Most of thermal models used to estimate the natural ventilation of greenhouses ((m) over dot(a)) use an average value for the cover transmittance (tau(c)) to estimate the input solar energy to the greenhouse. These models failed to predict rha at low solar radiation flux due to the spatial variation of tau(c) in the greenhouse and the thermal inertia of soil. This study is to develop and validate a thermal model predict (m) over dot(a) precisely. All modes of energy were treated, under the unsteady state conditions, at the outer surface of the cover to avoid the sources of error. The required parameters were measured for a naturally ventilated glasshouse. The predicted values of (m) over dot(a) were in the range between 0.36 kg s(-1) and 1.65 kg s(-1) and showed good accordance with measured and simulated values in the literature. The results also confirmed that outside wind at a speed <2.5 m s(-1) has no significant effect on the value of (m) over dot(a) and the temperature difference of air between inside and outside the greenhouse (Delta T) is the main driving force of ventilation. A correlation between (m) over dot(a) and Delta T was provided, it can be used to estimate (m) over dot(a) that required to maintain the air temperature in the greenhouse at a desired level. (C) 2015 Elsevier B.V. All rights reserved.