The thesis is focused on comparison of heat flow through windows at different boundary conditions. Research on heat flow is used experimental measuring system with samples of two insulating glass and triple glass. The first chapter is a summary of theoretical knowledge on the state solved the issue of heat transfer. The second chapter of the work deals with the thermal properties of hole construction of heat transfer, thermal conductivity of glass and gas filling between insulating glass. The third chapter deals with the thermal balance of insulating glass and the values that affect it. The fourth chapter is part of the experimental measuring system in which the insulating glass measured and surveyed them heat loss and heat flows. The final chapter consists of a practical part, which summarizes the measurement procedure, the sampling insulating glass, results, evaluation and comparison of heat flows measured constructions.