How to Make a Foam Board Cornice for a Corner Window

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I recently did a DIY overhaul on my master bathroom, and I wanted to create a cornice window treatment for the window. I found lots of great tutorials on creating one out of foam board, but couldn’t find a single tutorial for a corner window.

So, I took on the project anyhow and decided to learn as I went.

Now I’m sharing my steps with you, and I hope you’ll create one too!

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WARNING: Many tutorials say “easy” and “10 minutes” – this was time consuming and thought-intensive. I’d hardly describe it as easy or quick – I think I spent a good 4 hours on this, but it was worth it.

Here’s how you can create a cheap foam board cornice for a corner window:

Determine the size of your cornice board. I decided to make mine 12″ tall, have it extend 3″ beyond each window edge, and sit 2″ off the wall. All measurements in this post reflect this – adjust for your needs ūüôā

Sketch Out Pieces & Take Measurements:

You’ll need to cut the following 5 pieces for a corner window cornice:

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Once pieces together, I’ll refer to these 2 pieces:

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Piece #1:

Since it’s going in a corner, you’ll want to add some extra support where the 2 boards meet.

If both windows are the same length, choose 1 that will have a support piece that goes all the way to the adjacent wall. If one window is wider than the other (in my case, the right one is wider), then be sure to make that piece (piece #1 in my example) long enough to reach the wall.

Measuring Piece 1:¬†Distance from adjacent wall to¬†3″¬†beyond window. (Adjust if you choose a different distance beyond the window). With the width of the board (3/4″) you’ll only see 2.25″ between the window edge and the cornice if you measure 3″ beyond the window edge which looks nice.

If your window is 60″ wide, it’s 8″ from the wall, and you plan to go 3″ beyond the window with your edge, you’d cut this piece 71″ rectangle, then notch out the bottom area…

See how piece #1 is notched ¬†on the left side? I didn’t want the support piece to be visible when looking up at the window, so I cut the bottom of the support area off (2″ x 8″ removed).

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Piece #2:

This will be for your second window.

Measuring Piece 2:¬†Distance from wall to edge of window + 1/4″ (if going 3″ beyond window).

Since it will butt up against piece number 1, which is already 2″ off the wall (plus 3/4″ for thickness of board), you only¬†need to add .25 more inches to include the distance beyond the window¬†that it will reach. (Again, this is an example – adjust if you have different plans).

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Piece #3:

This is a support piece for Piece #2, cut it 2″ x 4″.

Piece #4 & #5:

These are the edges that will attach to the wall. Cut them 12″ x 2″

Supplies Needed

Supplies:

  • Fabric (enough to cover front, sides and wrap around to the back)
  • Batting¬†(enough to cover front, sides and wrap around to the back)
  • Foam Board (aka “Foam Board Insulation” or “Expanded Polystyrene”)
  • Duct tape
  • Nails (16 guage x 1-1/14″ – see image below)
  • Sewing Pins (with balls on end)
  • 3 L Brackets
  • 6 screws for mounting L Brackets to wall
  • Scissors (for cutting fabric)
  • Box Cutter Knife / Rotary Knife (for cutting foam board)

The fabric I used was actually curtain panels I got on clearance at Hobby Lobby.

Lowe’s is my nearest home improvement store, so I went there to pick up some foam board, along with nails, L Brackets & screws.

The foam board is 3/4″ thick and comes in sheets of 4′ x 8′ for only $8.35! Super cheap!

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Make sure your nails have a large enough head that they don’t slip through the boards. Here’s the ones I used:

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Cutting Your Foam Board:

Once you have all your measurements mark your foam board and cut to size.

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I used a rotary blade to cut the board which worked really well. Unfortunately, the blade wasn’t thick enough to cut all the way through, so I had to cut 1 side, then flip it over and cut the other side. Still, I think it was easier than using a box cutter blade, so if you have one of these, use it!

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Assembling Your Cornice Board:

Now, attach pieces #1 & #4 using wire nails. So simple but it works:

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Reinforce both the inside & outside with duct tape:

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Do the same for pieces #2 & #5.

Attaching Batting to Your Foam Board:

Lay piece #2 (along with attached piece #5) on your batting, cut enough to wrap around the edges, and tape into place. You do not need to cover the edge that will butt up against piece #1.

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Here is piece 2 covered with batting:

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Now, add batting to piece #1. You’ll want to leave no batting on the “support piece section” – as well as the last 3/4″ of the full length section of your board where piece #2 will be attached.

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You can pin this edge of batting into place.

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Attaching Fabric to Your Cornice Boards:

Lay your pieces on your fabric and cut large enough to wrap around the edges of your board. Then, iron your fabric if needed.

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Lay piece #2 out on the back side of the fabric, and cut inward at each end so you can easily fold it over.

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Wrap fabric around board and pin into place, pushing the pins in at an angle.

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Cut out squares at each corner.

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Note: I chose to cover a lot of the back of my boards with fabric for 2 reasons. One, I didn’t want to be laying in my bathtub looking up at the boards and see the ugly backing. Two, I didn’t want it to look ugly from the outside when the shades are up. If you don’t care about the back, there’s no need to cover so much of it.

At this point, you’ll want to attach our mini support piece of foam board (piece #3) to the top back of piece #2.

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Near the base, drive pins down at an angle to attach the two pieces on each side.

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Then, secure with duct tape.

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Cut and trim fabric so that it folds over the edge at the bottom and covers the side of the support piece. Secure with pins or duct tape.

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OK, piece #2 is done for now. Set it aside and get to work on piece #1.

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Add fabric to areas covered with batting on piece #1.

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Assemble Your Cornice Board

At this point, I was getting pretty excited!

Next, set up your 2 pieces as they’ll be assembled and hung.

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Now, it’s time to connect the support pieces to each other.

Line them up.

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You might think of a better way to attach these pieces – perhaps using glue! I just drove nails through the pieces, drove angled pins in along the edges, and added duct tape.

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And ta-da! It’s ready to hang!

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Hanging Your Corner Window Cornice Board

Hanging it was a little bit tricky, and definitely required 2 people, so taking pictures wasn’t really an option.

Here’s the steps we took:

  • We held it up in place at the height we wanted.
  • I positioned¬†the L brackets on the board as my husband held it up to determine where they would go, and marked the screw holes on the wall.
  • We set the cornice¬†aside.
  • I drilled the holes and mounted the 3 L brackets on the walls.
  • We held the cornice up against the brackets, and I used nails through the screw holes to secure the cornice to the brackets.

Here are some pics of 1 of the L-brackets once hung:

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You can see how I really angled the nails to keep it secure:

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Now a sigh of relief – it’s done and it looks great!

When I told my husband I was going to build a cornice out of foam board (we’ve built them before out of wood) he was very skeptical, but he was very impressed with how these came out!

This has been up for several months now and hasn’t budged.

I’ll definitely make some more again soon with my leftover foam board!

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Would you try this project? If you do, I hope you’ll share pics!

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