A building envelope is the physical separator between the conditioned and unconditioned environment of a

building A building, or edifice, is an enclosed structure with a roof and walls standing more or less permanently in one place, such as a house or factory (although there's also portable buildings). Buildings come in a variety of sizes, shapes, and fu ...

including the resistance to air, water, heat, light, and noiseSyed, Asif. ''Advanced building technologies for sustainability''. Hoboken, N.J.: John Wiley & Sons, Inc., 2012. 115. Print. 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 Heating, ventilation, and air conditioning (HVAC) is the use of various technologies to control the temperature, humidity, and purity of the air in an enclosed space. Its goal is to provide thermal comfort and acceptable indoor air quality. HV ...

. 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:Straube, J.F., Burnett, E.F.P. ''Building Science for Building Enclosures''. Building Science Press, Westford, 2005. * 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 A roof ( : roofs or rooves) is the top covering of a building, including all materials and constructions necessary to support it on the walls of the building or on uprights, providing protection against rain, snow, sunlight, extremes of te ...

is to resist water. Two broad categories of roofs are flat and pitched.

Flat roof A flat roof is a roof which is almost level in contrast to the many types of sloped roofs. The slope of a roof is properly known as its pitch and flat roofs have up to approximately 10°. Flat roofs are an ancient form mostly used in arid c ...

s 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 Domestic roof construction is the framing and roof covering which is found on most detached houses in cold and temperate climates. Such roofs are built with mostly timber, take a number of different shapes, and are covered with a variety of mat ...

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 wall A cavity wall is a type of wall that has a hollow center. They can be described as consisting of two "skins" separated by a hollow space (cavity). The skins typically are masonry, such as brick or cinder block. Masonry is an absorbent material ...

s. 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 residential construction often treats walls as sealed-surface systems relying on the siding and an underlayment layer sometimes called

housewrap Housewrap (or house wrap), also known by the genericized trademark homewrap (or home wrap), generally denotes a modern synthetic material used to protect buildings. Housewrap functions as a weather-resistant barrier, preventing rain or other fo ...

. Moisture can enter basements through the walls or floor. Basement waterproofing and

drainage Drainage is the natural or artificial removal of a surface's water and sub-surface water from an area with excess of water. The internal drainage of most agricultural soils is good enough to prevent severe waterlogging (anaerobic condition ...

keep the walls dry and a moisture barrier is needed under the floor.

Air control

Control of airflow 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 windwashing (cold air passing through insulation) and convective loops which are air movements within a wall or ceiling that may result in 10% to 20% of the heat loss alone. The physical components of the envelope include the

foundation Foundation may refer to: * Foundation (nonprofit), a type of charitable organization ** Foundation (United States law), a type of charitable organization in the U.S. ** Private foundation, a charitable organization that, while serving a good cause ...

, walls, doors,

window A window is an opening in a wall, door, roof, or vehicle that allows the exchange of light and may also allow the passage of sound and sometimes air. Modern windows are usually glazed or covered in some other transparent or translucent mat ...

s, ceiling, and their related barriers and insulation. The dimensions, performance and compatibility of materials, fabrication process and details, 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 Damp proofing in construction is a type of moisture control applied to building walls and floors to prevent moisture from passing into the interior spaces. Dampness problems are among the most frequent problems encountered in residences. ''Damp p ...

) is essential in all climates, but cold climates and hot-humid climates are especially demanding. Air sealing can improve the energy efficiency of a building by minimizing the amount of energy needed to heat or cool the building. This is especially pertinent in cold-climate buildings where space heating consumes the largest amount of energy. A

blower door A blower door is a machine used to measure the airtightness of buildings. It can also be used to measure airflow between building zones, to test ductwork airtightness and to help physically locate air leakage sites in the building envelope. Ther ...

test can be used to test the quality of the air sealing of the building envelope. Smoke pencils can be used to detect gaps and caulking and weather-stripping can be used to improve air sealing. HVAC systems can ensure that a building’s air intake is both adequate, safe, and energy efficient.

Thermal envelope

The thermal envelope, or heat flow control layer, is part of a building envelope but may be in a different location such as in a ceiling. The difference can be illustrated by the fact 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 roofing material to the interior paint finish on the ceiling) is part of the building envelope. Building envelope

thermography Infrared thermography (IRT), thermal video and/or thermal imaging, is a process where a thermal camera captures and creates an image of an object by using infrared radiation emitted from the object in a process, which are examples of infrared i ...

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 interstitial condensation. Other types of anomalies that can be detected are thermal bridging, continuity of insulation and air leakage, however this requires a temperature differential between the inside and outside ambient temperatures.Faulkner, Ray. ''Infrared Building Surveys''. Portsmouth, United Kingdom: iRed, 2017.
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