How to re-insulate space between window trim and wall?

Question

Just took off old window mouldings (need to change them) and figure I will need to better insulate the space between the window trim and the wall. The "wall" consists of:

• a painted layer of thin 3mm (1/8 in) plywood or similar, followed by
• 2cm (3/4 inch) of styrofoam,
• after which there's something more solid that crumbles easily.

All are visible on the photo from right to left: plywood, styrofoam, crumbly solid stuff, some of the pink fiberglass insulation, white window trim.

The pink fiberglass insulation was either used rather sparingly or it somehow shrank with time, so there are gaps.

What's the most appropriate course of action? Add fiberglass insulation? Remove it and spray foam into the gap? Some combination of the two? Something else altogether?

Finally, there's already this question, but it's not about the trim/wall at all.

Answer 1

In the case of the fibreglass, it likely looks shrunken because of the way the batting was pressed into the shim space (gap between the window and wall frame) when the window was installed. As far as insulating the window goes, you can choose to remove the batting and replace it with batting that has a better R-rating (i.e. Roxul) or fill the void with low expansion spray foam insulation (i.e. Tremco Exoair L.E.F., or Nuco PF-100) (based on methods of install provided by my window company's Install team). Combining the two would be redundant as the batt can displace the expansion area of the foam and vice versa. Either option can work well on its own.