Best Approach to Basement Window

A homeowner with a 1970s house in a cold climate zone is asking for advice on replacing four basement windows set in cinder block walls with wood frames. The basement isn't heated or cooled but houses the home's furnace and other HVAC equipment, and the owner is also dealing with insulation and radon concerns down there. The question is a practical one: is it better to buy a custom window that fits inside the existing wood frame, leaving the old frame and exterior untouched, or to remove the wood frame entirely and install a standard-sized window directly into the masonry opening.

This is a discussion post on a building-science forum, not a finished how-to guide, and no replies had been posted yet. So there's no expert verdict here on which method seals and insulates better. The homeowner's own instinct leans toward removing the old wood frame and mounting a window straight into the cinder block opening, using shims, spray foam, and concrete screws to secure it, then caulking the outside and finishing the interior and exterior trim. The alternative, simpler approach would keep the existing frame in place and just swap in a new sash sized to fit inside it.

For anyone with similar old basement windows, the tradeoffs are the same: keeping the original frame is less work but may leave old wood and gaps that leak air, while removing it entirely and sealing directly against the masonry can give a tighter, better-insulated result at the cost of more demolition and a less standard installation.