How to waterproof residential windows, and lessons learned from fixing window leaks

In residential projects, window leaks may look like a minor issue, but they are a huge hassle to fix. I once encountered a new house where, after the windows were installed and the exterior walls were sealed, the edge of the interior windowsill became damp after the first heavy rain. Upon opening it up, we found that the window itself wasn't defective; rather, the rough opening and the…