I don't understand where I'm supposed to add the casing. I just put in a replacement window. There's the old window frame, the new replacement window jamb and a little space between them. Am I supposed to add the casing to the old frame, or do I add a stop in the space between the new window and old frame and add the casing to this stop? I want the casing to be white so if I added to old frame, I'd have to paint the frame white

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Casing should almost always lap onto the current window jamb. It's the final edge for the wall finish. In your case, you have a slight level variation, which is usually handled with an extension jamb. You can also create a rabbet in your casing, which would result in an extension integrated into the casing.

In any case, the gap between the jambs should be insulated with fiberglass or low-expansion spray foam, and the casing should lap over the gap and onto the new jamb.

Ideally no joints are left flush. Each step should have a "reveal", creating a multi-level appearance. Flush joints are odd looking and difficult to get perfect.

  • The extension jamb can just be a 1x piece of lumber, right? I don't know if the gap between the two jambs is big enough to stuff insulation in there. Do you have an example of the rabbit casing? I'm assuming you can't do this with the casing you buy at like Menards?
    – user204588
    May 3 '18 at 16:46
  • You could rip extensions from any paintable material. "Rabbet". It just means taking a notch out of a board.
    – isherwood
    May 3 '18 at 18:26

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