Working for Dr. Energy Saver of Hudson Valley I have learned a lot about building science. My name is David Vasseur and I'd like to talk about how heat moves. Some people think that heat rises. This is only partially correct as it is only one aspect of heat movement. See, heat energy can travel three ways.
Heat energy from the sun reaches our planet through a very cold outer space by "radiation". When you stand in the sunshine, you can feel the warmth on your skin from the sun. The heat energy travels from a heat source, through a cold space, and warms up objects on the other side. That is radiation.
Heat energy can pass through objects by "conduction". Feeling the heat from your coffee or hot chocolate right through the mug is conduction. The R-value stamped on fiberglass batts stands for the "resistance" of heat energy to pass through it. That's the opposite of conduction, the higher the R-value the less heat is conducting through.
Now, thirdly, heat energy travels with air molecules. This is called "convection". Where ever the air moves, it takes the heat energy with it. When air receives heat energy, the molecules moves faster and move away from each other; making it less dense than colder air. Cold, dense air sinks, or descends; while warm air always rises or ascends upward. This is how hot air balloons work.
So heat energy by means of radiation or conduction does not only rise, it always goes from the higher concentration to the lower concentration; or simply put, from more to less.
But this third aspect of how heat energy travels, of warm air rising, is very important when it comes to making your home more comfortable and energy efficient.