Heat Loss & Insulation
How much heat leaks through a wall, roof or window, and how much does extra insulation save? Build the wall layer by layer from inside to outside and see the U-value, the heat loss, the yearly cost and the temperature inside every layer.
How Does Insulation Work?
Heat flows from warm to cold. Each layer of a wall resists that flow with a thermal resistance R = d / k: thicker layers and materials with a low conductivity k resist more. Because the heat has to pass through every layer in turn, the resistances simply add up, like resistors in series.
The U-value is one over the total resistance: the heat that passes through one square metre for every degree of temperature difference. Good insulation traps still air in tiny pockets — which is why mineral wool, foam and even a down jacket work.
Temperature Through the Wall
profileThermal Conductivity
| Material | k (W/m·K) | Thickness for R = 1 |
|---|
Worked Techniques
Add the resistance of every layer and the two surface films. One good insulation layer usually dominates the total.
The same heat flux passes through every layer, so the temperature drops most across the layer with the largest resistance — the insulation.
The first 5 cm of insulation saves the most. Each further centimetre still helps, but by less and less — doubling the insulation roughly halves the remaining loss.
Sources & Verification
- T. L. Bergman, A. S. Lavine, F. P. Incropera & D. P. DeWitt, Fundamentals of Heat and Mass Transfer, 7th ed., Wiley, 2011 — Ch. 3 (One-Dimensional, Steady-State Conduction).
- Y. A. Çengel & A. J. Ghajar, Heat and Mass Transfer: Fundamentals and Applications, 5th ed., McGraw-Hill, 2015 — Ch. 3 (Steady Heat Conduction).