XPS around windows?

A homeowner installing new triple-pane windows ran into an unusually wide gap — up to 2 inches in places — between the window frames and the rough openings in the wall. That gap matters because it has to be sealed properly to keep the home airtight; gaps like this are where outside air can leak in and out, driving up heating and cooling costs. The homeowner had already stuffed the gaps with mineral wool insulation (a fire-safe alternative to spray foam) and was looking for an affordable way to seal the inside edge. They asked whether rigid foam board (XPS) left over from a basement project, combined with a flexible sealant tape, could work as well as the specialty foam backer rod and tape products usually used for this job, since those products add up in cost on a house with many large windows.

This is a hands-on construction question posted to an online builder forum, not a product or policy announcement, so there is no deadline or rebate tied to it. For most homeowners, the takeaway is simply a reminder that gaps around new windows need a continuous air seal, not just insulation stuffed into the opening — insulation alone does not stop air movement the way a taped or sealed barrier does. If you are planning a window replacement, it is worth asking whoever does the installation how they plan to air seal the rough opening, since larger-than-expected gaps are apparently not unusual and can need extra material and labor to handle correctly.