What Is a Cornice Box? (Crown Pelmets)
A cornice box, also called a crown pelmet, is a rigid cover fitted above a window to hide a curtain or blind headrail, brackets, and gathered fabric. Its size depends on the treatment’s full operating space, not the window alone. Measure installed parts, allow product-specified clearance, mount securely, and keep grilles, egress, and electrical access clear.
Cornice boxes have a long history in interior trim because they give a window a finished, framed look. The style may change, but the practical job stays much the same: conceal the hardware and fabric stack without getting in the way of the window treatment. A well-planned box looks built in. A poorly planned one can rub, block light, or restrict airflow.
I start with the curtain or blind, not the trim design. The treatment’s brackets, headrail, moving fabric, and full range of travel set the space the cornice needs. There is no universal code dimension or clearance that suits every product, so the treatment manufacturer’s instructions matter.
What Is a Cornice Box?
A cornice box is a fixed, rigid cover mounted above a window treatment. It hides the headrail, brackets, and upper fabric stack, and may be finished with paint, fabric, or decorative trim. “Crown pelmet” is another name for this feature; the precise term can vary by region and trade.
Unlike a soft valance, a cornice is built around a firm frame or panel. It may be a simple flat-front box, a shaped timber assembly, or a board wrapped in fabric. Crown moulding can be added, but it is decorative trim, not the box itself.
A cornice also differs from a curtain board or valance in how it is designed. The important question is whether the cover hides the treatment while leaving enough room for it to work. A neat front face is not useful if the blind catches behind it.
Planning the Cornice Around the Treatment
Planning means identifying everything the cornice must cover and checking the conditions around the window before choosing its shape. I first note the treatment type, mounting position, window operation, wall or ceiling surface, and nearby equipment. These details guide the box layout and help avoid rework.
List the parts that define the treatment’s full operating envelope: its headrail, brackets, side returns, moving fabric, and, for a blind, the stack when fully raised. The operating envelope is the space those parts occupy while installed and in use. Measure actual components where possible; window width alone does not describe that space.
Check the mounting surface and locate framing before deciding how the box will attach. Also look for HVAC grilles, electrical equipment, and any window or door used for required egress. A cornice must not conceal a supply or return grille or interfere with the required operation of an egress opening.
There is no prescriptive IRC or IBC dimension for a cornice box. Follow the treatment and fastener manufacturers’ instructions, plus locally adopted code. For example, NEC 314.29 requires covered electrical boxes to remain accessible without removing building finish or structural elements. Do not build a cornice that blocks required access.
Measuring Operating and Window Clearances
Measure the installed treatment rather than estimating from the window opening. Record the full width, depth, and height of the hardware and fabric in their relevant positions. Then apply clearance specified by the treatment manufacturer; no single clearance value applies to all products.
- Width: Measure across the installed treatment, including brackets and side returns. The box’s clear inside width must exceed that measured envelope by the manufacturer-required operating clearance.
- Depth: Measure the farthest projection of the headrail, brackets, and moving fabric. Make the clear inside depth greater by the specified clearance, then test the blind through its full travel.
- Height: Measure the fully raised blind or curtain stack, including brackets and trim. Provide enough cover to hide it without contact. Stack height depends on the product and fabric.
“Clear inside” means the usable space between the inner faces of the box, not its outside dimensions. A 3/4-inch-thick board on each side reduces the inside width by 1 1/2 inches compared with the outside width. Account for that thickness when laying out parts.
| Measurement | What to include | Sizing rule |
|---|---|---|
| Clear inside width | Headrail, brackets, returns | Measured envelope plus product-specified clearance |
| Clear inside depth | Farthest hardware and moving fabric projection | Projection plus product-specified clearance |
| Clear inside height | Fully raised stack and brackets | Enough to conceal the stack without contact |
Write down each measurement and the source of any required clearance. If the treatment maker does not give a clearance figure, ask the manufacturer or installer rather than treating a guessed number as a standard. Then confirm that the proposed box also leaves the window and nearby equipment usable.
Building and Mounting the Cornice Box
Building starts with a layout based on the measured interior space, chosen material thickness, and attachment method. A common paint-grade option is 3/4-inch plywood, but that is a practical choice, not a code minimum. Select the substrate, edge treatment, and finish for the span, weight, and humidity conditions.
A basic box may have a front panel with top and side pieces, or a front with returns that conceal the ends. Cut and assemble it square so the face sits evenly against the wall or ceiling. Seal or finish exposed edges as needed for the chosen material and room conditions.
Before mounting, locate suitable framing or specify an engineered anchoring system. Fasteners and brackets must be rated for the assembled box and treatment load and installed as their manufacturer requires. There is no generic load rating for cornice hardware. Do not rely on drywall alone unless the anchor system is explicitly rated for the actual load and installation.
A practical sequence is:
- Make a full-size layout from the measured operating envelope.
- Add material thickness to calculate outside dimensions.
- Dry-fit the box around the installed treatment, where practical.
- Attach the box to verified framing or properly specified anchors.
- Finish edges and surfaces without filling access points or covering equipment.
If the box is long or heavy, plan how it will be lifted and held during installation. A second person or temporary support can reduce the chance of a dropped assembly or damaged wall. Keep fasteners clear of wiring and piping, and use suitable detection and safe work practices before drilling.
Verifying Operation, Access, and Airflow
Verification means checking the finished cornice against how the room and treatment must work, not only how the front looks. Operate the blind or curtains through their full range, then inspect the window, nearby equipment, grille openings, and required access. Correct contact or obstruction before treating the installation as complete.
Raise and lower blinds fully. Draw curtains open and closed, checking that fabric does not snag on an edge or bracket. Open the window as intended and confirm that the cornice does not obstruct a required egress opening or its operation.
Look above and beside the cornice for supply and return grilles. A cover that hides or restricts a grille can impair HVAC performance. There is no universal CFM allowance for a cornice; retain the grille’s required free area and clearances as specified by the HVAC designer or grille manufacturer.
Also confirm that electrical equipment requiring access remains accessible. NEC 314.29 applies to covered electrical boxes: access cannot depend on removing building finish or structural elements. If the layout conflicts with access, revise the cornice instead of burying the equipment.
Cornice Box Examples for Common Window Setups
These examples show how the measuring method changes with the treatment. They are planning scenarios, not standard dimensions: the actual box size must come from the installed product, its clearances, and the site conditions. I use them to focus the survey on likely interference points.
- Roller blind: Measure the headrail and brackets, then check the full fabric path and any projecting chain or motor parts. Confirm that the blind can travel without rubbing the front or bottom edge.
- Curtain track: Include the track, brackets, returns, and the gathered curtain stack. Check that the fabric can draw fully to the planned open position without being trapped by a side return.
- Window with a nearby grille: Map the grille and its required free area before drawing the box. If the cornice would cover or restrict it, change the layout or choose another treatment arrangement.
- Deep window trim: Measure from the mounting surface to the farthest treatment part, not just to the glass. Confirm that the cornice clears the trim and does not obstruct window operation.
These cases show why a standard “one-size” depth or height can fail. The treatment, brackets, and room layout determine the needed clear space.
Conclusion and FAQs
A cornice box is a rigid, finished cover for curtain or blind hardware, but its success depends on accurate planning. Measure the treatment’s operating envelope, use product-specific clearances, mount for the real load, and check airflow, egress, and access before finishing. The window dimensions alone are not enough.
What is a cornice box used for?
A cornice box hides a curtain or blind headrail, brackets, and gathered fabric. It can also create a finished architectural detail above a window.
Is a cornice box the same as a crown pelmet?
Yes. “Crown pelmet” is another name for a rigid decorative cover above a window treatment. Names differ by region and trade.
How do I measure a cornice box?
Measure the installed treatment’s full width, farthest projection, and raised stack height. Add the operating clearances required by the treatment manufacturer.
How deep should a cornice box be?
Its clear inside depth must exceed the farthest projection of the headrail, brackets, and moving fabric by the manufacturer-specified clearance. There is no universal depth.
How high should a cornice box be?
Make the clear inside height sufficient to conceal the fully raised blind or curtain stack without touching it. Stack height varies by product and fabric.
What material is commonly used?
Three-quarter-inch plywood is a practical option for a rigid, paint-grade box, but it is not a code requirement. Choose material for the span, load, and humidity.
Can I attach a cornice box to drywall?
Do not rely on drywall alone unless the selected anchor system is rated for the actual load and installed as specified. Suitable framing or a properly designed anchoring system is preferred.
Can a cornice cover an HVAC grille?
No. Do not cover or restrict a supply or return grille. Keep its required free area and clearances as directed by the HVAC designer or grille manufacturer.
Does a cornice box need to meet a standard code size?
No universal IRC or IBC cornice-box dimension applies. Follow local code, product instructions, and applicable rules for access, egress, and building systems.
How can I check that the box fits?
Operate the blind or curtains through their full range, open the window as intended, and inspect nearby grilles and equipment. Make sure required electrical access remains clear.
(This article was written by one of our staff writers, Garrett Vance. Visit our Meet the Team page.)
