Trimming Existing Windows for Continuous Insulation

This isn't news you can act on at home — it's a homeowner posting a question on an online building forum, asking other builders for advice about a technical detail on his own renovation.

The project: he's re-siding his house and adding 2 inches of rigid wood-fiber insulation (a board that wraps the outside of the walls to cut drafts and heat loss) over a water-resistant barrier, with a gap behind the siding for drainage and drying, then new fiber-cement siding. He's replacing some windows but keeping most of his existing vinyl windows, which sit in their original wood frames. Because the new insulation adds 2 inches of thickness to the wall, the trim around the kept windows needs to be built out to match, and window sills need to shed water properly so rot doesn't start again — he'd already cut back some old sills that were rotting or had lead paint. He's asking whether to build out the trim with plywood and dimensional lumber, whether to tape and reuse the old sill after flashing it, or to plane the old sill down to good wood and add fresh material, all while keeping the small weep holes (drainage gaps built into vinyl windows) clear.

For a homeowner reading this, the takeaway is more conceptual than actionable: adding thick exterior insulation during a re-siding job changes how existing windows have to be trimmed and flashed, and getting the sill detail wrong risks water and rot problems down the line.