Answer:
Add the thermal R values of each wall layer.
Step-by-step explanation:
Fiberglass insulation R value, sheetrock, sheathing, siding etc. All sum together to one composite opposition to heat transfer/loss.
Q= U x A x Delta T
Q= heat transfer btuh
U= 1/R inverse of resistance
A= area of surface
Delta T is temerature difference across the wall.