How does insulation thickness affect the U-value of a building envelope?

• A. Increasing thickness raises U-value
• B. U-value depends only on surface area
• C. Increasing thickness lowers U-value
• D. Thickness does not affect U-value

Answer: (C)

Increasing insulation thickness increases the wall’s thermal resistance, which lowers the U-value. U-value measures how much heat flows per area for a given temperature difference, so it is essentially the reciprocal of the total thermal resistance across the envelope. For a simple layer, resistance is proportional to thickness (R ≈ L/k), so U ≈ k/L. In a multi-layer wall, U = 1 / (R_s + Σ L_i/k_i + R_b). As you add thickness to insulation, the total resistance grows, cutting the U-value. The effect is strongest when the insulation layer is the dominant resistance, but even when other resistances are significant, increasing thickness still reduces U, just with diminishing returns.