After removing the old units, I take out the weights and pulleys and insulate the cavity.
I cut some fiberglass insulation to size, then use a strip of flexible metal to push it up into the pocket from the bottom access opening until I can see it at the top.
I pull the batt material back down snugly against the bottom of the pocket and fill the top of the pocket with small pieces. I replace the pocket covers, then cover the holes with foil-faced tape.
Next I provide a level bearing across the sill by using screws as shims.
For a typical unit, I’ll use three screws, one in the middle and one about an inch from each end.
I then apply a generous bead of caulk to the back edges of the casing and to the sill.
I fill the gaps at the base of each jamb and caulking between and on top of the shim screws.
At this point I prep the window for installation by tapping in the bottom expander and insulating the hollow extrusion as much as possible with low-expansion spray foam. I do the same with the top expander before fitting it to the top of the unit.