Between one of our wooden windows and the wall there is a gap of about 5 cm that is only very loosely filled with plaster (see below). The craftsman who did some other plastering work in our house suggested to cover the gap with a thin wooden panel instead of fully filling the gap with plaster. At that time I thought it would be a good idea and did not ask for the reason, but now I am wondering why, especially since I expect the plaster would provide better insulation.
Is there a reason to not fill the gap with plaster?
Edit: In Germany, I am used to windows that are recessed. In such a configuration, there is no gap as shown in the following photo.
Best Answer
I don't know where in the world you are, but in the U.S. no one fills that gap with plaster. We install "casing", or trim frames, around the window. Casing laps onto both the window jamb and the wall.
Image source
If you were to fill with plaster you'd want to reinforce the joint with fiberglass tape or similar. The seams between the new plaster, the wall, and the window are sure to crack.