I'm installing a vinyl picture window that has a nailing flange. I'm using spray foam to insulate the gaps. I have a sill pan installed which is designed to collect and throw out any errant moisture. Is filling the gap on the lower edge going to interfere with this function? How would you prevent air leaks from the lower edge of the window otherwise? The window only insets into the framing about 2 inches so the foam would be hard to control.