Final answer:
To calculate the heat flow rate through a wall, add the thermal resistances (R-values) of all layers and apply the formula Q = ∆T / (R_total). Part (a) requires summing the R-values of drywall and siding plus an estimation for fiberglass. Part (b) requires adjusting for the 2-by-4 studs which act as a thermal bridge, affecting the overall R-value.
Step-by-step explanation:
The student's question pertains to the physical principles of heat transfer through a solid masonry wall of specific construction and dimensions. To determine the rate of heat flow through the wall, we must look at the thermal resistance of each layer and apply the concept of thermal resistance in series. The formula for heat flow rate (Q) through a composite wall is given by Q = ∆T / (R_total), where ∆T is the temperature difference between the inside and outside of the wall, and R_total is the sum of the individual R-values for each layer of the wall.
For part (a), the total thermal resistance (R_total) is the sum of the R-values of drywall (0.56) and insulated siding (2.6). Since there is a 3.5-inch thick layer of fiberglass, we estimate its R-value based on typical values for fiberglass {this is where specific data about the insulating properties of the fiberglass would be helpful, but since it's not provided, an estimate or assumption might be discussed}. With the given values, the rate of heat flow is calculated using the temperatures 22 °C (inside) and -2 °C (outside).
For part (b), the presence of 2-by-4 studs introduces additional complexity, as the wood has a different R-value compared to the insulated space. Because the studs take up space within the wall, they alter the effective R-value of the wall wherever they are located. The heat current calculation for this modified wall will differ and must include the consideration of the thermal bridge effect caused by the studs.