Double-glazed windows have two panes of glass instead of one, with a small air space between them, which makes them more energy-efficient than regular single-sheet windows. If you’re replacing your old double-glazed windows, and you’re not using double-glazed replacement windows, you’re going backward, efficiency-wise. Double-glazed replacement windows install in the same manner as other kinds.
Stand the new double-glazed window unit in the opening, bottom-first. Push the top end back into the opening until the exterior border of trim stops it.
Shoot spray foam insulation into the spaces around the edge of the window unit, filling the spaces.