The current research presents an architectural design approach to determine an optimal proportion of the glazing areas in regard to energy-efficiency of prefabricated timber-frame buildings, with a special focus on the south-oriented glazing surfaces. A parametric analysis is performed on the variation of the glazing-to-wall area ratio (AGAW) from 0% to 80% for six different exterior wall elements with different thermal properties. Modifications are performed for the main cardinal directions, while a detailed analysis is carried out only for the south facade. The impact of the presented variable parameters on the energy demand for heating and cooling is analysed with the use of the PHPP software. A basic theoretical contribution of the present research is transformation of a complex energy related problem to only one single independent variable - that of thermal transmittance of the wall elements (11, U-wall-value), with a view to determining the optimal glazing area size (AGAW(opt)) for all contemporary prefabricated timber construction systems. The main aim of the current study is to offer architects a simple and useful short-cut to energy-efficient design of prefabricated timber-frame buildings. The use of mathematical linear interpolation is therefore presented as a simple method for predicting an approximate energy demand with respect to AGAW and U-wall-values. (C) 2011 Elsevier B.V. All rights reserved.