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Save $$ this Winter -- Read our article: The Basics of Insulating Attic
Energy Basics and Heat Transfer
TIP: Insulation and air leakage both play a big role in the energy usage of most houses. Always seal air leaks when you insulate a house.
IN DETAIL: The greater the temperature difference between one area and another, the faster heat moves. A cup of hot tea cools quickly at room temperature because the temperature difference is large: about 90°E Iced tea takes about twice as long to warm up, because the temperature difference is about half (40°F). If you were trying to keep a cup of tea hot with a heating element, the heater would need to produce twice as much heat to maintain the cup at 160°F as it would to keep it at 115°F (assuming the room is 70°F).
IN DETAIL: The largest energy use in most houses is heating and cooling (about 55%). Therefore, improvements to the building envelope and mechanical systems have the most potential to save energy. Next are hot water heating (15%), refrigeration of food (10%), and lighting (7%). It’s easy to notice when lights are left on or when someone stands in front of the refrigerator with the door open, but the things we don’t see have the biggest impact on our home energy use.
Understanding Heat Transfer
You don’t need to be a heating engineer to know how to install insulation. But by under standing how the mechanisms of heat transfer work and their relative importance in your home, you can better decide how to approach to any weatherization project that you undertake.
Heat transfer is the movement of heat from indoors to outdoors in the winter, and from out doors to indoors in the summer. If heat transfer didn’t occur, your house would always keep you cozy and warm in the winter and cool and comfortable in the summer, without the use of a furnace or air conditioner. In a way, the function of your furnace or air conditioner is less to “make heat” or “make cold air” than it’s to replace the heat that escapes in the winter and remove the heat that enters in the summer.
There are three forms of heat transfer: conduction, convection, and radiation. Of the three, conduction and convection are larger in magnitude— how fast the heat moves—and radiation can have a big influence on the comfort inside a house. Let’s look at how each of these mechanisms operates and affects your home.
Image: It’s important to seal air leaks when you insulate.
Image: Steel studs, joists, and headers are straight, strong, and superb
conductors of heat. Unless they have an effective thermal break, such
as rigid insulation, they conduct far more heat than conventional wood
framed walls.
Image: These infrared scans show a house before and after it was insulated
(above and below, respectively). In the “before” image, the white areas
represent the most heat loss; the black areas, the least. Note the hot
areas in the walls and gable of the left dormer, in the walls below the
dormer (under the porch roof), and under the rake overhangs, as well
as the plume of heat coming out of the main gable vent.
Image: Although this house is insulated, there’s still some heat loss;
the windows haven’t changed. The chimney is quite warm because of the
steam boiler, which stays hot all the time. Note the wall of the shed
dormer on the right appears to have lightened. It had been insulated
when the dormer was added and wasn’t touched. Because the scanner auto
ranges for good contrast, most of the walls in this “after” image show
up a bit lighter than they would other wise.
Heat flows toward cold
Conduction refers to the movement of heat through solid materials. Conductive heat loss always moves from the warm side to the cold side of a material. For example, if you have a cup of hot coffee outside on a cold day, the heat will move through the cup and out into the cold air around it (or into your hands to help keep them warm). On the other hand, if the weather is hot and you have a cup of iced tea, the heat will move from the warm air, through the cup, and into the tea, warming it.
Different materials allow conduction to happen at different rates. For example, we may use a steel pan on the stove to efficiently transfer heat from the stove burner to our food because steel is a very good conductor of heat. For the same reason, steel framing in an exterior wall performs poorly from an energy-efficiency standpoint. Thermal insulation describes a number of products designed to slow conductive heat loss in walls, ceilings, and floors. Common types of insulation, such as fiberglass, cellulose, and polystyrene foam, are poor conductors of heat (for more on insulation, see chapter 4).
It’s important to remember that heat flows through solid materials toward cold in any direction. A house may lose more heat down through an uninsulated floor in a sunroom than it loses up through the moderately insulated ceiling of the same room, The reason attics are usually insulated to a higher degree than walls or floors is not because heat rises; it’s simply because the ceiling usually contains more space to hold thicker layers of insulation more cheaply and easily than any where else in a house. The old saying “heat rises” is misguided—heat actually moves in every direction. However, that saying does have some truth to it, which brings us to the next subject: convection.
Heat moves on air
To an engineer, convection describes heat transfer through the movement of fluid. Convection, to an engineer, is a much more complex subject than we need to understand for weatherizing homes. For our purposes, convection can be thought of as heat transfer through air movement. What causes air to move? Three main forces cause air to move through your house: the stack effect, mechanical systems, and wind. In a typical house, convection is almost as important as conduction and, because it’s often misunderstood, I will go into some detail discussing it here.
The stack effect. The first convective force, the stack effect, is what is meant by the saying “heat rises.” W is really meant is, “warm air rises when surrounded by cold air.” In the winter, a house is very much like a hot—air balloon that is too heavy to lift off from the ground. If you were the pilot and wanted the balloon to go up, you would turn on the burner and add more heat; this would intensify the force pushing up the balloon by increasing the temperature of the air inside the balloon. Similarly, the amount of force pushing air through your house is proportional to the temperature difference between the indoor air and the outdoor air. This is an important concept for understanding the basics of air leakage in relation to indoor air quality and fresh—air ventilation. Because the stack effect is driven by temperature, it’s a more important force in severe climates than it’s in mild climates.
If you were piloting a hot-air balloon and wanted to descend, you would let some hot air escape by opening a flap at the top of the balloon. When you open the flap, warm air escapes from the top and cooler air rushes in from the bottom to replace it. Similarly, when you heat your house in the winter, warm air leaks out of holes at the top, and cold air leaks in at the bottom to replace it. Another way to picture this is to imagine holding a cup of air upside-down in a pan of water. The air in the cup, like the warm air in your house, is lighter—more buoyant—than the water, which is heavy like the cool, outdoor air. If you poke a small hole near the top of the cup, the air will leak out slowly, and the water will come in from the bottom at the same rate to replace the air (see below).
Image: This photo of a building under construction displays the air
pressure pushing the tarp out at the top and in at the bottom. This clearly
shows the pressures that move air through a building in winter.
TIP: Don’t just pack the ceiling with insulation because you can access the space. Remember that heat can just as easily escape through a poorly insulated floor.
IN DETAIL: Most combustion appliances, such as furnaces, water heaters, and fireplaces, draw their combustion air from inside the house. When they’re working properly, they act like exhaust fans, drawing in air to supply oxygen for combustion, then exhausting combustion gases up the chimney or out the vent pipe. Some appliances, known as power vent or induced draft, have blowers that push the combustion gases out. These blowers (shown below) are even more similar to the simple exhaust fans found elsewhere in the home.
Wind. People think that the wind causes most drafts, but in most houses, the effect of wind is quite small. Although the pressure may be greater than the stack effect when the wind is blowing, it’s a part—time occurrence; the stack effect operates 24 hours a day, 7 days a week, all year long. If your house is perched on a cliff or smack in the middle of a treeless plain, the wind may have a larger effect but, for most homes, the stack effect dominates the air movement.
One way to compare the seasonal impact of the stack effect versus the wind effect is to ask yourself whether you would rather have me give you $10 a month or $1 a day. The $10 feels like more on the day I give it to you, but you’ll be about three times richer if I give you $1 every day. So it’s with the stack effect: Although it’s per haps not as dramatic a source of energy loss as windy days are, it adds up over time to be a far greater energy cost.
Mechanical systems. In addition to the stack effect and wind, fans move air through houses. Exhaust fans, combustion appliances, and furnace air handlers move air in predictable and unpredictable ways. Exhaust fans push air out of a house—when they are working properly—and that air is replaced by outdoor air leaking in through openings in the building. Typical exhaust fans include kitchen and bathroom exhaust fans, dryers, and occasionally central vacuum systems (when they are vented to the outside). Combustion appliances, especially furnaces and boilers, contribute even more to air exchange when they are operating, which is when the weather is at its coldest.
Even more significant than exhaust fans are duct systems. Furnace and air conditioner fans are not intended to move air in and out of a house; how ever, if the ducts are leaky—and many are—your furnace fan may push a lot of air through your home. Like the furnace combustion air, an air handler runs the most when the temperatures are extreme, so ducts leak the most when it costs the most to reheat all the air that leaks out.
Image: A central vacuum system, if it vents to the outside, can also
act as an exhaust appliance in the home.
Whether the mechanical systems or the stack effect moves more air in a year varies from house to house, depending on the climate, the construction of the home, the size and location of leaks in the ductwork, and many other factors. In many homes, the air exchange caused by mechanical systems, though mostly unintentional, is the dominant force of convective heat transfer. Later, we will discuss controlling air movement in homes; combustion equipment; and sealing duct leaks.
The Stack Effect in Summer
One misunderstood aspect of the stack effect is that it reverses in hot weather. If the outdoor air is hotter than the indoor air, the heavier air in the house tends to sink and leak out through the bottom; it’s then replaced by warmer outdoor air that comes in through the top of the house.
To use the cup analogy, a house in the summer is like a cup full of water that is held upright. If you poke a hole in the bottom of the cup, the water (which is heavier) dribbles out the bottom and the cup fills from the top with air (which is lighter) at the same rate. People who leave upstairs windows open on a hot day thinking that the hot air will escape because “heat rises” are mistaken—they are actually opening a large hole through which hot outside air can be drawn into the house.
Although it’s true that the upper floor of a home tends to be much warmer on a summer afternoon, that is not because the heat is rising inside the house. It’s because superheated air is being drawn from the attic and roof deck into the upper part of the house through hidden air leaks—and through open windows and skylights.
Image: The Stack Effect Is Reversed in Hot Weather: Lighter, hot outdoor
air is drawn in at the top, heating the upstairs rooms (red arrows).
Heavier, cooler air leaks out the bottom (blue arrows).
Heat moves through space
Radiation is heat transfer from one object to another through space. Like conduction and convection, radiation depends on a temperature difference, but this time between the surfaces of objects rather than across a material. While conduction moves heat through solid materials and convection depends on air movement, radiation happens only when there is a direct line of sight between two objects of differing temperatures.
Image: A room with lots of glass is likely to be uncomfortable in hot
or cold weather, especially if it has a southwestern exposure.
Radiation plays a much smaller role than conduction or convection in the heat loss from a house in the winter. The type of window glass can influence radiative heat loss to some extent (see chapter 5). Radiation is, however, the primary factor in solar heat gain. Solar gain is a good thing in the winter (it adds some heat from the sun to your house for free), but it’s also the largest driver of air-conditioning loads in the summer. In any season, radiation has a large impact on comfort. A person’s comfort level actually depends more on the average temperatures of the surrounding surfaces than it does on the air temperature in the room (see the top drawing on the facing page).
The biggest role radiation plays in a home’s comfort is due to the surface temperatures of glass. In the winter, glass is usually the coldest surface in a house; in summer, it’s the warmest. A room with lots of glass may be uncomfortable in both hot and cold weather. Uninsulated or poorly insulated walls, ceilings, and floors also create cold surface temperatures in the winter (or hot ones in the summer). Cold surfaces add to the problem of conductive heat loss by making people feel less comfortable. When people are chilly, they turn up the thermostat, driving the heat loss even faster and costing them even more.
TIP: Glass is usually the coldest interior surface in winter and the warmest in summer. Rooms with a lot of glass may be uncomfortable in both seasons
WHAT CAN GO WRONG: Furnace air handlers should not move air in and out of a house; they are designed to move air around the house. Normally, as air is heated, it’s released into the house through supply ducts; house air is brought in through return ducts to begin the cycle again. However, leaky ductwork in attics, basements, and other areas can push air out side the house, acting like an unintentional exhaust fan. In fact, research has found that, in many homes, duct leakage accounts for the majority of air moving in and out of the house when the furnace blower is running.
Image: Mechanical Systems Contribute to Air Movement Makeup air is drawn
in from the outside through cracks and gaps (blue arrows). House air
is drawn into combustion equipment and exhaust fans (black arrows). Exhaust
appliances, combustion equipment, and leaky ductwork all push air out
of the house (red arrows).
Next: The Thermal Boundary
All Articles:
- Heat Transfer
- Air Leaks