Doubling the wall thickness and insulation has what effect on conductive heat transfer rates?

Doubling the wall thickness and insulation has what effect on conductive heat transfer rates?

• A. Reduces by one half
• B. Increases
• C. No change
• D. Doubles

Answer: (A)

Heat conduction through a flat wall depends on thickness: the thicker the wall, the greater the thermal resistance, and the smaller the heat flow for a given temperature difference. The relationship can be written as Q = k A ΔT / L, where k is the material’s thermal conductivity, A is area, ΔT is the temperature difference, and L is the wall thickness. If you double the thickness while keeping k, A, and ΔT the same, the numerator stays the same but the denominator doubles, so the heat transfer rate drops by a factor of two. Doubling insulation contributes to the same idea by increasing thermal resistance (or reducing k), further lowering Q, but the key result here is that the rate decreases by about half due to the thickness increase.

Heat conduction through a flat wall depends on thickness: the thicker the wall, the greater the thermal resistance, and the smaller the heat flow for a given temperature difference. The relationship can be written as Q = k A ΔT / L, where k is the material’s thermal conductivity, A is area, ΔT is the temperature difference, and L is the wall thickness. If you double the thickness while keeping k, A, and ΔT the same, the numerator stays the same but the denominator doubles, so the heat transfer rate drops by a factor of two. Doubling insulation contributes to the same idea by increasing thermal resistance (or reducing k), further lowering Q, but the key result here is that the rate decreases by about half due to the thickness increase.