The building envelope (or building enclosure) is the physical separator between the interior and the exterior environments of a building. It serves as the outer shell to help maintain the indoor environment (together with the mechanical conditioning systems) 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 mechanical loads)
- Control (the flow of matter and energy of all types)
- Finish (to meet human desires 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.
Control of rain is most fundamental, and there are numerous strategies to this end, namely, perfect barriers, drained screens, and mass / storage systems.
Control of air flow is important to ensure indoor air quality, control energy consumption, avoid condensation (and thus help ensure durability), and to provide comfort. Control of air movement includes flow through the enclosure (the assembly of materials that perform this function is termed the air barrier system) or through components of the building envelope (interstitial) itself, as well as into and out of the interior space, (which can affect building insulation performance greatly). Hence, air control includes the control of wind washing and convective loops.
The physical components of the envelope include the foundation, roof, walls, doors and windows. The dimensions, performance and compatibility of materials, fabrication process and details, their connections and interactions are the main factors that determine the effectiveness and durability of the building enclosure system.
Common measures of the effectiveness of a building envelope include physical protection from weather and climate (comfort), indoor air quality (hygiene and public health), durability and energy efficiency. In order to achieve these objectives, all building enclosure systems must include a solid structure, a drainage plane, an air barrier, a thermal barrier, and may include a vapor barrier. Moisture control (e.g. damp proofing) is essential in all climates, but cold climates and hot-humid climates are especially demanding.
The thermal envelope (or heat flow control layer) is usually different than the building envelope. The difference can be illustrated by understanding that an insulated attic floor is the primary thermal control layer between the inside of the house and the exterior while the entire roof (from the surface of the shingles to the interior paint finish on the ceiling) comprises the building envelope.
Building envelope thermography involves using an infrared camera to view temperature anomalies on the interior and exterior surfaces of the structure. Analysis of infrared images can be useful in identifying moisture issues from water intrusion, or internal condensation.