Question

(PLEASE HELP ME IF YOU CAN)

A wall is made up of four elements, as follows: ½” wood siding R = 0.81 ½” plywood sheathing R = 0.62 3 ½” fiberglass R = 10.9 ½” sheetrock R = 0.45 (Units of R-value are ft2-hr-oF/Btu)
A. How many Btu per hour would be would be transferred through a 1 sq. foot section of the wall if the outside is 50oF colder than the inside.
B. In which direction (in or out) would the heat flow? (Units of R-value are ft2-hr-ºF/Btu)
C. What is the heat transfer through the wall (in Btu/hr) if the insulation was not installed?

Answer 1

For heat transfer by conduction, it is calculated by dividing the temperature with all of the resistances present in the system. We do as follows:

q = ΔT / ∑R

We have 3 resistances involve.

Wood = 0.81 ft2-hr-oF/Btu
Plywood sheathing = 0.62 ft2-hr-oF/Btu
Sheetrock = 0.45 ft2-hr-oF/Btu
∑R = 0.81 + 0.62 + 0.45 = 1.88 ft2-hr-oF/Btu

q = 50 / 1.88 = 26.60 BTU / hr

The heat flow would from the hotter side to the colder side.