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How to Make a Window Smaller

Updated February 21, 2017

Perhaps it is time to upgrade to energy-efficient windows instead of the old single-pane glass, and you picture a smaller, cosier window instead of floor to ceiling expanses. You can install a new, smaller window with little time and patience. Removing the old window or installing the new will vary slightly depending on the window installation style, but the frame is standard regardless of your window type.

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  1. Remove interior window trim from the wall, gently prying up with a crowbar or other tool to avoid damaging the wall. Set trim aside for installation around your new window, if desired; it will need to be trimmed to the new, smaller size before reinstalling, however.

  2. Take out the window sashes, or the frames surrounding the glass portion, working from inside your home. Work carefully, eliminating any stops that hold the sashes in first, to avoid breaking the glass. Tilting the sash slightly may aid in its removal. Some windows may not have sashes but consist of a frame that is screwed into place from the outside. These windows must have the outside siding removed before taking the window out.

  3. Pry the casing trim from the jamb frame -- the interior walls of the window opening -- if you are removing a traditional window with sashes. Pull up the jambs using a hacksaw or reciprocating saw to cut through the nails to aid in removal, if necessary.

  4. Pull out the weights present inside the jambs in older windows; these are not part of modern windows or windows without sashes.

  5. Remove exterior trim, along with enough exterior siding or finish to reveal the flange in a sash-less window; if the window has a sash, it will already be removed. Unscrew or pry the flange away from the outside wall if present, then take the window out if necessary.

  6. Cut 2-by-4 boards to fit inside your current window opening, according to the current opening and the opening you require. Consult the manufacturer's specifications for your new window to obtain the dimensions of the opening needed, and increase the opening by 1/2 to 3/4 inch to allow for insulation between the frames.

  7. Assemble the cut boards into a box shape, overlapping the end of one board with the end of the next. Nail together through the ends.

  8. Set the new, smaller frame into place in your window opening, adjusting as desired. Use a level to check positioning, then place blocks or shims as a spacer as needed. Use nails or screws to attach spacers to the old frame, then repeat to secure the new frame to either the old frame or the spacers.

  9. Add your choice of insulation in the gap between the old and new frames. Spray expanding foam insulation into a smaller gap; it will expand further than it is installed. Push fibreglass insulation into larger gaps, or use a combination approach. Cut off excess spray foam after it hardens to provide a flush outer surface.

  10. Nail felt or other waterproofing material around the new opening from the new frame back to the old to prevent water seepage inside your wall. Run a bead of caulk around the new opening, according to manufacturer's installation specifications.

  11. Set your new window in place inside the frame. Check for level again, and adjust as needed with wood shims placed under the window frame.

  12. Secure the window in place, nailing the window's flange to the rough frame or going through the built-in side jambs into the new frame, according to the style window you are installing. Check for level constantly during installation and adjust as needed.

  13. Add drywall or other wall finishing as needed around the window inside to extend the wall to the new frame. Trim as desired using old or new trim pieces.

  14. Finish the exterior, adding siding or other finish as required. Complete with caulking as appropriate and top with exterior trim.

  15. Tip

    Add wood shims as needed to bring the surface of the new frame flush with the exterior or interior walls.


    Consult local building code for any restrictions or requirements.

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Things You'll Need

  • Crowbar
  • Hacksaw or reciprocating saw (optional)
  • Screwdriver
  • Hammer
  • Circular saw
  • 2-by-4 lumber
  • Tape measure
  • 10d nails
  • Screws
  • Level
  • Wood blocks
  • Wood shims
  • Expanding foam insulation or fibreglass insulation
  • Felt or other waterproofing material
  • Caulk
  • Interior wall covering
  • Exterior wall covering
  • Window trim

About the Author

Karie Fay earned a Bachelor of Science in psychology with a minor in law from the University of Arkansas at Monticello. After growing up in construction and with more than 30 years in the field, she believes a girl can swing a hammer with the best of them. She enjoys "green" or innovative solutions and unusual construction.

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