Interior window inserts are easy and affordable to make. They greatly reduce heat loss between the inside and outside. After installing them, your rooms will feel much more comfortable. Your utility bills will drop too. Learning how to build window inserts is a manageable project for anyone comfortable using a saw and power drill. The project uses the clear film insulation kits sold at hardware stores. Instead of attaching the film to your existing window, you build a frame to support the clear film. You insert this frame inside your window to add an air gap that slows down window heat loss.

How to Build Window Inserts Step by Step

1. Measure the height and width of each window from the inside edge of each existing window frame.
2. Multiply the height and width to find the approximate square footage of clear film needed.
3. Buy enough clear film insulation kits to provide double the square footage. Doubling the square footage allows you to build two-pane window inserts.
4. Buy enough 1 x 2 lumber to go around the inside edges of the window. For example, a window that measures 3 feet by 3 feet will need 12 feet of lumber.
5. Buy enough weather-stripping to go around the edges of the insert.
6. Make note of the size of the weather-stripping. It will be 1/4 inch or 1/2 inch.
7. Prepare to make your cuts on the lumber.
8. Reduce the height dimensions taken from your windows by 1/2 inch or 1 inch depending on the weather-stripping size. For example, a window height of 3 feet with 1/4-inch weather-stripping needs lumber cut at 2 feet 11-1/2 inches. This leaves space for weather-stripping on both ends.
9. Reduce the width dimensions for lumber to leave space for the vertical frame pieces and weather-stripping.
10. Mark your boards with a pencil.
11. Place a board in the miter box and cut it with a saw.
12. Assemble the frame by drilling two screw holes at each corner. Go through the vertical pieces into the ends of the horizontal pieces.
13. Apply wood glue to the connecting ends.
14. Drive wood screws through the holes.
15. Allow the glue to dry for 24 hours.
16. The next step for how to build window inserts is applying the plastic film.
17. Take the double-sided tape from the window insulation kit.
18. Apply the tape to the frame’s edges on both sides.
19. Set the frame on the sheet of clear film.
20. Wrap the film over the frame to create a sheet of film across both sides of the frame.
21. Trim the film with scissors near the exterior edge of the tape.
22. Secure all outer edges of the plastic wrap with clear shipping tape.
23. Hold the hairdryer a few inches from the film.
24. Apply heat in sweeping motions and watch the film contract.
25. Stop when wrinkles disappear. You don’t want to keep shrinking it and make it tear or melt.
26. Expose the self-adhesive edges of the weatherstripping and stick it around the outer frame edges.
27. Position pull tabs so that you can grab them from the inside of the room.
28. Glue a pull tab to two sides of the frame about halfway up.
29. Insert the completed frame into the window.