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The Top Two Reasons Powered Attic Ventilators Are a Waste of Money

Despite how fervently some people believe in them, powered attic ventilators don’t pass building science muster

Posted on Jun 4 2014 by Allison A. Bailes III, PhD, GBA Advisor

Three years ago I wrote an article titled, Don’t Let Your Attic Suck: Power Attic Ventilators Are a Bad Idea. Nearly a hundred thousand page views and 93 comments later, it's still generating lots of heat. I don't know why so many people are so defensive about powered attic ventilators (PAVs), but here are a few of the things they've said to me in the comments:

You really should do more research before you post blogs like this.

Common Sense!!!! is in the building world, you really ought to check it out sometimes.

I challenge you to a battle of applied knowledge in this field any day, it's people like you who make people who need an attic fan second guess themselves out of speaking with a true professional.

Oh, and let's not forget: "You just are not getting it lady..." Last week I got another one so clever and witty I was at a loss as to how to respond. As it has mild profanity, I'll post it at the end so you can avoid it if you'd like. With so many delicate construction types hanging around here at GBAGreenBuildingAdvisor.com, I try to be sensitive to your needs. ;~)

One reason powered attic ventilators don't help

In my original article, I focused on makeup air. That is, I said that when you run a powered attic ventilator in a typical house, yes, it will cool the attic down. A significant amount of that cooling is likely to come from conditioned air being sucked up from the house below. Most ceilings aren't air-sealed well, so putting a negative pressure on the attic will do that. I quoted Peter Yost in the earlier article, and in the comments, David Butler made a similar remark:

"In a particular home, if a PAV truly reduces cooling costs enough to pay for itself (don't forget to consider the energy the fan consumes), then that tells me there are issues with ceiling insulation and/or attic venting."

That's as true today as it was three years ago when I wrote the first article. And it's part of the reason that my state, Georgia, has banned power attic ventilators (unless they're solar-powered, which was a concession needed to get the grid-powered fans banned). But there's really a more fundamental reason that powered attic ventilators won't help a lot, and for some reason, I didn't mention that in the original article.

The #1 reason

How does heat get into the attic? Well, it starts at the sun and radiates down to the rooftop. We make sure that most rooftops can soak up as much heat as possible by using asphalt shingles. They're often dark-colored. They're granular. And many roof surfaces are tilted toward the sun for enhanced absorption.

That heat then conducts down through the roofing materials. The underside of the roof deck can get very hot — so hot you can't keep your hand on it. At temperatures of 150°F or more, that's a lot of heat sitting there in that plywood or OSB. Some of it will go directly into the attic air by conductionMovement of heat through a material as kinetic energy is transferred from molecule to molecule; the handle of an iron skillet on the stove gets hot due to heat conduction. R-value is a measure of resistance to conductive heat flow., but that's a small amount because air isn't a good conductor.

The main way that heat gets into the attic is through radiation. That hot roof deck radiates heat down into the attic. But that radiant heat passes through the attic air and hits the solid materials. It gets absorbed by the framing, insulation, the stuff you're storing up there, and, unfortunately, any ductwork and HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. systems that are up there.

Those materials heat up. They give up most of their heat by conducting it downwards into the house or into the ductwork and HVAC system and then into the house from there. Some of that heat gets into the air above the hot materials on the attic floor, but the attic air getting heated up is a secondary effect. See it now? Here it is: Using a fan to blow hot air out of the attic doesn't address the radiant heat flow from the roof to the attic floor. Much of that heat then conducts downward and finds its way into the house.

Trying to solve the heat gainIncrease in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss. problem in your attic by using a fan is like lying out at the beach with a fan blowing over you and thinking you're not going to get a sunburn. A radiant barrier would be a better way to attack this problem, but the cost-effectiveness of radiant barriers is marginal.

The clever comment

Well, I promised I'd share the wisdom of the latest commenter with you, so here it is: "OK... I'm going to say it because no one else will... Allison Bailes, you're an ASS!! :-)"

It makes me feel so much better that he used that smiley face at the end. ;~)

Powered attic ventilators cause problems

I've been in lots of attics. I've seen lots of powered attic ventilators, including the one in the top photo. That one was hooked up at one of the gable vents. The other seven fans in the attic were spread across the roof, and they were naturally fighting against each other. There's no way there was enough open vent area in that attic to supply all eight of those fans.

In another home, I found three powered attic ventilators in the roof. We had been called in to solve a mold problem in two of the bathrooms, and those three fans turned out to be the main problem. We turned them off and the negative pressure pulling humid outdoor air into the bathrooms has gone away.

Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. Check out his in-depth course, Mastering Building Science at Heatspring Learning Institute, and follow him on Twitter at @EnergyVanguard.

Footnote:

My friend Mike Barcik gets credit for this analogy, except that when he tells it, you're lying naked in the sun. Naturally, this being a family blog and all, I don't talk about naked people here. (And no - that's not me in the photo.)


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  1. Energy Vanguard
1.
Fri, 06/06/2014 - 03:22

Edited Fri, 06/06/2014 - 03:23.

.... keep it cool in the first place ...
by flitch plate

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Instead of money and mechanicals (and the other problems outlined by Allison) to exhaust overheated attics, why not keep them cool in the first place. This is a perfect reason for a cold roof design (i.e. finished top isolated from deck by airspace and a ventilation plain), as well as other considerations such as insulated rafters, and either low solar absorption (reflective) tops or fast heat releasing materials (steel).

The Cold Roof assembly being the most significant and obvious way to not have this and a bunch of other problems in the first place.


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