What is the total r value for a wall made of 4 inches of wood, 2 inches of fiberglass, and 3 inches of foam? assume that the r values for a 1-inch thickness of these materials is given by: r (1" wood) = 2, r (1" fiberglass) = 4, and r (1" foam) = 5?

The total R value is computed from the R-values of the components in the stack as ...

(inches of wood) * (R-value of wood) + (inches of fiberglass) * (R-value of fiberglass) + (inches of foam) * (R-value of foam)

= (4 in) (2 / in) + (2 in) (4 / in) + (3 in) (5 / in)

= 8 + 8 + 15

= 31

The total R-value of the wall is 31.