to isolate the window assembly 74 from the interior 76 the modules may be readily inserted within the window opening and extended to proper height and if their natural firm interconnection will not hold them then it is merely necessary to apply a drop of solvent or other household cement to provide the proper modular length and tight fit between the sill and upper frame 68 the double dead air space provides still further heat control effects thus in cold weather the plurality of insulative modules 10 may be disposed as shown in fig4 with the respective lower cut out 34 and upper cut out 32 directed into the building interior 76 thus while the modular insulator provides a double dead air space within exterior window assembly 74 it provides a heat trap and energy return as interior air entering cut out 34 can be warmed by convection for travel up through the module 10 and release through cut out 32 back into the interior room thus there is not only provision of a double dead air space but a convection current of warmed air input to interior 76 in warm weather insulation is enhanced by simply reversing each of the insulative modules 10 and the sizing module 40 so that the respective upper and lower cut out portions 32 and 34 are directed outward from interior 76.