A recurring problem in retrofitting historic windows is that most traditional means of improving their energy efficiency, e.g. the use of IG units, is often unavailable due to structural and esthetic concerns. Through a refurbishment project of traditional Central-European box windows we aim to investigate the possibility of using less intrusive methods to improve the windows' heat balance with comparable results. Many different approaches to calculating the energy balance of windows in historic buildings are reviewed and various modelling assumptions and algorithms analyzed to highlight the depth of modelling needed for fenestration heat balance calculation in a historic context. We introduce our new research program for the task: EPICAC-BE, a MATLAB based dynamic energy simulation software. After a validation with the BESTEST suite it is used to compare different retrofit options for a prototype window with and without controlled shading devices. Results indicate the use of double glazing units is not always required or the most favorable. Utilizing the dynamic window concept by combining higher U value with higher solar transmittance with automatic interpane shading can achieve better overall results. Simulations should have a key role as they can lead to designs better satisfying both energy saving and conservational requirements. (C) 2016 Elsevier B.V. All rights reserved.