Heat loss
Fabric heat loss through an element (Q = U A ΔT)
- where
- fabric heat loss (W)
- U-value of the element (W/m²·K)
- area of the element (m²)
- internal design temperature (°C)
- external design temperature (°C)
Explain this equation
Works out the heat lost through one building element, such as a wall, roof or window, at the design outside temperature. Add up every element in a room, then add ventilation loss, to size a heat emitter.
U is how easily heat passes through one square metre of the element for each degree of temperature difference. A is its area. t_i − t_e is how much warmer inside is than outside.
Conducted heat is proportional to area and to the temperature difference, and the U-value bundles together the thermal resistance of every layer and its surfaces.
Sense check: a 20 m² modern wall at U = 0.28 with a 24 K difference loses about 134 W. A single-glazed window (U about 5) loses roughly eighteen times more per square metre.
Watch out: walls to unheated spaces or ground floors see a smaller temperature difference than outside walls, so use adjusted values from BS EN 12831-1.
Heat lost by conduction through one wall, roof, floor or window at design conditions. Add the elements up for a room, then add ventilation loss.
Enter your values to see the result.
Good to know
Use the element’s U-value from the specification or a BS EN ISO 6946 calculation. External design temperatures for UK locations are given in CIBSE Guide A; BS EN 12831-1 sets out the full method, including adjustments for ground floors and unheated spaces. Internal design temperatures are typically 18–21 °C for living spaces.
Common mistake
Using the full outside temperature difference for elements next to unheated spaces or the ground, which overstates their loss.
Limits
Steady-state one-dimensional conduction. Thermal bridges, ground-floor corrections and adjacent-space temperatures are not included; see BS EN 12831-1.
Sources
- BS EN 12831-1:2017 Energy performance of buildings — Method for calculation of the design heat load (transmission heat loss H = ΣU·A, ventilation heat loss from air volume flow)
- CIBSE Guide A: Environmental design (2015), chapters 5 and 6 (thermal design, internal heat gains from people, lighting and equipment)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the equipment manufacturer’s data, CIBSE guidance, BS EN 378, the F-gas Regulations and your company procedures. Refrigerant work needs the correct F-gas certification.