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R value | ='''R value'''= | ||
==Definitions== | |||
The "resistance" of a substance to [[heat transfer]] by [[conduction]] | |||
The following information is obtained from Energy, Its Use and the Environment: | The following information is obtained from Energy, Its Use and the Environment: | ||
"A measure of the [[resistance]] of the material to heat flow is the | |||
[[thermal resistance]] R, often called its | "A measure of the [[resistance]] of the material to heat flow is the [[thermal resistance]] R, often called its "R value" given by R=delta/k. The higher the R-value is, the better are the insulating properties of the material. Using this notation, the rate of heat flow is given by | ||
properties of the material. Using this notation, the rate of heat flow is | |||
given by | |||
Qc/t = 1/R x A x change in T | Qc/t = 1/R x A x change in T | ||
L/k = R | R is a function of both the type of material and its thickness. | ||
L/k = R value of insulation | |||
R-value according to Physics by Cutnell and Johnson: | |||
R | * The R value expresses in a single number the combined effects of thermal conductivity and thickness. | ||
* Larger R values reduce the heat per unit time flowing through the material and therefore mean better insulation. It also convenient to use R values to describe layered slabs formed by sandwiching toget a number of materials with different thermal conductivities and different thicknesses. The R values for the individual layers can be added to give a single R value for the entire slabe. It should be noted, however, that R values are expressed using units of feet, hours, F and BTU for thickness, time, temperature, and heat, respectively. | |||
[[Category:PH261]] | |||
[[Category:Heating and cooling]] | |||
Revision as of 22:10, 17 October 2007
R value
Definitions
The "resistance" of a substance to heat transfer by conduction
The following information is obtained from Energy, Its Use and the Environment:
"A measure of the resistance of the material to heat flow is the thermal resistance R, often called its "R value" given by R=delta/k. The higher the R-value is, the better are the insulating properties of the material. Using this notation, the rate of heat flow is given by
Qc/t = 1/R x A x change in T
R is a function of both the type of material and its thickness.
L/k = R value of insulation
R-value according to Physics by Cutnell and Johnson:
- The R value expresses in a single number the combined effects of thermal conductivity and thickness.
- Larger R values reduce the heat per unit time flowing through the material and therefore mean better insulation. It also convenient to use R values to describe layered slabs formed by sandwiching toget a number of materials with different thermal conductivities and different thicknesses. The R values for the individual layers can be added to give a single R value for the entire slabe. It should be noted, however, that R values are expressed using units of feet, hours, F and BTU for thickness, time, temperature, and heat, respectively.