A novel and useful frameless supplemental window for fenestration incorporating infiltration blockers suitable for use with existing windows. The supplemental window, in one embodiment, comprises plastic sheet material with bullnose edging around it. Corner braces add rigidity and strength to corners in several embodiments. An attachment mechanism secured either to the sheet material or the bullnose edge functions to fasten and/or seal the supplemental window to an existing window. Infiltration blockers fastened to the sheet or bullnose prevent or minimize air leakage around various window elements. The bullnose edging and infiltration blockers function to substantially enclose (i.e. trap) a volume of air between the window pane and the plastic sheet material. The supplemental window is configured such that the layer of air enclosed is of an optimum thickness within a preferred range of 0.15 to 0.75 inches to maximize thermal insulation properties of the supplemental window.