How to Install Octagonal Windows

Measure the outside of the octagonal window's sleeve and the building's window opening, using a tape measure. Compare the two sets of measurements. The window sleeve's measurements should measure about 1/2 inch smaller than the opening's corresponding measurements. The window sleeve holds the window pane in place and slides into the exterior side of the building's window opening.

Lay a 1/4-inch-thick shim on the window opening's bottom horizontal board.

Run a 1/4-inch-thick bead of exterior-grade caulk around the perimeter of the window opening, using a caulking gun to apply the caulk. Keep the caulking about 3/4 inch away from the edge of the opening.

Hold the top of the octagonal window's sleeve against the top of the window opening. Slide about half of the sleeve into the opening. Lower the window until the sleeve rests on the shim. Push the window into the opening until the window's fin, the flange that surrounds the perimeter of the window, touches the exterior-grade caulking.

Run a 1-inch wood screw through the center screw hole found in the fin attached to the top of the window. Each side of the fin contains at least three screw holes. Twist the screw clockwise with a Phillips screwdriver until the screw head touches the window fin.

Enter the building and inspect the window sleeve's clearance in the window opening. The window should rest centered in the opening. Adjust the window's position as needed. The screw holding the window's top fin against the building's sheathing acts like a hinge and will allow the window to shift side-to-side in the opening.

Secure the window's fin to the sheathing with additional 1-inch wood screws, using the Phillips screwdriver to tighten the screws. Run a screw through each of the window fin's screw holes.

Cut a piece of flashing tape about 4 inches longer than one of the window fins, using scissors to cut the tape. Peel the paper backing from the flashing tape's sticky side. Press the sticky side of the flashing tape onto its window fin. Repeat this process with the remaining window fins.

Enter the building and pull the shim out from under the window's sleeve.

Fill the gap between the interior side of the window's sleeve and the window opening with a non-expanding foam, using the foam manufacturer's instructions. The foam insulates the sleeve and eliminates drafts.

Install the building's drywall and exterior siding before continuing. The drywall covers the interior wall. The exterior siding covers the octagonal window's fin and butts against the window's exterior trim.

Slide the octagonal window's interior trim over the sleeve's interior edge and secure the trim with finish nails, using the hammer to drive a nail every six to eight inches around the window trim. The trim covers the drywall's cut edge and completes the interior side of the octagonal window.

Fill the gap between the siding and the octagonal window's exterior trim with an exterior-grade caulking. Smooth the caulking with a damp finger.