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 power attic ventilators, 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 that it's not just the cooling effect we are after here." Yesterday I got another one so clever and witty I was at a loss as to how to respond: "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. ;~)
One reason power attic ventilators don't help
In my original article, I focused on makeup air. That is, I said that when you run a power 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 power 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 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 conduction, 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 systems that are up there.