This article explains the second of three fundamental concepts that you should take a few minutes to understand before starting your next energy efficiency project in your old home. The short time it takes you to read these three articles could save you quite a bit of money, energy, and frustration. If you haven’t already, read about Concept 1. At first, Concept 2 might seem unrelated, but we’ll tie it all together in the final article in the series.
I’ll present the second concept to you by combining two phrases you’ve heard before:
Heat rises, and nature abhors a vacuum.
These two sayings pretty much sum up what’s known as “stack effect,” or the chimney effect. Technically stack effect only describes the “heat rises” part, but it is so linked to the “nature abhors a vacuum” part (as you’ll see, that for practical purposes it’s best to combine them. As you might remember from grade school, warm air rises because it’s lighter—less dense—than cooler air:
In the thermal image above (that’s a joke), you can see how the warmer (red) air molecules are more spaced out at the top of the room. They’ve got more energy (from heat!) than the cooler (blue) air molecules that are all lazily packed together at the bottom of the room. Packed together = more dense, so the cool air sinks to the bottom while that energetic warm air rises to the top. (The idea that warm air is “less dense” may feel counterintuitive if you live in a part of the country with humid summers—that’s because warmer air is also capable of holding more moisture. We’ll get to that in another article.)
OK, so heat rises. Anybody who’s ever climbed a ladder to change a lightbulb and noticed hey, it’s hotter up here, knows that. It’s why the top floor of your house is probably warmer than your bottom floor, year-round. What we’re more interested in here is, well, a side-effect of stack effect, one that occurs because you rarely find perfectly sealed boxes full of air like the one in the picture above. And one of the best illustrations of this side-effect is a charcoal grill:
When you’re grilling, the hot air quite obviously rises, which you can feel in your face as you cook and see illustrated in the smoke wafting away. But when that air floats away, something has to take its place. That’s why I mentioned the phrase “nature abhors a vacuum.” And that’s why there are air intakes at the bottom of a charcoal grill: cooler air rushes in to fill the void left by the escaping hot air. As part of a beautifully simple system, this air intake provides the oxygen to fuel the burning coals, which keeps the whole cycle going—but don’t think of it as a cycle. Think of it as constant. You intuitively know that the hot air is constantly rising up from the grill. Force yourself to think about the cooler air that is constantly rushing in.
Ever use a chimney starter to get your coals going? If you haven’t, I recommend it—no lighter fluid necessary, and it takes the same amount of time (that’s an affiliate link, by the way). And a chimney starter might be an even more elegant example of the process. Hot air up and out, cooler air in through the bottom, filling the void and fueling the coals:
You can probably see where I’m going with this, but I’ll spell it out for you anyway: this same process is going on inside your old home, every day of the year. Because like it or not, your leaky house is much more like a charcoal grill than an idealized box of air. In the winter, stack effect and its side-effects are most dramatic, because—like in a charcoal grill—the temperature differences between inside and outside are the most extreme, which speeds up the inflows and outflows. That heated air (that you’re paying for) is rising to the top of your house, finding its way outside any way it can, and that air is being instantaneously replaced by cold air rushing in somewhere else in your house. And yes, that’s basically the scientific explanation for what we call a “draft.”
In the summer, the same thing is happening, just more slowly—and if you’re running air conditioning, it’s probably actually happening in reverse. That effect is a little more complicated, or at least less intuitive, but essentially the hotter and more humid outside air is pushing its way inside your leaky house, and forcing your conditioned air out the bottom! In any case, the key takeaway is the same: the air you are paying to make more comfortable is running away from you, and it’s being constantly replaced by uncomfortable outside air (that you then have to pay extra to condition).
There are situations when you can harness these natural forces for good, such as opening windows at strategic locations around your house to take advantage of stack effect for natural ventilation and cooling. We’ll talk more about those in the future. The important factor is control: right now, you probably have very little over this phenomenon. And if you want to seriously save money and energy, you’re going to want to change that.
You’ve made it this far, don’t stop now: last up, we’re going to introduce the final crucial concept on your road to serious energy savings.