Re: [Phys-l] Insulation R-value

["ALVIN BACHMAN" <bachman_28@msn.com>]

R is related to the reciprocal of k.

  dQ/dt= k A (T1-T0) / L  = A (T1-T0) / R

For a set of slabs (same R) , of conductivity Ki and thickness Xi, R = SUM( 
1/KiXi )

Units of k are  W/(mK)  and common units of R are BTU per sq-ft per hour per 
F-deg,
which is why everyone drops the units and says  R-19 or R-30, etc. )

Al Bachman

> The book I am looking into first defines the heat conductivity , k. It
> is used to calculate H  ( energy lost per square meter and per second).
> By definition,
>
> H=k*dt/d
>
> where d is the wall thickness and dt is the temperature difference on
> its two sides. Thus the SI unit of k is J*m/C. The R of a material is
> the same thing as k but in British units (BTU*ft/F). The conversion
> factor is 6.93. I have never measured k but the definition suggests
> what could be done.