Project Guide

How To Install a Window

Remove the Existing Window and Test-Fit the New One
Illustration of two men holding window in place using levels and wood blocks to test-fit new window before installing.
  • If replacing a damaged window, be careful to avoid jagged glass. Use painter's tape to cover any cracks to prevent shards from falling during the removal process.
  • Center the new window in the opening, supporting it with wood blocks and shims placed below the horizontal jambs. 
  • Make sure the window is plumb and level and adjust the shims if necessary.

Safety: When handling glass, always wear hand and eye protection to avoid injury caused by broken glass.

Trace Around the Moulding
Illustration of person tracing around the molding of a window with a pencil.
  • Clamp or have a friend hold the moulding in place. 
  • If you have vinyl or aluminum siding, you may need to install a J-channel to hold the trim.
Cut the Siding Along the Outline
Illustration of person cutting the siding of a home exterior for new window installation.
  • Use a circular saw adjusted so that the blade depth equals the thickness of the siding. 
  • Start the cut with the toe of the saw plate on the siding, but with the blade and heel of the plate above the surface. 
  • Slide the saw guard back, start the saw, and ease the blade into the wood. 
  • To avoid splintering, stop before the corner. 
  • Complete the corner cuts with a sharp chisel.
Cut the Drip Edge
Illustration of person cutting the length of a drip edge to fit over top of window.

Cut a length of drip edge to fit over the top of the window, then slide it between the siding and the building paper.

Caulk Around the Moulding
Illustration of person caulking the molding of window installation area.
  • If it has brick mold, apply a continuous bead of caulk around the opening. 
  • If it has a nailing fin, apply the caulk to the back of the fin.
Set the Window Into the Opening
Illustration of person setting window inside window cutout area.

Set the window into the opening and push the brick mold or nailing fin tight against the sheathing.

Level the Window
Illustration of person leveling a window with a large level.

Check to see how much adjustment will be necessary to level and plumb the window unit.

Tack Window in Place
Illustration of person shimming a window with a side jamb before tacking in place.
  • On a level window with brick mold, tack the window in place by pre-drilling and then driving 16d casing nails partway into the top corners of the moulding.
  • If the window is uneven, correct it by shimming below the side jambs and then tacking in place.
Use Shims to Fill the Opening Between the Jambs and Framing
Illustration of person's hand placing centering a window sham with a shim.
  • Despite the nails you just put in, the window will still move somewhat. 
  • Use the shims to center the window in the opening.
Ensure Correct Window Size
Illustration of person measuring the diagonals of an exterior window.
  • Measure the diagonals of the window. 
  • If they are equal, the window is square. If not, adjust the shims until the window is square. Always double-check that the window is level and square after you adjust the shims.
Pre-Drill Holes
Illustration of person's hand holding a hammer and nail.
  • If the window has a brick mold, pre-drill holes in it and drive 16d nails through it and into the framing. 
  • If the window has a nailing fin, nail through it into the framing. 
  • In either case, start at the corners and space the remaining nails as recommended by the manufacturer. 
  • Drive all nail heads below the wood surface with a nail set.
Apply Moulding Over Nailing Fin
Illustration of person's hand applying special-order moulding to window with nailing fin.
  • A window with a nailing fin often has a special-order moulding that you apply over it.
  • Install as directed by the manufacturer.
Fill with Insulation
Illustration of gloved hand filling gaps in window frame with fiberglass insulation.

Fill the gaps between the window jambs and the framing members with loosely packed fiberglass insulation.

Trim the Shims
Illustration of person's hand holding a utility knife to trim off excess insulation from shims.

Trim the shims flush with the framing by scoring them with a utility knife and then snapping off the excess.

Apply Caulk
Illustration of person caulking window frame with bottle of caulk.
  • Apply paintable silicone caulk around the entire window unit and fill the nail holes with caulk. 
  • When the caulk dries, paint to match the trim on the rest of your house.

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