and insulation will.

Jul 8, 2010 1:13 PM ET

Solar Powered Fans are bass-ackwards
by Kevin Dickson, MSME

The thing that bugs me about the solar powered attic fans is that they only run during the hot part of the day. Trying to cool down an attic with hot air is less effective than using cool air which you have plenty of at night. Also, the problem of pulling conditioned air out of the house is moot at night in many climates, because the ambient air temperature drops to 75F or less.

Peter, I think most folks faced with evaluating attic air sealing vs. installing a hard-wired fan will choose the fan. It's a much simpler task. But as you say, it doesn't help in wintertime.

Jul 8, 2010 2:13 PM ET

Whole House Fans
by Frank Bell

Any thoughts on or research done on the use of whole house fans? Any links or thoughts would be appreciated.

Jul 8, 2010 4:39 PM ET

This works like a charm
by Ian on the Trent

My +125 year old, 2-story home has a mansard roof. The height of the attic is approx 3ft. The plan area is 18x 33ft. Fifteen years ago, we gutted the upstairs, insulated, and installed a vapour barrier.There is a ventilation channel between each vertical roof joist. These empty into the attic. There is a single 'whirlybird-type attic ventilator on the flat roof area.

My wife and I decided to go green a few years ago so we ditched the old air conditioners. This is when we noticed how hot the upstairs got even with window fans turned to max. It would take until the wee hours of the morning to bring the upstairs room temperature down to the outside air temperature. It reminded me of when I work at a GM forge. This was unacceptable.

I read somewhere online that a green cooling strategy was to make sure that the attic cavity temperature was as close as possible to the outside air temperature throughout the day. In our situation it meant constantly removing air from the attic since the flat roof was naturally heating up and radiating heat into the low volume attic cavity. And then into our upstairs rooms (despite the 6" of batt insulation).

As money was tight I couldn't afford to install an electric roof vent. Solar roof vents were even more expensive. As a short-term solution, I rigged up a 10-inch portable fan directly below the whirlybird vent and plugged it into a mechanical timer. (This is used just for the summer months.

Guess what? It works like a bloody charm. Even today with the sun beaming down and the outside temperature in the neighbourhood of 33degC (91degF) the upstairs is still the same as the outside air temperature. And in combination with the window fans, the room temperature will match the outside temperature as the evening progresses. Yes, on the exceptional hot days the evening temperatures are still hot, however, most of the time our upstairs is remarkably cool by bedtime or earlier. I cannot believe how well a "powered vent" works in my situation.

What I do wonder is if a solar vent can displace the same volume of air as my portable house fan.



Jul 9, 2010 12:03 PM ET

Solar attic fans
by Anonymous

I agree with Travis Hipp and I'm definitely not a physicist, here's my input. Attic fans can and will work with proper sizing and airflow, also solar attic fans at there peak performance can only move on average of 1200 CF/m that's nothing, but enough to do something!
Thank you all

Jul 29, 2010 10:08 PM ET

hurricane question
by D Rev

Any thoughts on the value of having a solar powered attic vent when the power goes out for an extended period - such as in the wake of a hurricane.

Jul 30, 2010 4:38 AM ET

Response to D. Rev
by Martin Holladay

D Rev,
Since a solar-powered attic fan is a solution in search of a problem, I have to ask: what problem would you be trying to solve in your post-hurricane scenario?

I'm guessing that you are afraid that your house will be hot. (The air conditioner will be off.) If your attic makes your house hot, the solution is: (1) Seal the air leaks between your attic and your house, and (2) Install insulation on your attic floor. Then you'll be fine.

If you want a way to stay cool during a power outage, put the solar panel in your front yard and run a wire from the panel to a fan in your living room. Then you can sit in front of the fan and feel the moving air on your face.

Jul 30, 2010 3:14 PM ET

Powered Attic Vent
by Carolina

We recently had our roof replaced and instead of having the 'whirley birds' installed again, we opted for powered attic vents. We have a total of three installed. I live south of Dallas so have plenty of hot, humid weather to contend with. Since the installation, I've noticed the downstairs to feel more humid but still registering at 76 degrees. We have a masonry fireplace with no glass doors and was wondering if the attic fan was pulling air down the chimney and into the house. I decided to tape the fireplace opening shut with some plastic and could see how the plastic was being sucked inward into the family room. Do we have a potential problem here and/or should we consider having the powered attic vents removed and the 'whirley birds' installed after all? Thanks for your help!

Jul 30, 2010 3:53 PM ET

Response to Carolina
by Martin Holladay

If the plastic is being sucked inward toward your living room, that is a sign that your house is under negative pressure. Your attic fans could certainly be the cause of the problem, although other factors (including problems with HVAC ducting) can also cause depressurization.

You need to take three steps:
1. Determine the cause of the depressurization.
2. Seal the leaks causing the depressurization (for example, leaks in the ceiling under your attic, or leaks in your HVAC ducts).
3. Seal your fireplace or chimney to prevent outdoor air from entering your living room when the fireplace isn't being used.

You will probably have to hire a home performance specialist (for example, a BPI-certified or RESNET certified home rater) to help diagnose your problem.

Jul 30, 2010 4:49 PM ET

Powered Attic Vent
by Carolina

Martin, thanks for answering so quickly! I will certainly see about hiring a home performance specialist.

Aug 1, 2010 12:34 PM ET

by Anonymous

If you guys are so oppossed to solar vents, why do you let them advertise on your site?

Aug 3, 2010 1:38 PM ET

Google serves the ads
by Daniel Morrison

We don't decide who can advertise on Lakesideca Advisor; the ads are served through a Google algorithm.

For more on our advertising policy, see this page:

Aug 14, 2010 7:33 PM ET

Solar-powered attic vents
by Sambini

OK, I'll bite. Full disclosure: we install solar-powered vents as part of our product offering.

Your article correctly & predictably zeroes in on ‘sealing & insulating’ but gives scant mention to proper air intake requirements, which is also a preventative for depressurization. For all the reasons you state, and also to preserve motor-life, we insist on bringing the attic intake-venting up to the proper specs. We understand negative pressure and do our best to avoid contributing to it. Thank you for advocating a careful approach; we'll continue to be careful.

The Florida study is a good starting point. However, it was done on a ranch on a slab, and as many have mentioned, with a radiant barrier on the rafters and an attic-mounted HVAC unit. It did not address the tremendous variety of building styles, attic volume, the science of 'delta-T', multiple stories, the importance of air intake, etc. A much more comprehensive study—which might include proper controls on CFM, available intake, sizing, pitch, roofing material, # of stories, geography, shingle-life, tax credits, hvac zoning, etc.—might give a more complete picture.

For example, in addition to attic-heat reduction (the biggest area of energy savings), no mention was made of any of the other benefits/payback from this green technology, which strengthen its case:
--Counteracting cold-weather condensation (e.g., from bath fans or heat rising through inadequate insulation) which can/does lead to mold & mildew problems. Wet insulation is conducting heat and cold, not insulating; Dry insulation saves on heating costs.
--Extending the life of roofing: Your dismissiveness of the real problem of attic heat is curious (‘If the attic ceiling is airtight you don’t need the fan’). The composition-shingle manufacturers are now pro-rating their warranties, tied directly to attic ventilation. Isn’t that a measurable plus?
--Reduced ice damming—yes I know, usually another insulation problem.
--Tax Credits—same as solar, wind and geothermal. No limit, through 2016, includes the install.

As to "Money better spent,” you suggest “take the money you would have spent on the PV-powered attic ventilation and upgrade the air sealing and insulation in your attic and on your HVAC system.” PLEASE tell me where you can have the same dramatic results in comfort and efficiency—especially on two-story homes—for the $600-$850 for a solar-powered fan! In truth, it takes thousands of dollars in 'sealing and insulation' (not to mention upgrading the HVAC) just to counteract the reality of 'delta-T'.

Proper attic ventilation is a legitimate part of a green solution. Maybe the solar-powered attic vents can be seen as a stop-gap until a homeowner can afford the kind of expense for the "right" solution.

The science does not condemn these fans, and truly, neither did your article. It was just "kinda" negative, and zero'd-in on the holy grail of 'sealing & insulation'.

Aug 23, 2010 2:22 PM ET

We are Happy
by PeterM

I live in Las Cruces, NM in a newly built home of around 2,000 square feet. The temp's here during the summer range from 95 - 105 every day of the summer. I decided to go green in an attempt to cool our flat-roofed home. I first coated the flat roof with LO-MIT (aluminum/silicon based thermal coating. It helped a bit. I then installed radiant barrier foil within the attic... That also helped a bit. My garage (non-cooled) air temp never got above 88 degrees F. I then decided to try using two Honeywell 10w solar powered attic fans. The day before fan installation my garage temp was 88 with an outside temp of 103. The day after installation the temp outside was also 103 degrees F, but my garage temp was at 82 degrees F. A six degree drop due to the introduction of the fans.

My ac is on the flat roof with all the ducting in the attic. I am of the opinion that I should see a substantial drop in my ac costs. The Sam's Club Honeywell fans, plus tax, only run $200 each and have a 5 yr warranty on the motor with a 20 yr warranty on the solar panel.

The only real precaution I took is to make sure that I had at least 2 times passive ventilation rooftop openings vs the diameters of my fans. This should be sufficient in ensuring that we will not have any major depressurization problems.

Aug 23, 2010 2:29 PM ET

Response to Peter M
by Martin Holladay

Two observations:

1. You never told us how much insulation you have on your attic floor, or what kind of insulation, or whether you have made any attempt to seal air leaks under your attic insulation.

2. Your $400 worth of fans are ventilating your attic with 103°F air.

Aug 23, 2010 2:36 PM ET

Venting 160 degree attic air with 103 degree outside air.
by PeterM

The outside air was 57 degrees F cooler that the 160 degree F attic air. The attic floor is well insulated with about 6" blown in. House is pretty airtight but did not bring in anyone to test it.

Aug 23, 2010 2:44 PM ET

Bingo! There's your problem
by Martin Holladay

Peter M,
Loose-fill fiberglass has a lower R-value per inch (2.0 to 2.7) than any other common insulation product. If you have 6 inches of blown-in fiberglass, you've got no more than R-16. That's dreadful.

You're in Climate Zone 3, where the code calls for attic floors to be insulated with a minimum of R-30 insulation. Your measly 6 inches is a long way from meeting the minimum code requirement.

Imagine how much more comfortable your house would be if you had installed $400 of additional attic insulation instead of $400 of unnecessary ventilating equipment.

Aug 23, 2010 2:55 PM ET

I am cooking the attic NOT my house
by PeterM

I did not measure the insulation. I can if you like. It is to code, i.e. R-30, I was at the builidng site when the inspectors went through it.

Imagine the decrease in my AC costs with the ac ducting in the attic residing in a much cooler attic environment. I am looking to decrease my ac cycle times. Dropping the temp in the attic will do that.

Aug 23, 2010 3:39 PM ET

Cose when house was built
by PeterM

Just want to correct your impressions. Our climate zone here is Zone 7-8 not Zone 3. Also, I checked with the builder here (now a friend). He claims I have R-42 in the living areas (about 17") and R13 over the garage (that is where I came up with the lower insulation thickness), i.e. my crawl space entry is in the garage. My garage walls have R11 and the living area walls have R13. All in all a very well insulated executive home on a golf course.

Aug 23, 2010 3:52 PM ET

Climate zones
by Martin Holladay

Your local code evidently uses a different climate zone map than the DOE. Using the DOE climate zone map, which is referenced in the International Residential Code, I believe that Las Cruces, NM is in Climate Zone 3.

If you have ducts in your attic, you are quite right that your attic ducts can get very hot. I strongly urge you to improve the insulation on your attic ductwork.

Aug 23, 2010 4:03 PM ET

Ducts are wrapped
by PeterM

I am an avid gardener. I and most people, I believe, go by the climatic zones that gardeners use. My zip is between Zone 7 and Zone 8..

All my heater/ac ducts are wrapped with super thick glass insulation already.

What I was fighting was the temp's in the attic that can reach over 160 degrees F during the heat of a summer's day.

Aug 23, 2010 4:09 PM ET

Hot attic woes
by Martin Holladay

You wrote, "temperatures in the attic can reach over 160 degrees F during the heat of a summer's day."

With all due respect, that's why your ducts probably need to be protected with more insulation.

Aug 23, 2010 4:18 PM ET

I say 'attic' but mean 'crawl space'
by PeterM

In the Southwest most homes have flat roofs and crawl spaces that only an ant can crawl around in. There are few true attics here. You have to make sure things are done at the time of construction. Mine are insulated now, but if I had it to do over again I would have doubled the insulation on the ductwork. However, I am finding that the solar powered attic fans are doing there thing. I will know for sure when I can compare electric bills over the next couple of months compared to last year.... The drop in the garage temp is only taken as a leading indicator that the fans are going to make a significant difference. I will update you as the data come in. :)

Sep 1, 2010 12:47 AM ET

Question for the attic fan proponents
by Danny Kelly

I assume you would agree that attic fans move heat through convection.
I think we would all agree that the majority of the heat in an attic during the summer is from radiation.

Are you aware that radiant heat cannot be moved via convection?

Sep 1, 2010 5:44 AM ET

Danny, Danny, Danny
by Martin Holladay

Heat does not come with name tags. There aren't three kinds of heat in your attic, each separated into three cliques like teenagers at a party. Heat is heat.

Fans move hot air. If (this is a big if) there is a source of cool air to supply makeup air, moving hot air out of a room can lower the temperature of the room.

Just because the temperature of an attic is increased due to radiant heat from the underside of the roof sheathing, doesn't mean that the warm objects in the room have been forever branded with a special sign called "radiant heat." The objects are simply hot.

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