Are solar-powered attic ventilators green?

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.

Dear Solar Fan,
by Martin Holladay, GBA Advisor

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.

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.

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,
http://www.BuildEnergy.org

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.

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!

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).

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.

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.

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.

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.

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.