With the humidity at about 100% today–my glasses steam up when I step outside–I figured today was the perfect day to revisit a reader question from a few years ago. It was about whether it makes more financial sense to spring for central air conditioning or use window a/c units. (We have central air and I couldn’t live without it.)
Q: How energy efficient is central air vs. window units? I bought a house that can be easily converted to central air, but have been living with window units for the last two summers. Would I save energy by making the switch? I don’t cool the whole house at the same time — in fact, when it’s really hot out (like last week) I move my home office downstairs and stay in the dining room, which is the only downstairs room with a/c.
A: Turns out the answer isn’t as simple as one might think. There are a number of factors that affect whether or not you should use window units versus central air. According to experts what really matters is the climate where you live (hot and humid Houston versus dry Denver), how your home is insulated, and what your personal preferences are. Let’s start with climate.
If you’re looking to cool your home and lower the humidity, a central air system is your best bet, hands down, as far as efficiency goes. “Well-designed central systems win out in terms of being able to filter the air for allergens and pollutants, and for controlling humidity,” says Mike Rogers, a senior vice president with Green Homes America, a company that provides residential home performance contracting solutions that increase in-home comfort, health and safety, and significantly reduce energy usage and utility bills.
Two reasons that window units are all wet when it comes to reducing humidity are that they tend to sweat (especially if they are overworked) and therefore introduce moisture into the room. And, adds Rogers, “it’s harder to seal window units, and thus you do get some of that hot, humid air sucked in from the outside.”
That said, if humidity isn’t your problem but you’d just like to cool your home, you could get away with one window unit on each floor, if your home is well insulated. (Check out this Energy Star website to figure out the right-sized air conditioner based on a room’s size.) Insulation isn’t just in the walls, by the way. We’re also talking about well-insulated windows and, believe it or not, a well-insulated attic. “In the summer, temperatures in the attic often climb to more than 140 degrees,” says Rogers. “This tremendous heat conducts down into your home. Even when the first floor is comfortable, this constant flow of heat from the attic can bake your bedrooms upstairs. Although most people don’t think of insulation as a way to keep cool, increasing levels in your attic can make a huge difference.”
Keep in mind that while window units are cheaper in the short run, if you’re looking to purchase more than a few of them, you’re probably better off going with a central air system. Says Rogers, a central air unit will use less energy overall and cool more efficiently than a series of window units running in tandem. On the other hand, he says, “if you’re just trying to cool one or two rooms—not the whole house—a window unit or two can use less energy than a central system, and a couple of window units are much cheaper than a central unit. And, if you have a very energy-efficient house, or small house, they may be all you need.”
Finally, it’s all about personal preferences. The reader who sent in this question doesn’t seem to mind moving into a single room when it’s really hot out. And if it doesn’t get hot that often that she needs to disrupt her life regularly like this, then she can probably continue to get away with window units. However, if she’s fighting humidity and she seems to be losing the battle, then maybe it’s time to make the leap to central air.
Keep in mind that just like appliances with Energy Star ratings, air conditioning units come with their own efficiency ratings. This Council for an Energy-Efficient Economy website offers a guide to understanding central air efficiency ratings so you can make the best choice for your budget and the environment.