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Insulation R-Value & Heat Loss Calculator
See exactly how many watts a wall, roof or floor is losing, and how much a better R-value would save.
How heat loss is calculated
Steady-state conduction heat loss through a building surface follows Q = A × ΔT ÷ R, where A is the surface area in square metres, ΔT is the indoor-to-outdoor temperature difference in degrees Celsius, and R is the surface's thermal resistance in m²·K/W. A lower R-value means more heat escapes for the same area and temperature difference.
Why the comparison matters
Doubling the R-value of a surface roughly halves the rate of heat loss through it, because R sits in the denominator of the equation. Entering a second, upgraded R-value alongside your current one shows the real-world watt and kilowatt-hour reduction an insulation upgrade would deliver, which is far more useful than the R-value number alone.
Frequently asked questions
What is R-value, exactly?
R-value (thermal resistance, measured in m²·K/W) tells you how well a material resists conductive heat flow. Higher R-value means better insulation and slower heat loss. It's the inverse of U-value (W/m²K), which measures how much heat passes through.
Why does the temperature difference matter so much?
Heat loss is directly proportional to the indoor-outdoor temperature difference — double the temperature gap and you double the heat loss through the same surface, which is why the same wall loses far more heat on a cold winter night than on a mild autumn day.
Does this include air leakage or thermal bridging?
No — this is pure conductive heat loss through the surface's insulated area only. Draughts, gaps, thermal bridges at studs or joists, and uninsulated edges all add extra heat loss on top of this figure in a real building.
How do I turn the watt figure into an energy bill saving?
Multiply the wattage difference by the number of hours per year your heating system is actively working against that temperature difference, then convert to kilowatt-hours (divide by 1000) — that's what the heating-hours field and the kWh/year rows below are doing for you automatically.