Question

Find the R value (ft2 F h/Btu) of a brick wall 3.3 inches thick

Answer 1

The R-value of a given material is given by the following formula:

`R=(L)/(k)`

Where:

`\begin{gathered} R=\text{ R-value} \\ L=\text{ thickness} \\ k=\text{ thermal conductivity} \end{gathered}`

The thermal conductivity is a property of the material, in the case of the common building brick it is:

`k=0.347\frac{Btu}{ft\text{ h F}}`

Before we determine the R-value we will convert the thickness from 3.3 inches to feet using the following conversion factor:

`1ft=12\text{in}`

Multiplying by the conversion factor:

`3.3in*(1ft)/(12in)=0.275ft`

Now, we plug in the value in the formula for the R-value:

`R=\frac{0.275ft}{0.347\frac{Btu}{ft\text{ h F}}}`

Solving the operations:

`R=0.79\frac{ft^2\text{ h F}}{Btu}`

Therefore, the R-value is 0.79