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Explanation of energy terms
- BTUs – BTU or British Thermal Unit is a term used to measure the quantity of heat. One BTU is enough heat to raise one pound of water one degree Fahrenheit. Heat is the only commodity, there is not a substance called cold. Cold is just the absence of heat. When heat transfer is measured over time, it is called BTUH or BTU’s per hour.
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- R-Value – R-value is a number that measures the resistance to heat transfer through a material. The higher the R-value the more effective the resistance. R-Value is a direct correlation to the amount of heat transferred. If the R-value is doubled, the heat that is transmitted is cut in half.
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- U-Value – U-Value is the inverse of R-Value. U-Value = 1/R-Value and R-Value = 1/U-Value.
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- Conduction - Conduction refers to the transmission of heat from one body of material to the adjoining one by direct contact. Much like water that seeks its own level, heat will also seek to balance out. If one material, the outside air, is hotter than an adjoining material, the exterior wall, the heat will migrate, transfer or “conduct” from the higher heat material to the lower heat material. This will continue until they are both at the same heat content or temperature. R-Value directly affects the rate that heat is conducted through a material.
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- Infiltration - Air infiltration is when the air “carries” the heat or cold with it into a new area. When the door is open, the outside hot air travels directly into the inside cooler air and warms the inside temperature.
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- Radiation or solar energy - Radiated energy is different than conducted energy as the heat energy is not conducted by direct contact but by line of site, much like light. The sun emits radiated energy that does not directly warm the air that it travels through but warms the surface that it strikes. That surface conducts heat to the air that it is in contact with and warms the air. The air temperature in the direct sunlight and in the shade are the same, but when we stand in the direct sunlight we feel the affects of the increased heat from the radiated energy. All items can radiate energy, walls, furniture and even people. The amount of energy that is absorbed from radiated heat depends upon the surface conditions. Light colored or white surfaces reflect more radiated energy than dark surfaces.
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- Thermal Mass – When a building material has the characteristics that it can absorb and hold a lot of BTUs or heat energy, this material is referred to as solar mass. Materials of high density are usually thermal mass, such as concrete or marble. Solar Mass has very little R-value. The effect of solar mass is that that material can absorb a lot of heat before the temperature will rise. Solar Mass acts like a “storage bin” for heat. It will have the effect to stabilize the room temperature and will delay heat transfer. A brick or block home will delay the heat about 5 hours but will not reduce the heat being transferred like R-Value. If the solar mass is thick enough, the heat will not get all the way through the material before the outside temperature drops and the heat flow is reversed.
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