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'''R value''' | '''R value''' | ||
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* The R value expresses in a single number the combined effects of thermal conductivity and thickness. | * 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 slab. 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. | * 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 slab. 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. | ||
== Interwiki links == | |||
* [[Wikipedia:R value]] | |||
[[Category:PH261]] | [[Category:PH261]] | ||
[[Category:Heating and cooling]] | [[Category:Heating and cooling]] | ||
Revision as of 06:24, 11 April 2010
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 slab. 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.