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R value
'''R value'''
  - Definition - the "resistance" of a substance to heat transfer by conduction (according to Energy, Its Use and the Environment)


The following information is obtained from Energy, Its Use and the Environment:
==Definitions==
    "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 over k.  The higher the R-value is, the bettwer 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
The materials ability to resist the [[heat transfer]] by [[conduction]] is known as thermal resistance or R-Value. Materials with high R-values have greater insulating capabilities than  materials with low R-value numbers. Most construction materials have easy to find R-values, such as insulation you may find at a local hardware store may have R-values ranging from R-19 to R-32. As it would be expected the R-32 insulation has better insulation capabilities than the R-19, but the R-32 is usually more expensive. 
R is a funciton of both the type of material and its thickness.
 
R-values can be expressed using basic thermodynamic properties of heat transfer, [[conductivity]] and amount of material in question:
 
Sine R-values are "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*A*change in T
where;
*R is a function of both the type of material and its thickness.
*T=Time
*Qc=Heat transfer
*AL/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 together a number of materials with different thermal ratings 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.
 
According to the [http://www.engext.ksu.edu/ees/henergy/envelope/basement.html Kansas State University Extension Service] the R-value of soil is about 1 per foot.
 
==See also==
* [[Thermal mass]]
* [[Earth sheltering]]
 
== External links ==
* [[Wikipedia:R value]]
 
[[Category:PH261]]
[[Category:Heating and cooling]]
[[Category:Thermal insulation]]

Revision as of 17:26, 2 May 2015

R value

Definitions

The materials ability to resist the heat transfer by conduction is known as thermal resistance or R-Value. Materials with high R-values have greater insulating capabilities than materials with low R-value numbers. Most construction materials have easy to find R-values, such as insulation you may find at a local hardware store may have R-values ranging from R-19 to R-32. As it would be expected the R-32 insulation has better insulation capabilities than the R-19, but the R-32 is usually more expensive.

R-values can be expressed using basic thermodynamic properties of heat transfer, conductivity and amount of material in question:

Sine R-values are "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*A*change in T

where;

  • R is a function of both the type of material and its thickness.
  • T=Time
  • Qc=Heat transfer
  • AL/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 together a number of materials with different thermal ratings 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.

According to the Kansas State University Extension Service the R-value of soil is about 1 per foot.

See also

External links

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