Heat Loss Rate Calculator
Calculate the rate of heat loss per square meter based on temperature difference and U-values.
How to Calculate the Rate of Heat Loss Per Square Meter
Understanding heat loss is critical for building design, HVAC sizing, and energy efficiency audits. The rate of heat loss per square meter ($q$) measures how much thermal energy (in Watts) escapes through one square meter of a building element (like a wall, window, or roof) for every degree of temperature difference between the inside and outside.
The Heat Loss Formula
The calculation is based on the fundamental law of thermal conduction, often expressed using the U-value (Thermal Transmittance):
q = U × (Tinside – Toutside)
- q: Heat loss rate (Watts per square meter, W/m²)
- U: U-value of the material (W/m²·K)
- Tinside: Desired indoor temperature (°C)
- Toutside: Ambient outdoor temperature (°C)
What is a U-Value?
The U-value measures how effective a material is as an insulator. It is the reciprocal of the R-value (Thermal Resistance). A lower U-value means better insulation and less heat loss. For example, a modern triple-glazed window might have a U-value of 0.8, whereas an old single-pane window might be as high as 5.0.
Typical U-Values for Common Building Elements
| Building Element | Typical U-Value (W/m²·K) |
|---|---|
| Solid Brick Wall (Uninsulated) | 2.0 – 2.5 |
| Insulated Cavity Wall | 0.15 – 0.30 |
| Double Glazed Window | 1.2 – 2.8 |
| Loft Insulation (300mm) | 0.11 – 0.15 |
| Solid Wood Door | 2.5 – 3.0 |
Practical Example
Imagine you have a living room wall with a U-value of 0.30 W/m²·K. It is a cold winter day, so the outside temperature is -2°C, and you want to keep the inside at a comfortable 20°C.
Step 1: Find the temperature difference (ΔT). 20 – (-2) = 22°C.
Step 2: Multiply by the U-value. 0.30 × 22 = 6.6 W/m².
This means every square meter of that wall is losing 6.6 Watts of energy. If the wall is 10 square meters, the total heat loss through that wall is 66 Watts.