In winter, the distance D between the window head and the underside of the eave or overhang, is critical to the window’s performance. If D is close to zero, meaning the window head is directly under the overhang, and A is greater than zero, the top of the window will never admit sun, and this area of the window will suffer net heat loss during a sunny day. Height D must be designed to ensure that winter sun is admitted through the entire window.