I’m asked all the time about attic fans. I get their appeal. You see them on your neighbors’ roofs, you can buy them in the store, and they look like an easy (cheap) solution to your hot-house problem. Even the state’s energy-efficiency program promotes them, so they must work, right?
…Right?
Take a look at this map of attic insulation jobs I’ve done:
More than half of these houses already had an attic fan installed. Sometimes more than one. So then why did they call me?
They called me because the attic fan doesn’t do what they hoped. The thought was to suck the hot air out of the attic, but you don’t live in your attic. They hoped an attic fan would keep their house cooler but it doesn’t, because it doesn’t address the core issue, which I wrote about here.
I’m not saying they don’t do anything. There are air molecules in your attic and when they’re hot they have more energy. The fan removes the hot molecules and their associated energy. I typically get calls from hot areas, so if you live outside of where I normally work, then an attic fan may help you. But if you live near any of my past clients, an attic fan isn’t where I’d start. I saw a proposal recently where someone charged $1200 to install one, and the rebate on them isn’t that big.
Attic Fans Suck
There’s also the issue that when an attic fan turns on and starts sucking, it doesn’t care where it sucks the air from. Most of the houses I visit have these little puck vents around the perimeter, in the bird blocking.
The thing about these little vents is that they don’t have much “net free area.” Each of these 2” pucks only has 0.66 square inches of NFA. That’s not very much. It’s not uncommon for normal houses to only have a couple of these vents in every other rafter bay, adding up to much less than a square foot of soffit ventilation area. Look how few there are in this attic:
That matters because not all fans are the same and the chances that your fan installer is doing any fan-math are very slim. But I do love a good exercise so I found this fan for sale at HPM:
You can buy it right off the shelf, which is what people do. It moves 1825 cubic feet of air per minute. The guideline is that for every 300 CFM of fan capacity, the fan needs 1 square foot of NFA. Therefore this HPM fan needs 1825/300 = 6.1 square feet of NFA.
I can say pretty confidently that almost every one of the houses I visit doesn’t have anywhere near that amount of ventilation. So when the fan turns on, and it can’t get the air it needs from the attic vents, where does it get the air from?
It sucks the air out of your house. If you have air conditioning, it’s sucking out cold air that you paid for. That’s expensive air. And now your attic fan is forcing your air conditioning to work harder for longer, costing you money, for no appreciable benefit.
I know that if you ask around, people will tell you how much they love their attic fans. Just remember that their house is different from yours. Maybe they have a lot of insulation that’s actually doing the job, or they’re on solar so they don’t notice the higher electrical usage. Maybe they live in a different climate. Every house is different, even if they’re next door to each other.
But if your house is hot, and doesn’t have any insulation in it, installing insulation works all the time! If you’d like to explore different options, get in touch.
And if you’d like to read more, Allison Bailes is also a physicist, but he’s got a PhD:
Don’t Let Your Attic Suck
The Top Two Reasons Powered Attic Ventilators Are a Waste of Money
Are Powered Attic Ventilators Ever a Good Idea?
