Heat Movement

Heat, energy and insulation are all related to each other. As inspectors we should understand how heat moves around inside a home, and how insulation can control that movement. One main reason to understand how heat moves is because warm air can carry moisture, and warm moist air needs to be controlled in relation to a building envelope. Uncontrolled, moving warm air and moisture can cause lots of problems. Another reason to learn about heat is that insulation provides a resistance to the flow of heat, and the more insulation there is, the less energy is needed to heat and cool the house.

Heat needs to be controlled to keep the occupants of the home comfortable. When a home is well insulated, your client will save on energy costs. After learning the information in the next few sections about heat, moisture, air, and insulation, you’ll be able to perform a great inspection, and speak to your client about how the building envelope is functioning.

Now, let’s talk heat. There are essentially three ways that heat moves from one area to another. When bodies of unequal temperatures are near each other, heat leaves one body and goes to the other. Heat moves from the hotter body, and the colder body absorbs it. The greater the difference in temperature, the greater the rate of flow of the heat.

Heat moves from one body to another by the following ways:

· Radiation;

· Conduction; and

· Convection.


Radiation is the transfer of heat energy by electromagnetic wave motion. Heat is transferred in direct rays. It travels in a straight line from the source to the body. The closer you are to the hot object, the warmer you feel. The intensity of the heat radiated from the object decreases as the distance from the object increases.

You feel cool in a room that has a cold floor, walls and ceiling. The amount of heat loss from your body in that room depends upon the relative temperature of the objects in that room. The colder the floor is (relative to the temperature of your feet), the great the heat loss from your body will be standing there. If the floor, walls and ceiling of that room are relatively warmer than your body temperature, then heat will be radiated to your body from those objects or surfaces.

When you step into a cold room, you can immediately feel the heat energy leaving your body. Use all of your senses as an inspector when moving about the house. Just entering a space with your body can tell you a lot of information about that space, the insulation, the heat, air movement, and even moisture or humidity levels. Some inspectors can give a good estimate on the temperature of an attic space simply by entering it. Keep aware of your surroundings when moving about the interior of the house.

Radiant heat emits in all directions. Radiant heating in residential buildings include piping and electrical wiring in floors, walls and ceilings. Reflective materials are commonly used in a radiant heat emitting system in order to direct or control where the heat is emitted.

Radiation happens when heat moves as energy waves, called infrared waves, directly from its source to something else. This is how the heat from the Sun gets to Earth. In fact, all hot things radiate heat to cooler things. When the heat waves hit the cooler thing, they make the molecules of the cooler object speed up. When the molecules of that object speed up, the object becomes hotter.


Conduction is the transfer of heat from one molecule to another, or through one substance to another. It is heat that moves from one body to another by direct contact. For example, heat is transferred by conduction from a boiler heat exchanger to the water passing through it. When you touch a suction lines of an air conditioner and it feels warm, that’s heat energy moving from the warm copper pipe to your cooler hand – by conduction.

Heat is a form of energy, and when that heat comes into contact with matter, it makes the atoms and molecules move. When atoms or molecules move, they collide with other atoms or molecules and make them move too. This movement transfers heat through matter.

This is demonstrated when touching a ceramic coffee cup. The exterior surface of the cup is warm to the touch because the heat of the hot coffee transferred through the cup material.


Convection is known by most people by the using the phrase “Heat rises.” Convection is the transfer of heat by warming the air next to a hot surface and then moving that warm air. It’s the transfer of heat by the motion of the heated matter itself. The air moves from one place to another, carrying heat along with it. Since warm air is lighter than the cool air around it, the warm air (or heat) rises.

Warm fluids tend to rise while the surrounding cool fluids fall. This rising and falling tends to form loops or convective loops, where warm air, for example rises and cool air falls. Early warm-air furnaces, gravity furnaces, used principles of convective loops. In a gravity system the warm air rises and cool air falls, and this is how the gravity warm-air heating system circulated air.

When a certain amount of air is heated up, it expands and takes up more space.  In other words, hot air is less dense than cold air.  Any substance that is less dense than the fluid (gas or liquid) of its surroundings will float.  Hot air floats on cold air because it is less dense, just as a piece of wood floats because it is less dense than water.  Warm air is often described as weighing less than cool air. Warm air rises, and cool air falls. The weight per unit volume of air decreases as its temperature increases. And conversely, the weight per unit volume of air increases as its temperature decreases.

Inside a wall cavity, there can be convective loops, where cool and warm air are moving about inside the wall cavity. If warm, moist air comes in contact with a cold surface of that wall assembly, then condensation may form inside the wall. And that’s not good.

For another example, an old gravity furnace heats the air; the air gets lighter and rises out of the heating system. Cool air enters the heating system and pushes or displaces the warm, rising air. The warm air rises up through warm-air ducts or pipes (often called stacks) that are inside the walls. The warm air rises up through the building. The warm air enters a room through the supply registers on the wall or floor. The cool air falls out of the room and might return through a return grille, and travel back through return ducts to the heating system.

Some houses with old gravity heating systems may not have a lot of ducts and pipes, but might rely on large openings (covered with iron grates or grilles) in the floors that allow the cool air to fall down through the building. The cool air is allowed to simply fall back to the furnace – hence the name gravity warm-air heating system.

The air circulation in a house with a gravity warm-air heating system will depend upon the temperature difference between the warm air rising and the cool air falling. The greater the difference, the greater the speed of the air circulating.


So, heat moves from one body to another by the following three ways:

· Radiation;

· Conduction; and

· Convection.

Understanding how heat moves will help you understand how moisture moves too.  You can find information about how to inspect for moisture and other subjects at http://www.bengromicko.com.