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Window Casings

Exterior and Interior Window Casings Provide the Finishing Touches to a Window

By Mark J. Donovan



Window casings offer a number of unique benefits to a window and home. Exterior window casings provide an accent piece around the window and act as a transition piece between the window and the house siding. Interior window casings help to cover the transition where drywall abuts the window frame, and function as insulation barriers to outside air drafts. Interior window casings also serve to dress up and provide the finishing touches to a window. Often interior window casings are installed that complement the baseboard trim, crown molding and other finish carpentry in a room or home.


And in some window casing styles, in particular when window sills are included, they offer functionality. Interior window casings also enable a surface area for securing window treatments.


Installing Window Casings

Instead of purchasing pre-fabricated window casings with the windows you can alternatively install your own window casings around your new windows. Installing window casings is actually not that difficult, however, I highly recommend the use of a miter saw.

Interior window casings come in a variety of materials and styles. The most inexpensive interior window casing moldings are made from plastic, MDF and pine wood. More expensive interior window casings are fabricated out of hardwoods or exotic woods. Colonial casing is very commonly used in trimming out interior window frames, however flat 1”x 3” or 1”x4” trim lumber is also commonly used. Most window casing installations involve using mitered 45 degree cuts, however straight cuts (butt cuts) are also used, particularly when flat 1”xN” trim lumber is used.

As part of every interior window casing installation project there are side, top and bottom (apron) pieces that must be installed. In many cases finish carpenters and homeowners also elect to use window stool to create a window sill. On the apron piece, the edges are usually tapered inwards 15 to 22.5 degrees, or they are mitered at a 45 degree angle and corresponding small return pieces are installed on each end.


Pre-primed 1”x4” wood or Azek PVC trim lumber is often used for exterior window casing.


If using wood exterior trim you may want to consider encasing the window casing pieces in aluminum prior to installing them around the window frame to protect them from ultra-violet and water damage. Also, it is important that flashing be installed over the exterior window casing to ensure water does not penetrate behind the window casing, windows and siding.

Often when you order the installation of windows they include window casings and extension jambs. Window extension jambs are used commonly for 2x6 wall framing. They effectively extend the window jamb of the standard 2”x4” manufactured window frame to the width of the 2”x6” framed wall.

Here is an old window with interior window casing.

Photo by Mark Donovan

Finishing Window Casings

Window casings can be finished in a number of different ways. They can be simply primed and painted for example. Alternatively they can be stained and then polyurethaned. Regardless of how you plan to finish the window casings, I highly recommended pre-priming/painting/staining the window casing prior to cutting and installing it on the window frame. In addition, after each cut, dab the cut ends with paint or stain to hide any seams.

For information on Installing a New Window, see the Installing a New Window Ebook from HomeAdditionPlus.com.  The Installing a New Window Ebook provides easy to understand, step-by-step instructions, on how to remove an old window and install a new one. Pictures are included for every key step in the process.

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