Why Whole House Fans Do More Harm Than Good
In theory, whole house fans make a lot of sense. When the conditions are right, cooling your home with a whole house fan consumes less energy than most air conditioning systems. It's efficient and effective. Your house probably came with one, so why not use it?
Here's the thing about whole house fans. While they might be useful on a handful of days throughout the year, they're a burden for the rest of the year. In most homes, you're better off not having a whole house fan at all and just running your air conditioner instead.
Want to know why? Let's explore.
How whole house fans work
A whole house fan brings outdoor air into your home and exhausts it through your attic. Typically, you need to crack some windows open for the fan to be effective.
When the following conditions are met, a whole house fan might cool your home more efficiently than your air conditioner:
- It's warmer inside your home than outside
- You want your home to be cooler
- Outdoor dew points are lower than indoor dew points (in other words, the air is drier outside than inside)
Here in Atlanta, these conditions only occur every so often. On some days during the spring and fall, you'll wake up to an uncomfortably warm home and cool, dry-ish conditions outdoors. This is pretty much the only time it makes sense to run the whole house fan.
During the summer, it's usually too hot and humid to run the fan. During the winter, you don't want to cool your home - you want to heat it!
Out of every 365 days, you might experience optimal whole house fan conditions on 25 days, max.
That's 340 days each year where the fan does nothing at all.
Except that it does do something. It serves as a gaping hole in your home's thermal and air barrier every single day.
Every day, conditioned air escapes from your home through the slats that cover the fan, forcing your HVAC system to work harder.
Every day during the summer, hot, humid air leaks into your home through via the fan. This makes it harder to keep cool on the hottest days.
Every day during the winter, warm air escapes through the fan. As a result, it's harder to stay toasty and you run your furnace more often.
That's 25 days of fan use and 340 days of crippling energy loss and sub-par indoor comfort.
And because of the way insulation gaps affect overall thermal performance, your whole house fan could be reducing the effectiveness of your attic insulation - up to 27% by some estimates! Let's say you've insulated your attic to R-40 everywhere except above your whole house fan. It's only going to perform as if you had R-29 insulation.
The bottom line? For all the good whole house fans do, they cause more problems than they solve. On the whole, you're better off not having a whole house fan at all and simply running your AC on days when you'd otherwise use the fan.
Got a whole house fan? Here's what to do.
There are two ways to deal with whole house fans:
- Covering, sealing, and insulating around the whole house fan
- Removing the whole house fan, patching the hole with drywall, and adding insulation on top
The first method involves building an insulated box around the whole house fan inside the attic. First, we seal the joints and edges around the fan to prevent air leakage. Then we remove power to the fan, fill the gaps with loose fill insulation, and build a box around the fan using foam board.
This process is very similar to how everybody should treat their attic hatch. Much like a whole-house fan, your attic hatch is a significant source of air leakage and heat loss. Sealing it off and insulating it can help you stay comfortable and reduce your energy bills!
The second method involves drywall work, which can be kind of messy. Many people prefer it, though, because the end result is a smooth, unblemished ceiling. You won't have to look at the fan anymore, which is a nice plus.
But, but… I like my whole house fan! You guys are harshing my mellow.
You can keep it! Just be aware of the likely energy and comfort penalties you're experiencing.
More often than not, a whole house fan causes more problems than it solves. By sealing up the ceiling cavity and adding insulation, you can avoid those problems, keep conditioned air where it belongs, and save money on your utility bills.
In the end, we don't think you'll miss the whole house fan. You can run your AC instead and still consume less energy overall.
Image Credit: Piercetheorganist