Skip to Content

How to Trim Interior Windows

Having recently purchased a “new” home, we were faced with the job of completing the interior work of the house which amongst other things, included trimming the windows.

wood cabinet with flowers next to a window with trim.

How to Trim Windows

As you may have noticed, the word “new” was in quotations. The house itself is about 12 years old yet the interior of the home was never fully completed. This included things like adding baseboards, painting and trimming the windows. All of the windows in the house looked like this photo, without any jambs, casings or baseboards, only the unfinished 2×4 wood pieces holding the window in place were visible.

Windows with trim removed.

Get updates on the latest posts and more from Sustain My Craft Habit straight to your inbox by joining our weekly newsletter. We promise to send you only the good stuff!

Knowing it was going to be a big undertaking to finish all the windows, hubby started the job as soon as we moved in. And what a difference a finished window can make!

Finished window trim.

Materials Needed:

Disclosure: This post contains affiliate links for your shopping convenience. Should you choose to purchase through them, we will make a small commission at no extra cost to you. You can read our full disclosure policy here.

  • Window jambs Jambs (since we needed ⅝” thickness to fit inside window sills, we used a sheet of good one-side plywood and cut them down to size using a table saw)
  • Casing (we used modern profile pre primed wood casing, 4” wide)
  • Level
  • Shims
  • Nail Gun with nails (1 ½” long)
  • Wood Glue
  • Mitre Saw
  • Tape Measure
  • Wood filler and 200grit fine gauge sandpaper
  • Utility knife
  • Safety glasses

How to Trim Interior Windows:

During the building stage, the windows were installed and fastened to the frame, those ugly 2×4 pieces of wood you can see in the photos. The exterior of the house and the windows were fully finished keeping them sealed and free of weather and little critters from getting in. Whereas on the other hand, the trim work on the interior was left unfinished, this step is purely for aesthetic purposes.

The interior casing covers the frame and any spray foam that is visible. The spray foam in case you’re wondering insulates the area around the windows to keep the cold air out. Much of our foam had expanded into the sills so this needed to be cut off with a utility knife first.

Installing the Interior Window Jambs

The interior window jambs are the pieces around the window that cover the ugly 2×4 frame. Depending on the type of window, it may or may not have a sill in which the jambs fit into.

As I mentioned earlier, our window sills measured ⅝” thick so we needed to use trim that would fit inside the sill. This proved to be harder to find than we expected! In the end, we decided to use ⅝” thick sheets of plywood to rip down to the width we needed for each window.

If your windows don’t have sills, you can use any thickness of trim but I’d recommend ¾” thickness is probably best. 

Start by measuring the height, width and depth of the areas needing the jambs. 

Add the bottom jamb in place first, adding shims underneath to help hold and level. In addition to a level, we used a right angle ruler to ensure the corners were square.

Do the same with the side jambs and then add the top jamb last. Measure all areas twice to be sure the lengths are correct. 

Once the jambs are in place and level, use the nail gun and nail the pieces in place. We nailed the jambs together at the corners.  

Man using a nail gun onto the window casing.

Adding the jambs already makes a huge transformation, especially when the jambs are primed and/or painted beforehand.

Installing the Window Trim

Now that the jambs are in place, determine the length you will need for the top and bottom pieces. The side edges of the jambs will be mostly covered by the trim. Measure across from inner corner to inner corner, keeping in mind that there should be a bit of a “reveal”. Reveal means the amount showing of the jamb, not exactly aligned to the edge of the jamb. We prefer an ⅛” reveal so we added that amount to the length of the trim. 

Cut the window trim using a miter saw at 45 degree angle.

Line up the window trim along the window jamb to reveal ⅛” on both sides and along the top length of the jamb. Place the level underneath the casing to help ensure the trimwork is straight before nailing to the wall.

Using the wood glue, glue the trim to the wall and then nail it with the nail gun.

Repeat the process for the vertical sides of the window, lining up the mitered corners of the top and bottom trim.

Man holding window trim.

Finishing the Trimwork

Once the trim around the window is installed, take the wood filler and fill in the nail holes and spaces along the mitered corner. Let the wood filler dry completely and then use a fine gauge sandpaper to smooth where the holes were filled. 

Wipe away the dust and sanded filler with a clean cloth and then paint or stain the trim to finish the project.

We’re so pleased with how the windows turned out. Now it’s time to think about DIY window coverings like as these easy to sew curtains I made for my sister’s dining room or DIY roman shades.

More DIY Home Updates

We’ve done plenty of renovations over the years, many of which we shared on the blog. Check out these great tutorials for your next DIY.

Thankyou so much for stopping by. Let us know if you have any questions as you begin trimming your interior windows. To stay in the know on our latest crafts, special offers and freebies, join our mailing list today. 

Like it? Pin it for later!

Collage showing hot to trim a window.

How to Trim Interior Windows

Jane and Sonja
Outlining the step by step instructions (with photos) on how to trim interior windows.
5 from 2 votes


  • Jambs
  • Casing
  • Level
  • Nail Gun
  • Nails 1 ½”
  • Wood Glue
  • Mitre Saw
  • Utiity knife
  • Shims
  • Tape Measure
  • Wood Filler for the Nail Holes


  • Using a utility knife, trim away any excess spray foam that's expanded on to the window sill.
  • Determine the length, width and depth you will need for the jambs to cover the exposed 2×4 frame.
  • Insert the bottom jamb into the sill and level the jamb with shims.
  • Repeat with the side jambs and then lastly with the top jamb.
  • Nail the jambs in place.
  • Measure the bottom width of the window from jamb edge to jamb edge. Add 1/8" on either side for the reveal.
  • Cut the window casing using a mitre saw at 45 degree angle on both ends.
  • Line up the window casing over the bottom jamb to reveal ⅛” (along the top and sides of the casing). Use the level to ensure the casing is straight.
  • Using the wood glue, glue the casing to the wall and nail the casing with the nail gun.
  • Repeat the process for the top and vertical sides of the window.
  • Once the casing has been installed, take the wood filler and fill in the nail holes and mitered corners. Let the wood filler dry.
  • Once the wood filler is dry, use a fine gauge sandpaper to smooth where the holes were filled.
  • Lastly, paint or stain the trim to finish.
Like this craft? Leave some stars!& mention @SustainMyCraftHabit or tag #sustainmycrafthabit on Instagram!

Tutorial Rating

This site uses Akismet to reduce spam. Learn how your comment data is processed.

This site uses Akismet to reduce spam. Learn how your comment data is processed.