A mull bar is a type of separator when installing two windows side-by-side in the same opening. Using a mull bar is an alternative to building a wood frame for the windows. The mull bar is a metal or plastic separator that is attached to the side of two windows in the window opening. You can use a mull bar when a bay window is to be replaced with two double-hung windows.

Step 1

Determine which side of the window that will be attached to the mull bar. Score along the inside edge of the fin on the side of the window with the utility knife. Pull the fin off from the side of the window. Repeat this step with the "J" leg on the window.

Step 2

Place the anchor clip on both ends of the mull bar. Insert the screws into the holes and secure with the power drill or use a screwdriver.

Step 3

Open the bottom section of the window so that is it 3 inches above the sill. Drill a 3/8-inch hole into the first layer or wall of the window. Open the top section of the window so that it is 3 inches below the top of the head. Drill a 3/8-inch hole into the first layer or wall of the window.

Step 4

Measure up 9 inches from the bottom hole and drill another hole. Measure down from the top hole and drill another hole. Repeat this step until the last two holes are separated by 9 inches or less.

Step 5

Close and lock the sash on both of the windows.

Step 6

Apply a bead of silicon caulk along the front edge and the back edge of the first window. Place the mull bar on the edge of the window and secure by tightening screws into the pre-drilled holes.

Step 7

Apply another bead of silicon caulk along the front edge and back edge of the other window. Align the window so that it is centered on the mull and tighten screws into the pre-drilled holes.

Step 8

Apply a bead of caulk in the groove on the left and right side of the mull bar. Place the cap for the mull bar in place and press it into the center groove. Repeat this process on the opposite side of the window.

Step 9

Insert screws into the bottom anchor clip and secure it to the sill with the power drill. Repeat this step with the top anchor clip.