In PVC windows, buyers most often divide the plane of the window sashes in a simulated manner with the use of muntins between the panes. The advantage of this solution is, above all, the ease of cleaning and maintenance of the window. The window sash is equipped with only one composite pane, the interior of which is divided into any number of quarters by means of aluminum muntins. Their colors are practically free, although white, brown, gold bars or in colors corresponding to the wood-like veneer of window profiles are most often used. The basic widths of the glazing bars are 8mm, 16mm, 26mm or 45mm, although the given dimensions may vary slightly depending on the manufacturer. However, before we use the simulated division of the sash plane with the use of muntins inside the glass panes, it is worth remembering that this type of muntins also affects the deterioration of the heat transfer coefficient of the insulating glass unit. Below are the graphics showing the profile of the glued-on mullion and the visualization of the glass division effect obtained through the use of glued-on and inter-pane muntins.