Donaldson R F Insulation Heat Loss Calculator
Work out the rate of heat loss through a wall, roof, floor or window, from its U-value, area and the temperature difference across it.
How it is worked out
Heat loss Q = U × A × ΔT, where U is the element's U-value (how easily heat passes through it — lower is better insulated), A is its area, and ΔT is the temperature difference between inside and outside. This gives the instantaneous rate of heat loss in watts through that one element; a whole building's heat loss is the sum of this calculation for every wall, roof, floor, window and door, plus ventilation losses.
Source: standard building-physics heat-loss equation (Fourier's law applied to a building element), as used throughout the industry, e.g. CIBSE Guide A.
Working out a greenhouse heater, or ventilation heat loss? Try the Greenhouse Heating & Ventilation Calculator or the Air Changes Per Hour Calculator.