Which factor most directly affects heat transfer through a multi-layer wall?

• A. The exterior color.
• B. The interior layout.
• C. The overall U-factor.
• D. The thickness of drywall.

Answer: (C)

Heat transfer through a multi-layer wall is governed by the total resistance to heat flow of all the layers combined. The best descriptor for that is the overall U-factor, which measures how much heat crosses the wall per unit area for every degree of temperature difference. This single value captures the effects of each layer’s material properties and thickness, as well as surface films and any thermal bridging, so it directly reflects how well or poorly the wall resists heat transfer. In other words, the lower the U-factor, the less heat passes through.

Exterior color can influence solar heat gain by absorbing or reflecting sunlight, but that effect is about radiant heat from the sun rather than the wall’s steady conductive transfer. Interior layout doesn’t change the wall’s conductive path. The thickness of drywall affects resistance, but it’s just one component; the U-factor encapsulates the entire assembly’s behavior.