Energy Concepts - Heat Transfer
About 70% of you annual utility bill is due to heat transfer in some manner. (Refer to the table on the Energy Use page. Space heating, space cooling, water heating, refrigeration, cooking were used for this number.) Knowing about this can help understand how your home uses energy.
Heat always flows hot to cold; there are three methods by which this is accomplished.
Conductive Heat Transfer |
Convective Heat Transfer |
Radiation Heat Transfer |
Conduction Heat Transfer
Conduction heat transfer is the transfer of heat through solid objects or within a solid object. If two objects are touching and at different temperatures, then heat transfers by conduction. Even if one side of a material is at a different temperature conduction will occur. Conduction occurs from molecule to molecule as long as there is a difference in temperature.
Heat will move by conduction out of your home in the winter through the walls, windows, doors, and ceiling. In the summer heat conducts from outside to inside through the same materials. This process cannot be totally stopped; but, it can be slowed dramatically. If the molecules in the material were far apart, then heat would have difficulty in conducting through the material. Gasses have molecules that are far apart, so gasses can be used to slow heat conduction. This is how insulation works. But care should be taken so the next method of heat transfer does not increase.
Convection Heat Transfer
Convection heat transfer is the transfer of heat by a gas or liquid moving past another material at a different temperature. The speed of the gas or liquid is a factor in the magnitude or amount of convective heat transfer. For example as air moves faster, more heat will be convectively transferred. If you are out in the cold winter air, you feel colder as the wind speed increases even if the air temperature does not change. This "wind chill" is convection heat transfer.
When air blows into your home through a crack, convective heat will be transferred. Infiltration is a convective process.
Convection is not all bad. Your furnace or air conditioner is located in one spot, but they can heat or cool your entire house by moving conditioned air throughout your home. This is convection.
Radiation Heat Transfer
Radiation heat transfer is the most bizarre method of heat transfer, but it is one you are very 
familiar with. Any two bodies at different temperatures can transfer heat if there is a unobstructed path between them. If the two bodies can "see" each other, then radiation heat transfer will occur. The distance between then can be quite large. The sun at 93 million miles can still radiate heat to the earth.
Summer sun can add a large amount of heat to your home through radiation heat transfer. This is called solar gain.
Your home will experience all three methods of heat transfer. Your energy saving actions can try to affect one or more of these methods. Sealing cracks reduces convective heat transfer, adding insulation to your attic reduces conductive heat transfer, and adding shades over your windows reduces radiation heat transfer.
