All openings in the building envelope, including curtain walls, windows, doors, and skylights, are referred to as fenestration systems. Fenestration is typically seen as an aesthetic element, but it also has an impact on building performance. For example, energy efficiency can be improved by using low-emission materials for windows and doors. Color can be an important factor in determining how people feel about a building; for example, a blue facade will reflect light into the interior while a white facade will transmit light into the room. There are two main types of fenestration systems: fixed and movable. A fixed system cannot be moved once installed, whereas with a movable system the window or door can be removed from its frame (usually by a professional glazier) and replaced with another unit. This allows for replacement of old property owners who may want to change the appearance of their building or replace broken windows without having to pay to have the original system removed and replaced. Fixed fenestration systems include: windows, sliding glass doors, and patio doors. They are installed before the structure is completed so they do not move when constructed over an opening such as a garage or alley. These fenestrations allow natural light into the building and provide views outside, but due to their shape, they are generally less efficient at keeping out heat than other fenestration types.