The design of fenestration (windows, skylights, etc) requires special attention because of the huge variety of available building components and the several important roles that windows play. Perhaps thermally most important, they admit solar radiation. This is often advantageous in the winter and disadvantageous in the summer. Also, despite dramatic improvements, glazing still usually has the lowest R value (highest U-factor) of all components of an envelope. Windows and skylights also admit daylight to buildings and often provide desired ventilation.