A building envelope is the physical separator between the conditioned and unconditioned environment of a building including the resistance to air, water, heat, light, and noise transfer. Discussion The building envelope (or the more modern term, building enclosure) is all of the elements of the outer shell that maintain a dry, heated, or cooled indoor environment and facilitate its climate control. Building envelope design is a specialized area of architectural and engineering practice that draws from all areas of building science and indoor climate control. The many functions of the building envelope can be separated into three categories: Support (to resist and transfer structural and dynamic loads) Control (the flow of matter and energy of all types) Finish (to meet desired aesthetics on the inside and outside) The control function is at the core of good performance, and in practice focuses, in order of importance, on rain control, air control, heat control, and vapor control. Water and water vapor control Control of rain is most fundamental, and there are numerous strategies to this end, namely, perfect barriers, drained screens, and mass / storage systems. One of the main purposes of a roof is to resist water. Two broad categories of roofs are flat and pitched. Flat roofs actually slope up to 10° or 15° but are built to resist intrusion from standing water. Pitched roofs are designed to shed water but not resist standing water intrusion which can occur during wind-driven rain or ice damming. Typically residential, pitched roofs are covered with an underlayment material beneath the roof covering material as a second line of defense. Domestic roof construction may also be ventilated to help remove moisture from leakage and condensation. Walls do not get as severe water exposure as roofs but still leak water. Types of wall systems with regard to water penetration are barrier, drainage and surface-sealed walls. Barrier walls are designed to allow water to be absorbed but not penetrate the wall, and include concrete and some masonry walls. Drainage walls allow water that leaks into the wall to drain out such as cavity walls. Drainage walls may also be ventilated to aid drying such as rainscreen and pressure equalization wall systems. Sealed-surface walls do not allow any water penetration at the exterior surface of the siding material. Generally most materials will not remain sealed over the long term and this system is very limited, but ordinary