1. The problem statement, all variables and given/known data How thick a concrete wall would be needed to give the same insulating value as 3.5 inches of fiberglass? 2. Relevant equations Q = -A·k·ΔT/Δx k = -Q·Δx/A·ΔT 3. The attempt at a solution So here, k is equal to the insulating value, and since I want the insulating value to be the same for the concrete wall and fiberglass, I would have the equation -Q·3.5/A·ΔT = -Q·Δx/A·ΔT Since it seems that area and change in temp are the same in both cases, I got Q·3.5 = Q·Δx Which is where I got stuck. Would the Q (heat flow) cancel?