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Heat Transfer

There are 3 types of heat transfer: conduction, convection and radiation.

Conduction is heat transfer through a material as thermal energy moves from molecule to molecule through a substance; or from one object to an adjoining object. If you pick up the handle of a cast iron frying pan from a hot stove you’ll experience conduction! The heat reaches your hand via conduction from the burner to the bottom of the pan through the metal handle to your hand.

Heat is conducted through the ceilings, walls and floors of homes. Effective insulation slows conduction by keeping heat out during summer and in during winter.

Convection is the transfer of heat by a liquid or gas (such as air). Circulatory air motion due to warmer air rising and cooler air falling is a common mechanism by which thermal energy is transferred. An open chimney flue provides a good example of convective heat loss during the winter. Warm air will rise up the chimney and cold air will fall down into the home. The energy used to warm the air that escapes is lost. The cold air must now be heated. The greater the temperature difference between the inside and outside of the home, and the larger the openings in the home, the easier it is for air to move and the greater losses you will have due to convection. Convective heat loss occurs through cracks and holes in the home and gaps and voids in ceilings, walls, and floors—and in the insulation. Convection also occurs if air can circulate through the insulation — if insulation is to be effective, it must prevent air from flowing easily through it. Properly applied insulation reduces convective heat loss by resisting and minimizing air movement.

Radiant heat transfer occurs between objects that are not touching. The sun heating the earth is an example of radiant heat transfer. The sun warms the earth without warming the space between the sun and the earth. An example of radiant heat transfer is found in a typical attic during the summer. The sun radiates heat to the roof, which in turn radiates heat down toward the ceiling. If the insulation covering the ceiling does not effectively resist radiant heat transfer, then the ceiling will become increasingly warm - radiate heat down into the home - and the home will be uncomfortable. Properly applied insulation arrests radiant heat transfer.

R-value

Insulation is rated by R-value. The higher the R-value the less heat is transferred through a material in a given period of time. (The R-value is the reciprocal of the U-value.) Attic insulation rated at R-40 will have a greater resistance to conductive heat transfer than attic insulation rated at R-19.

R-values are determined in laboratory conditions by placing carefully prepared test specimens between two plates and measuring heat flow by conduction through the insulation. It is widely believed that the higher the R-value, the better the insulation. This is not necessarily true unless all other factors (such as density or gaps and voids) are identical. Laboratory R-values do not take into consideration many factors (for instance, wind-wash on the outside walls, and less-than-perfect installation) that exist in real homes.

R-value is a good measure of insulating quality—as far as it goes. But remember that R-value is a laboratory measurement of a material’s resistance to conductive heat transfer only. And we don’t live in carefully controlled laboratories—and there are other methods of heat transfer than just conduction.

In other words, R-value can be a good measure for comparing different brands of the same type of insulation; but it can be a poor predictor of ability between different types of insulation. To get the insulating benefit you’ve paid for, know your choices!

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