DIY Wood Window Cornice

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A closed top window cornice increases the energy efficiency of window drapes as well as ads architectural interest to your windows. Building your own window cornice will save you a considerable of money over buying a pre-made cornice. They are relatively easy to make and don't require many tools. Making your own also ensures you get the perfect size without having to hire an expensive carpenter.

How Window Cornices Improve Drapery Insulation

Window drapes improve the insulative value of your windows by creating an additional layer of separation between the warm room air and the colder air near the window. Curtains however are open at the top which lets air pass through that insulated pocket. 

During the winter, when you run your heating system, air circulates due to convection. Hot air rises and cold air falls. As the air passes through the top of your drapes it passes in front of the window which cools the air even faster. By preventing the air from passing in front of the window less heat will be lost to the outside and less energy will be required to keep the room warm.

One way to keep the air from passing through the top of the drapes would be to run the drapes from the ceiling down to the floor. If you have very high ceilings, or just don't like the look of floor to ceiling curtains, a closed top window cornice might be a more attractive alternative.

What You'll Need

For this example we'll be making a window 36" wide cornice with a 34.5" opening. You'll need to size your cornices based on the size of your windows and how you mount your drapes. You'll want to have a couple of inches past your drapery rod on either side to allow space for the drapes to slide without binding against the cornice. Since the curtain rod will be covered by the cornice you can use more affordable curtain rods instead of an expensive decorative one.

Materials

All the materials below should be available at your local Home Depot. You can choose different moldings to suit your personal style and match your existing trim and decor.

• 1 - 1 x 6 x 8' S4S Board (appearance poplar is nice but #2 whitewood is much cheaper and good enough for painting)
• 5' - Crown Molding (fiberboard molding is much cheaper than solid wood moldings and accepts paint well, finger jointed is also an affordable option)
• 5' small beaded molding (such as WM163 11/16 in. x 1-3/8 in. x 8 ft. Primed Finger-Joint Poplar Base Cap Moulding)
• 12 - Kreg SML-C125-100 1-1/4-Inch 8-Coarse Washer-Head Pocket Screws, 100-Count
• 21 - #18 3/4" zinc wire brads.
• 2 corner braces (for mounting to wall ex: Everbilt 1-1/2 in. Zinc Plated Corner Braces)
• 4 drywall screws (for brace to wall)
• 4 3/4" wood screws (for brace to cornice)
• Wood glue
• Sandpaper assorted grits
• Sealing Primer
• Semi-gloss paint (to match other trim)

Tools

• Miter box or power miter saw for square and miter cuts.
• Power drill
• Kreg pocket hole jig
• 3/64" Drill Bit (to make pilot holes for brads)
• sanding block
• brushes and other painting supplies
• tape measure

Step 1: Cut Cornice Box

For our 36" wide (34.5" opening) window cornice we'll need to cut the following pieces out of the 1x6 boards:

• 1 x 36" long Front
• 1 x 34.5" long top
• 2 x 5.5" sides

Step 2: Attach Sides

In each side piece, drill 2 pocket holes about 1 1/2" away from the edges using the Kreg Jig and screw into the front piece using Kreg SML-C125 screws as shown below. 

Make sure the pocket holes are on the inside of the cornice box.

Step 3: Attach Top

Attach the top using 9 pocket holes and screws as shown below:

The top will fit inside the front and sides.

If you're willing to spend a bit more you can use a piece of clear acrylic sheet cut to 6 1/4" x 36" and screwed down onto the top instead of using wood. This will still give you the enclosed top for energy efficiency but will also allow light to pass through. It creates a nice uplight effect when your drapes are closed during the day.

At this point your basic cornice is done. If you'd like you can just paint it as is or cover it with fabric. To give it a little more visual interest we'll continue with adding trim.

Step 4: Sand

Sand the outside surfaces of the cornice using 120 grit sandpaper to smooth out any rough spots. Follow up with another sanding using 220 grit sandpaper. Make sure to sand the exposed end grain of the front piece to smooth out any roughness from sawing.

Step 5: Attach Bottom Trim

We've chosen a small decorative base cap molding to apply along the bottom of the cornice. Cut the molding at 45 degree angles in your miter box or power miter saw at the corners. Make sure the back of the molding is pressed up against the back fence of the miter box/saw when cutting.

Attach the trim to the bottom of the cornice box using wood glue and the 3/4" brad nails. Use the 3/64" drill bit to drill pilot holes so the trim doesn't split. Be careful because the bits are very thin and can span easily if not drilling straight. Follow directions and safety instructions on your tools, wearing safety glasses is also a good idea.

Step 6: Attach Crown Moulding

The crown molding give the window cornice a more interesting shape. It is important to get the mitered cuts right so you get nice crisp corners. To get the right angle set the miter saw to 45 degrees in the appropriate direction (it's a good idea to mark which direction the cut will be with a pencil) and place the crown molding in your miter box or saw upside down and backwards (shown right). It's important that the flat edge on bottom and top of the crown molding are flat up against the base of the miter saw and the back fence in the same position as they will be installed. This will insure you get the appropriate angle when you make the 45 degree cut.

The corners on the crown moulding are more critical than on the base moulding so double check your measurements and take your time. It's better to go a little long and then to recut. Especially for the side pieces or if you're using a power miter saw.

Attach the crown moulding to the top of the cornice box as shown, using wood glue and 3/4" brads. Use a 3/64" drill bit make pilot holes. Make sure to glue the crown moulding pieces together at the corners as well. One brad at the top will help keep the pieces joined together.

Step 7: Fill and Sand

Once the glue has fully dried as per the instructions on the bottle, fill any holes or gaps in the trim with a wood filler. Lightly sand with 220 grit sand paper to touch up.

Step 8: Prime, Sand and Paint

Brush and wipe up any sanding dust off the cornice and then brush on a good quality sandable sealing primer. After the primer dries hard, lightly sand with 220 grit sand paper to remove any brushstrokes and other imperfections.

Wipe down the cornice and with a good quality brush and a good quality semi-gloss paint (Cabinet Coat, Benjamin Moore Satin Impervo Oil Based, Benjamin Moore Aura Semi-Gloss) apply 2 coats of paint in the color of your choosing to the cornice following the paint manufacturer's instructions.

Step 9: Install Cornice Over Window

Allow the paint on the cornice to dry and cure fully before attempting to mount it. 

Determine where on the wall you'd like the cornice to be. Attach one of the corner braces on the wall using 2" or longer drywall screws about 10" in from one end of the cornice. Attach the 2nd corner brace level with the first in the same fasion 10" from the other end of the cornice. There should be a big header over the window to screw into but if you are mounting the cornice higher you will want to make sure you screw into studs using 2" or longer drywall screws.

Place the cornice on the brackets and secure to the underside of the top of the cornice box using 3/4" flat head wood screws.