Donaldson R F Greenhouse Heating & Ventilation Calculator
Work out roughly how much heater power keeps a greenhouse above a target temperature on a cold night, including both the glazing loss and air-change (ventilation/draught) loss.
How it is worked out
Glazing heat loss = U × glazing area × ΔT (the same building-physics formula as the Insulation Heat Loss Calculator). Ventilation/draught heat loss = 0.33 × air changes per hour × volume × ΔT, using air's volumetric heat capacity of about 0.33 Wh per m³ per °C (a standard building-physics constant). The two are added together, then a safety margin is applied, to give the heater wattage needed to hold the target temperature on the coldest night you specify.
Source: standard building-physics heat-loss and ventilation-loss equations (volumetric specific heat of air ≈ 1200 J/m³K ≈ 0.33 Wh/m³K), as used in CIBSE Guide A-style calculations.
Working out general air changes, or a wider building's insulation loss? Try the Air Changes Per Hour Calculator or the Insulation Heat Loss Calculator.