How to Install Interior Window Trim

Many of today's homeowners who are energy conscious are choosing to do their own interior window trim. If you are ready to install window trim in your house and you would like a few guidelines, here are 7 steps that will help you with your installation.

Step 1: Choosing Your Window Frame Style

Generally, you'll have 3 styles of window frame to choose from:

  • Corner Block (Victorian)
  • Butt-Jointed
  • Window Frame

Match the window into which you plan to install your new trim with your choice of frame styles

Step 2: Prepare Stool (the Piece Lying Flat in the Window Sill)

The stool and the bottom of the window will need to come into contact with each other when the window is closed, and the window bottom must extend ½" or more past the new trim on each side of the window.

Step 3: Replace Defective Caulking or Insulation

Examine your existing window. Look for any old or defective caulking or insulation. If you find any needing replacement, this will be the perfect time to replace it before installing your new trim. Examine the edge of the wall and window jam. These edges should be lined up to create a smooth common surface. Use a wood plane, sander or wood chisel to even the edges, if necessary.

Step 4: Measure

The frame top, or head casing, should fit the top window jam edge. The side casing positioned in the stool should butt up against the frame top. Carefully measure these pieces, so you'll know they are positioned correctly, allowing for the 1/8" reveal at the jamb's sides and top. Using a miter box, cut the corner pieces so they are at a 45-degree angle. If you need to adjust the corners to get the right angle, use a planer or sander to remove excess wood from the rear of each piece. Be sure the front edges of the corners match evenly.

Step 5: Stain and/or Seal the Wood

Before you attach the casing, seal each end where it has been cut or sanded. This will prevent moisture from gaining entrance and create warping of the wood. If you wish to stain the wood, or paint it, you should do it at this time.

Step 6: Fasten the Casing in Place

To keep the casing temporarily in place, use a wood glue or adhesive. Insert one of the finish nails you intend to use to permanently connect the corner pieces into the chuck of your power drill. Use this nail as a drill bit to create 2 guide holes in one of the corner pieces. Then insert into these guide holes 2 other finishing nails, and drive the nails into the other corner piece. This should permanently secure the corner pieces.

Step 7: Finish

With a nail set, drive the small nails deeper into the wood, so that the nail heads are below the wood's surface. Fill the nail holes with wood putty. When the putty is dry, sand it and finish sufaces with varnish or paint.