How to Trim Out a Window With Insulated Vinyl Siding

July 24, 2018



How to trim out a window using contoured insulation and vinyl siding


Select the right j-channel

There are two options for j-channel. The first is a standard channel, which ranges from 5/8 of an inch to 7/8 inch. The other channel from 1-1/8 inch to 1-1/2 inch. The wider j-channel must be used when installing insulated vinyl.

  • Have a cut edge going into the channel to maintain continuous insulation.
  • When installing FullbackV, slide the foam into the channel and then place the vinyl directly on top.
  • If you do not have access to a wider J, use a standard j-channel with a foam shim underneath. Slide the Fullback against the foam shim, and the siding into the pocket of the j-channel.


Capping the window

After choosing the j-channel to use, cap the window. When installing the j-channel around a window, push the j-channel against the window. If the j-channel sticks out past the window put a wood frame around the window to built it out.

  • Cap the wood frame with aluminum coil stock. The j-channel now is sitting flush with the window.
  • Install house wrap and a drip cap at the top of the window, properly overlapped.
  • Frame the widow with two by fours. Taper the bottom ledge to allow any water to drain away from the window.
  • Measure the width and height of the two by four frame. Cut a piece of coil two inches longer than the width for the top cap.
  • Cut two pieces one inch longer than the height for the side caps, and cut one piece to length for the bottom cap.


Bending the coil stock cap

To bend the coil, measure the inside, the face, and the outside of your frame.

  • Your first bend will be the inside measurement
  • The second bend will be the face measurement
  • The third bend will be the outside measurement.


Notching and mitering corners

Notch the corners on both edges of the bottom cap, and on both side caps on the top.

  • Start by laying the bottom cap flush against one side.
  • Mark the inside of the window, and then one-inch past that mark. Cut on each side all the way to the mark and remove the extra material.
  • Cut to the second mark and bend to create a tab. This creates a notched corner.

Miter the corners at the four remaining ends:

  • Mark the inside corner of the window and draw a 45 angle.
  • Leave a one-inch tab at the bottom and cut away the remaining material.
  • Place the cap on the window and fold the tab downward. Repeat for the other side.
  • Add the top piece with two mitered edges. Refer to the manufacturer's instructions and the local building codes for nail, caulk, and flashing specs.


Whether capping the window or not, the steps for installing J-channel are the same

  • Measure the width and height of the window.
  • Cut a piece of j-channel two inches longer than the width to use on the bottom.
  • Cut two pieces three inches longer than the height for the sides, and cut a piece four inches longer for the top.
  • Notch the corners on both ends of the bottom j-channel and at the tops of the side j-channels.
  • Miter the corners on the four remaining ends. 
  • For notched corners, remove material equal to the width of your j-channel face.
  • For mitered corners, mark the inside corner of the window and cut a 45-degree angle.
  • Remove a small piece from the J-channel to provide more wiggle room.
  • Insert the tab from the mitered corner into the notched corner. Repeat for the other side.
  • Then add the top piece with two mitered corners.

The window trim is now complete.

As you can see, this method creates a beautiful premium finished look. 


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—This video is from Progressive Foam Technologies, one of ProTradeCraft's advertisers. Founded in 1992, Progressive Foam invented and brought insulated vinyl siding to the marketplace and have since diversified to add siding insulation products that can be paired with fiber cement, engineered wood, steel, and others. See all of Progressive Foam Technology's videos.