Windows can be overshadowed in winter by trees, other buildings, fences, or the structure of the building itself which can significantly reduce solar access. For instance, east and west facing walls can shade adjacent north-facing windows, or side walls can also overshadow large windows in deep courtyards. Windows in courtyards need to be sized carefully in order not to lose heat through permanently shaded areas. In general, overshadowing will occur more on the lower part of walls than on the upper, hence full-height windows adjacent to side wall should be avoided and sill heights should be raised instead.