Things You'll Need
1 by 2s
Quarter round molding
Hollow core doors are used as passage doors between spaces in the interior of houses and little else. There are two main types. Hardboard doors are made of pressed wood fiber and can be found in flat panel or raised panel styles. Luanne, or plywood doors, are typically flat panel and will work best for adding glass. Due to the lighter weight and resistance to breaking, plexiglass, or one of the other excellent acrylic glasses is your best bet for a long lasting window in a hollow core door.
Mark the outline of the area you want to add your window, using a carpenter's square to ensure straight edges and square corners. The outline should be 1/8-inch taller and wider than the actual glass. Your glass will be 1-inch taller and wider than its visible surface to allow for trim to hold it in place. The overall outline will be 1 1/8-inch taller and wider than the visible glass with trim installed. E.g., for a 12-inch square window, mark a square 13 1/8-inches wide and tall.
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Cut along the four sides of this outline using a circular saw set at its deepest cut. Stop short of the corners and finish with a jig saw to prevent cutting too far. Remove the piece from the door.
Remove any cardboard or wooden spacers between the faces of the door. You will find these by looking into the door around the edge of the opening you cut. Cut the spacers loose with a utility knife, or sharp chisel, 1-inch back from all edges of the opening.
Measure the inside space between the faces of the door at the edge of your opening. This is typically 1 3/8, but may range from 1 to 1 1/2-inches. Cut enough 1 by 2 lumber to go around the opening to the same width as the space between the faces of the door. Cut 2 pieces to length 1 1/2-inches longer than the width of your opening. These are the horizontal members of the glass frame. Cut 2 pieces to length the same as the height of your opening. These are the verticals.
Apply wood glue along both edges of the horizontal frame pieces and position them inside the edge of the door opening at the top and bottom, flush with the edge of the cut. Use squeeze clamps to hold them in place. Glue and position the vertical pieces similarly and glue them in place. Allow the glue to set for one hour.
Cut 4 pieces of quarter round molding to the width of the opening with 45 degree miters at each end, one right, one left, using a miter saw. Cut 4 pieces to the height of the opening the same way. These will form 2 "picture frame" style trim rings to hold the glass.
Glue and nail quarter round at the top, bottom and both sides of the opening, flush with the face of the door on one side. Turn the door over, set the glass into the opening, resting on the quarter round. Glue and nail the remaining pieces into the opening, pressed against the glass.