How to Install Window Trim: Inside and Out

New window trim is one of the cheapest ways to make a room look finished, and installing window trim is a weekend job most homeowners can handle. The casing frames the glass, covers the gap where the jamb meets the drywall, and gives a plain opening some shape.

Doing it yourself keeps the labor cost, usually the biggest line on a pro’s bill, in your own pocket. You mainly need a miter saw, a nailer, and a patient hand with a tape measure.

Below is how to install window trim on both the interior and the exterior, including the reveal, the miter cuts, and the waterproofing that keeps outside casing from rotting.

Window Trim Installation Cost

Hiring a pro to trim out a window generally runs about $120 to $350 per window, and labor is the biggest part of that. Trim carpenters charge roughly $65 to $105 an hour and spend a little over an hour on a standard window, so a whole-house trim job often lands near $900.

Going the DIY route keeps that labor money in your pocket. You only budget for materials, meaning the wood or PVC, nails, and caulk. If you already own the saw and nailer, you can trim a single window for the cost of a board and a tube of caulk.

Parts of a Window

Before you start cutting wood, you need to know the anatomy of the window you are working on. Here are the key terms.

Sills

The sill is the shelf-like piece at the bottom of the frame. On the interior, this is technically called the “stool”, it’s the flat ledge where you might set a coffee cup or a plant.

On the exterior, the sill is angled downward to shed water away from your home’s foundation.

Jamb

The jambs are the vertical sides and the top horizontal board that form the “box” the window sits inside. Your trim will be nailed into the wall studs and the edge of these jambs.

Sash

The sash is the moving part of the window that holds the glass. If you have a double-hung window, you have two sashes that slide up and down. Fixed windows also have sashes, but they don’t move.

Head

The head (or head jamb) is the top horizontal piece of the window frame. When measuring height, you usually measure from the sill up to the head.

Stiles

Stiles are the vertical parts of the sash itself. You generally won’t be nailing into these, but knowing where they are helps visualize the window operation.

How to Install Interior Window Trim

For this guide, we are installing a “picture frame” style trim. This uses four pieces of casing joined by 45-degree miter cuts at the corners. It is a clean, modern look that works in almost any house.

What You’ll Need

• Utility knife.
• Pry bar.
• Measuring tape.
• Pencil.
• Miter saw (essential for clean angles).
• Brad nailer (18-gauge is standard).
• 1-inch and 2-inch brad nails.
• Wood glue.
• Spackling compound.
• Paintable Caulk.
• Stain or paint.

1. Remove Existing Trim

Use a utility knife to score the paint along the edges where the old trim meets the wall and the window jamb. This prevents the paint from peeling off the drywall when you pull the wood.

Slide a pry bar behind the trim near a nail stud. Gently wiggle it to loosen the board, working your way down the length of the trim until it pops free.

2. Mark Reveal Lines

The “reveal” is the small strip of the window jamb that remains visible after the trim is installed. It adds depth and keeps the trim from interfering with the window sash operation.

Set your combination square or tape measure to 1/8 or 3/16 inch. Mark this distance from the inside edge of the jamb on all four sides (top, bottom, left, right). These marks are your guide for installation.

3. Cut and Install the Top Trim

We start at the top to set the standard for the rest of the window.

1. Measure the distance between the two side reveal marks you made on the head jamb.
2. Transfer this measurement to your trim board. This is the “short point” of your miter cut.
3. Set your miter saw to 45 degrees. Cut outward from your marks so the long side of the board is wider than the short side.
4. Apply a little wood glue to the mitered ends. Align the bottom edge of the trim with your reveal line and nail it into place using the brad nailer.

4. Cut and Install the Side Trims

Now you connect the sides to the top piece.

1. Measure from the bottom of the top trim (the short point of the miter) down to the bottom reveal mark.
2. Cut the top of your side trim at a 45-degree angle and the bottom at a 45-degree angle (ensure they angle away from each other).
3. Test the fit. The corner should close tightly against the top trim.
4. Nail it in place with 2-inch 18-gauge brads spaced every 10 to 12 inches, alternating between the inner and outer edge to pull the casing flat, and keep the reveal consistent all the way down.
5. Repeat for the opposite side.

5. Cut and Install the Bottom Trim

This piece completes the picture frame.

1. Measure the distance between the inside short points of the two side trims.
2. Cut your bottom piece with 45-degree miters on both ends.
3. Apply glue to the corners, fit the piece in, and nail it secure.

6. Finishing the Trim

Now for the cosmetic work. Fill all nail holes with spackling compound. Once dry, sand it smooth.

Run a bead of painter’s caulk along the outer edge (where trim meets wall) and the inner edge (where trim meets jamb). This seals gaps and hides uneven walls. Once the caulk cures, you are ready to paint.

How to Install Exterior Window Trim

Exterior trim has a different job: it must look good and keep water out. Because of this, we rarely use mitered corners outside, as they tend to open up when wood expands and contracts with the weather. Instead, we use “butt joints” (square cuts).

While wood is traditional, cellular PVC trim is popular outside because it never rots and holds paint well. Just remember that PVC expands and contracts with heat far more than wood does, close to ten times as much. Fasten it with stainless-steel nails or screws and a PVC-specific adhesive, leave a small gap (up to 1/8 inch) at any butt joint, and avoid butting PVC directly against real wood, because the two move at very different rates and the seam will open.

What You’ll Need

• Measuring tape and pencil.
• Circular saw or miter saw.
• Galvanized casing nails (or exterior trim screws).
• Hammer or drill.
• Exterior wood putty.
• Exterior-grade paint.
• Rot-resistant lumber (Cedar, Redwood) or PVC trim.
• Exterior silicone sealant.

1. Measure the Window

Measure the height and width of your window frame. Note that exterior siding usually butts up against the window, so you may need to cut back siding to make room for wider trim.

Check the frame for squareness. If the window is out of whack, you might need to use shims, thin wedges of cedar, to level out your trim boards so they sit flat.

2. Cut the Trim

For a standard butt-joint installation:

1. Side pieces: Cut these square to the exact height of the window frame.
2. Top and bottom pieces: Cut these to the width of the window frame plus the width of your two side boards. The top and bottom pieces should extend over the side pieces to “cap” them.

3. Install the Trim

Start with the top piece (head casing). Center it and tack it loosely with a nail. Use a level to ensure it is perfectly horizontal.

Next, install the side pieces snugly underneath the top piece. Finally, install the bottom piece. This specific order (top capping the sides) helps shed water better than if the sides ran past the top.

4. Nail the Trim

Once you are happy with the fit, drive your galvanized nails in. Space them about 8 to 12 inches apart. If you are close to the end of a board, drill a pilot hole first to stop the wood from splitting.

5. Fill the Nail Holes

Countersink your nails slightly below the surface of the wood. Fill these divots with high-quality exterior wood filler.

Once the filler dries, sand it flush. This is critical for preventing water from pooling in the nail heads and causing rust or rot.

6. Seal and Paint

Apply a high-quality exterior sealant around the edges where the trim meets the siding and the window frame. I recommend GE Sealants and Adhesives for a bond that withstands temperature shifts. Finally, apply two coats of exterior-grade paint to protect the wood from UV rays.

Tips for Installing Trim

To make sure your trim installation yields pro-level results, keep these tips in mind:

• Measure long-to-long: When measuring for miters, always measure to the “long point” (the outer edge) of the trim. It is easier to mark and cut accurately.
• Use a “cheater” block: Cut a small scrap piece of trim with a perfect 45-degree angle. Use this to test the corners of your window frame before cutting your full boards.
• Safety first: If working on second-story exterior windows, ensure your ladder is secure. Never overreach.

FAQs

Here are the most common questions DIYers ask when tackling window trim for the first time.

What Type of Wood Should You Use for Window Trim?

The best window trim wood depends on where it goes. Outside, cedar and redwood are the top picks because they naturally resist rot and insects, with cellular PVC as a no-rot synthetic option. Inside, clear pine or poplar are the value choice for a painted finish, while oak, cherry, or walnut suit a stained, natural-wood look.

How Do You Install Window Trim Without a Nail Gun?

You can install window trim by hand with finish nails and a hammer. Pre-drill a pilot hole slightly smaller than the nail shank at each spot so the casing does not split, then drive the nail and sink its head just below the surface with a nail set so you can fill the hole with putty. It is slower than a brad nailer, but the finished result is the same.

Should You Paint Before or After Installing Trim?

Prime and paint the trim boards before you install them, then touch up afterward. Painting first means you never have to tape off the walls or window glass, and once the casing is up you only need a quick pass over the filled nail holes and caulk lines. A pre-primed board also seals the wood against moisture from the first day.

How Do You Install Trim on a Window That Is Not Square?

Install the trim level and square anyway, then hide the difference; do not twist the casing to chase an out-of-square frame. Setting each piece plumb with a level leaves small, even gaps against the jamb, which you close with tapered shims and flexible caulk. For a tight look against a wavy wall, “scribe” the back edge of the trim to match the wall’s contour before nailing.

What Is the Difference Between Casing and Trim?

Casing is one specific kind of trim: the molding that frames a door or window. “Trim” is the umbrella term for every finish molding in a house, including baseboards, crown molding, and chair rails. So all casing is trim, but not all trim is casing, which is why a window’s own boards are called casing.

How Do You Keep PVC Window Trim From Buckling?

PVC window trim buckles when it has no room to expand, so give it room. Cellular PVC moves with temperature about ten times as much as wood, so fasten it with stainless-steel nails or screws plus a PVC adhesive, leave up to a 1/8-inch gap at butt joints, and install long runs when the material sits around 60 to 65 degrees. Never butt PVC hard against real wood, because the two expand at different rates and the joint will open.

The Bottom Line

Installing window trim requires patience and a steady hand, but the visual payoff is huge. Remember the golden rule: measure twice, cut once. Start with a less visible window (like a closet or spare bedroom) to practice your cuts, and by the time you reach the living room, you’ll be cutting miters like a pro.