Using (and Countering) Convection

The principle of convection says that warm air always rises, and cool air always falls. Ditto with water or any fluid. Remember this principle; it will save you hours of frustration and more than a few dollars. If you don’t know why this happens, click here. If you already know why, read on.

Before you start building any home, take a moment and think like air. No, really. Imagine that you are a clump of air encountering your house. Where do you go in, and where do you go out, and what happens to you along your journey? If your house is typical of most built in the last 50 years, it was built as if convection didn’t exist. Why worry about convection, when cheap fossil fuels allow us to size a central air conditioning system as large as we want to keep all areas of the interior dust-free, at just the right level of humidity, and within 1.5 degrees F of the ideal temperature? Made sense in the old days, but no more. Now we will need to use our brains instead.

Understand your challenge. Are you in a region that’s cold more than it’s warm, so the challenge is to keep warm in the winter? Are you in a region where it’s hot more than it’s cool, so the challenge is to stay cool during the summer? Or are you in a region where it’s cool most nights and hot most days? Knowing this will guide the way you use convection.

Cold Climates

If you live where it’s cold and your challenge is to stay warm, in general you’ll want to minimize convection by keeping your ceilings relatively low. You’ll want to maximize insulation, of course, and you’ll want to keep your living space as close as possible to where warm air collects. And of course, you will want to make the most of passive solar principles.

Heat stratification (the collection of warm air above and of cool air below) is your enemy. If you’re stuck with high ceilings, consider a ceiling fan to keep the air circulating, or even better, a StratoJet. Or maybe move more of your normal daily activities, especially the most sedentary waking ones, to a point higher in the dwelling. Sleeping isn’t a problem, because you can address that with blankets and comforters. We’re talking about sitting and visiting, eating, reading, and watching television or listening to the radio.

Hot Climates

if your challenge is staying cool in the summer, you want to focus on two things: (1) keep cool air coming in and send warm air gently on its way, and (2) let warm air stay above where you conduct your normal daily activities. There’s a reason those grand old houses in the south had 12 foot ceilings!

If you have the luxury of designing your dwelling from scratch, consider a stack, an area specifically designed to exhaust the warmest air that collects at the ceiling level and therefore to pull in fresh and (we hope) cooler air at the lower level. You may be surprised at the robust quantities of air air you can keep moving through your house with a well-designed stack, all without air conditioning or even an attic fan. Convection is a powerful thing. And at night when the weather’s warm, the more air you can move through the house, the better.

You’ll want to deploy ceiling fans in every area where you’re going to be spending considerable time, because they will provide spot air movement. Moving air nearly always feels cooler than stationary air, even if you’re pulling some warm air down from the ceiling. Because you want to maximize convection, however, and also because you don’t want to waste energy, turn the attic fans off when you’re not in the area. They will actually make the air warmer wherever they’re running.

A note about stacks. They work great unless the air gets so hot you have to turn the air conditioning on. A well-designed stack can extend the time during the summer when you don’t need air conditioning, sometimes dramatically. Once you turn the air conditioning, however, stacks don’t do you any good at all; they interfere with the air conditioning, because they provide an area where heat collects and just place more load on the cooling equipment. We anticipate a day in the not-too-distant future when everyone will want to avoid running their air conditioning as long as they possibly can, so we’re not troubled by the extra load a stack places on air conditioning, but you may see it differently.

Swing Climates

If most nights are cool and most days are warm, you’ll want to do everything possible to control convection. In your climate, your main passive solar challenge is to use thermal mass effectively. You want heavy, heat absorbing materials that will heat up slowly during the day, taking on heat and making the house cooler, then give it back at night and keep the house warmer. Convection usually will need to be slow and gentle, because all rapid convection does is to reduce the effectiveness of your thermal mass.

Hot spots are a challenge for houses in your climate, because sun tends to be direct and unrelenting during the day, and at night your thermal mass will radiate heat. Consider a gentle aid like a StratoJet that simply redistributes warm air around the living space.

Resist the temptation to have massive windows, they almost always let in more heat than you want during the day and give up more of it than you want at night. Opt instead for strategically placed windows that you can open when you want to bring in fresh outside air just when you want it, and shade during the summer to limit the amount of heat they take on. If you’re already stuck with big windows, think about heavy insulating drapes you can use to reduce the sun’s action during the day (when you want to) and to keep warm air from escaping at night.