I don't know about basement windows, but when I wanted them to mount mine on the wall of the garage, they made a big deal about how the wall might need to be reinforced to handle the additional load. The installer traveled several hours to get here, and in the interest of keeping things moving, I agreed to have them sit on the floor, while just using the wall anchors to prevent them from falling over. Having seen many PWs mounted on walls, I was never aware of any special structural requirements, but I understand the concern.