Windows were no exception to the all-encompassing development trends; in their case, progress has also taken the path of minimising heat losses. We studied heat losses in various window systems with different frame constructions, fitted with window glass and spacing board. It was found out that wooden, aluminium and PVC windows with the new PVC spacer all featured reduced heat losses, compared to the windows with usual aluminium spacers. Another finding came from the comparison of window sizes. The smaller the window, the larger the proportion of glass with the spacer, which results in greater heat losses, and vice versa; the larger the window, the smaller its spacer contribution.