Use storm windows

Storm windows approximately double the insulation value of a window and reduce air leakage. Better-quality storms have independently tested air-leakage rates of as low as 0.1 cfm/ft. Although storms are not certified by the National FenestrationTechnically, any transparent or translucent material plus any sash, frame, mullion, or divider attached to it, including windows, skylights, glass doors, and curtain walls. Rating Council (NFRC), research has shown that they significantly improve single-pane window performance and, when installed on the exterior, shield the existing sashes from water and sunlight degradation. Storm windows, particularly airtight ones, can also substantially reduce outside noise.