Installing Interior Window Trim

Written by laura reynolds
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Installing Interior Window Trim
(Window and Door Manufacturers Association)

Replacing or rebuilding windows is a popular project and there are plenty of energy-efficient windows on the market. Some come with trim but most are sash and jamb units ready for installation in the wall. All that's necessary is to level the window by shimming it and attaching the frame to the studs surrounding the opening. Before trimming out your new window, remove any leftover caulking and check the insulation around the window. Upgrade or replace any old insulation and re-caulk. Check to make sure that the inner edge of the jamb and wall form a flat surface for the frame; reduce any imperfections with a planer or circular sander. Add moulding strips and sand or plane to build up a jamb that is too low. The new trim will form a "casing" around your window, set back a bit to reveal about 1/8 inch of the jamb around the sides and top of the window.

Know Your Style

Choose between a simple "picture frame" window with mitred corners, a Craftsman or "butt-jointed" frame or a Victorian "corner block" frame. Choose the style that best matches the window and your home's style. Before beginning, prepare the "stool" (the piece that lays flat in the sill) and "apron"---the pieces at the bottom of the frame that cover and finish the window's sill. Make the stool deep enough to meet the window when it's closed and wide enough to extend at least a 1/2-inch beyond the finished trim on either side of the window. Notch either side to slide into the window to sit on top of the stool and cut a shorter apron to put against the wall under the stool for support. The style you choose for your window casing will dictate how far on either side of the window the stool extends, what sort of trim to use for the apron and how the ends of each are finished.

Measure (At Least) Twice...

Begin with the "head" casing (top of the frame), measuring so that it fits the top edge of the jamb, adding the width of the framing on either side for a butt joint or cutting 45-degree angles for a picture frame. The side casing will sit in the stool and meet the head casing. Measure carefully and "dry fit" before attaching casing pieces. Remember to allow for the 1/8-inch reveal on the sides and top of the jamb. Use a mitre box to cut 45-degree corner pieces and a coping saw or sander to correct mismatches, always removing a little more wood from the back of the casing so that the front edges meet perfectly. Stain or seal the wood before attaching the casing, sealing the ends of each piece to keep moisture out. Use construction adhesive to hold the casing in place while you add finish nails. Add finish trim like bullseye corner blocks or casing moulding for more detail. Fill finish nail insets and paint or varnish to finish.

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