The lights in this project are plugged into an electrical receptacle located behind the crown molding. The power to the receptacle is drawn from the room's light switch and is controlled by a new switch added in the same box. For effect a rheostat can be used to allow for dimming the lights.
If no wall switch exists in the room where you are adding this lighting, go here for help finding an electrical source and adding the wiring required for this project.
Standard 52° crown molding is used in this project. To provide sufficient space behind and conceal the lights, a 1x2 board is added to the back and the crown is hung upside down. The crown is installed in reverse because the ceiling edge is thicker than the wall edge and can hold the weight of the "floating" molding. It also causes the angle to be more obtuse, providing more space behind for the string. In addition to crown molding, a piece of stop bead and a small squared strip are used to add dimension to the finished profile.
Plan to hang the molding a minimum of 5 inches from the ceiling to allow room for installing the lights. Final placement is not set in stone but 5 to 8 inches is usually a good location. Hang it lower if you want to, but don't go overboard and remember to allow for window and door casings when planning placement.
Measure and mark the wall using a board cut to the right size as a measuring tool, we use 5 inches here. First locate the studs and make a small pencil mark at each, a couple inches from the ceiling. Tap across the wall and listening for a difference in the sound produced, in general a hollow sound indicates a space behind the wallboard and a sharper sound indicates a stud location.
With a rough idea of the location of the framing use a drill and an eighth inch bit to bore a small hole where you expect to find the studs. If the drill passes straight through, missing the stud, keep drilling at one inch intervals until you hit wood. When you've located the framing, use the marking board to draw a straight line over each stud as a guide for hanging the molding.
Determine a convenient place to install a new outlet box for the receptacle so it will be near the electrical source. If you're using the existing switch as a source, and you can remove the box without damaging the wall, you can widen the hole and then use an old work outlet box for the two switches. If you have to damage the wall to get the old box out, you will have to use a nail-in box or one with a bracket to attach it to wood framing and then repair the wall when you're done.
For the receptacle use an old work box and locate it in the same hollow part of the wall as the existing switch box. Place the box horizontally so it will be hidden behind the new crown molding. Mark the wall for the new receptacle using the outlet box to draw the outline and then cut drywall using a drywall saw. Cut wood lath plaster with a keyhole saw or reciprocating saw. Run the new cable from the switch hole to the receptacle hole and push the wires into the new boxes before installing them in the wall.
If necessary, cut a notch in the 1x2 board to allow for the outlet box before attaching it to the wall. Hold the board up and drill a pilot hole over each stud using a countersink bit so the screw heads will sink below the wood surface when it's installed. Set the bottom edge of the 1x2 on the straight lines you drew with the measuring stick and then drive 2½ inch screws to secure it to the framing.
Connect the black wire from the new cable to the hot terminal on the receptacle. Connect the white wire to the neutral terminal and the ground wire to the ground terminal. Install the receptacle in the horizontal outlet box.
Measure and cut the crown molding using the surface of the 1x2 as the reference point. Cut it upside down and install it so the thick edge is against the 1x2. If possible use a pneumatic nail gun with 2 or 2 ½in. nails to attach the crown to the board. Otherwise, use a hammer and nail-sink to bury the nail heads below the surface so they can be filled with putty later.
Install the stop bead on the wall just under the 1x2. Fasten it to the wall studs using 2 or 2 ½in nails driven along the center of the bead where they will be hidden by the square strip molding. Sink the nail heads so the top molding fits tightly without any gaps.
Butt the square strip against the bottom of the crown so it covers the nail holes in the stop bead. Use small brads about an inch long to fasten the strip to the stop bead. Sink the head on the strip and fill them with putty. Caulk the seams between all the pieces and between the stop bead and wall. Putty any remaining nail holes.
Reinstall the original switch and add a second one to the circuit to control the new lighting. We use a dimmer here, but you can also use a simple single pole switch for this. Split the source to serve the two switches by splicing the hot wire to two short pieces of jumper wire using the proper wire gauge. Connect each jumper wire to the bottom terminal on one of the switches.
Reconnect the other wires to the original switch and then wire the new one by connecting the top terminal to the black wire that runs to the receptacle behind the molding. Splice the white wire from the receptacle with the other white wires in the box. Splice the ground wire with the existing ground wires in the box and a short jumper wire connected to the ground terminal on the new switch.
When all the wires are connected turn on the power to the circuit and test it using a lamp plugged into the receptacle. Make sure the dimmer is working and then plug the string lights into the receptacle and lay them out in the space behind the molding.