Building envelope

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A building envelope or building enclosure is the physical separator between the conditioned and unconditioned environment of a building, including the resistance to air, water, heat, [1] light, and noise [2] transfer.

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Discussion

The building envelope or 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. [2]

The many functions of the building envelope can be separated into three categories: [3]

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. [3]

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. [4]

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

Moisture can enter basements through the walls or floor. Basement waterproofing and drainage 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 [6] (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. [7]

The physical components of the envelope include the foundation, roof, walls, doors, windows, 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) is essential in all climates, but cold climates and hot-humid climates are especially demanding. [8]

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. [9] A blower door 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. [10] 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. [11]

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 interstitial condensation. [12] 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. [13]

See also

Related Research Articles

<span class="mw-page-title-main">Passive solar building design</span> Architectural engineering that uses the Suns heat without electric or mechanical systems

In passive solar building design, windows, walls, and floors are made to collect, store, reflect, and distribute solar energy, in the form of heat in the winter and reject solar heat in the summer. This is called passive solar design because, unlike active solar heating systems, it does not involve the use of mechanical and electrical devices.

<span class="mw-page-title-main">Thermal insulation</span> Minimization of heat transfer

Thermal insulation is the reduction of heat transfer between objects in thermal contact or in range of radiative influence. Thermal insulation can be achieved with specially engineered methods or processes, as well as with suitable object shapes and materials.

<span class="mw-page-title-main">Earth shelter</span> House partially or entirely surrounded by earth

An earth shelter, also called an earth house, earth bermed house, or underground house, is a structure with earth (soil) against the walls, on the roof, or that is entirely buried underground.

<i>R</i>-value (insulation) Measure of how well an object, per unit of area, resists conductive flow of heat

In the context of construction, the R-value is a measure of how well a two-dimensional barrier, such as a layer of insulation, a window or a complete wall or ceiling, resists the conductive flow of heat. R-value is the temperature difference per unit of heat flux needed to sustain one unit of heat flux between the warmer surface and colder surface of a barrier under steady-state conditions. The measure is therefore equally relevant for lowering energy bills for heating in the winter, for cooling in the summer, and for general comfort.

<span class="mw-page-title-main">Radiant barrier</span>

A radiant barrier is a type of building material that reflects thermal radiation and reduces heat transfer. Because thermal energy is also transferred by conduction and convection, in addition to radiation, radiant barriers are often supplemented with thermal insulation that slows down heat transfer by conduction or convection.

<span class="mw-page-title-main">Exterior insulation finishing system</span> Non-load bearing building cladding

Exterior insulation and finish system (EIFS) is a general class of non-load bearing building cladding systems that provides exterior walls with an insulated, water-resistant, finished surface in an integrated composite material system.

<span class="mw-page-title-main">Vapor barrier</span> Damp proofing material in sheet form

A vapor barrier is any material used for damp proofing, typically a plastic or foil sheet, that resists diffusion of moisture through the wall, floor, ceiling, or roof assemblies of buildings and of packaging to prevent interstitial condensation. Technically, many of these materials are only vapor retarders as they have varying degrees of permeability.

<span class="mw-page-title-main">Superinsulation</span> Method of insulating a building

Superinsulation is an approach to building design, construction, and retrofitting that dramatically reduces heat loss by using much higher insulation levels and airtightness than average. Superinsulation is one of the ancestors of the passive house approach.

<span class="mw-page-title-main">Domestic roof construction</span> The supporting structure of a roof

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 materials.

<span class="mw-page-title-main">Ground-coupled heat exchanger</span> Underground heat exchanger loop that can capture or dissipate heat to or from the ground

A ground-coupled heat exchanger is an underground heat exchanger that can capture heat from and/or dissipate heat to the ground. They use the Earth's near constant subterranean temperature to warm or cool air or other fluids for residential, agricultural or industrial uses. If building air is blown through the heat exchanger for heat recovery ventilation, they are called earth tubes.

<span class="mw-page-title-main">Passive cooling</span> Building design that reduces inside temperatures without air conditioning

Passive cooling is a building design approach that focuses on heat gain control and heat dissipation in a building in order to improve the indoor thermal comfort with low or no energy consumption. This approach works either by preventing heat from entering the interior or by removing heat from the building.

<span class="mw-page-title-main">Building insulation</span> Material to reduce heat transfer in structures

Building insulation is material used in a building to reduce the flow of thermal energy. While the majority of insulation in buildings is for thermal purposes, the term also applies to acoustic insulation, fire insulation, and impact insulation. Often an insulation material will be chosen for its ability to perform several of these functions at once.

<span class="mw-page-title-main">Building insulation material</span>

Building insulation materials are the building materials that form the thermal envelope of a building or otherwise reduce heat transfer.

<span class="mw-page-title-main">Thermal bridge</span>

A thermal bridge, also called a cold bridge, heat bridge, or thermal bypass, is an area or component of an object which has higher thermal conductivity than the surrounding materials, creating a path of least resistance for heat transfer. Thermal bridges result in an overall reduction in thermal resistance of the object. The term is frequently discussed in the context of a building's thermal envelope where thermal bridges result in heat transfer into or out of conditioned space.

<span class="mw-page-title-main">Spray foam</span> Building material

Spray foam is a chemical product created by two materials, isocyanate and polyol resin, which react when mixed with each other and expand up to 30-60 times its liquid volume after it is sprayed in place. This expansion makes it useful as a specialty packing material which forms to the shape of the product being packaged and produces a high thermal insulating value with virtually no air infiltration.

<span class="mw-page-title-main">Rainscreen</span>

A rainscreen is an exterior wall detail where the siding stands off from the moisture-resistant surface of an air/water barrier applied to the sheathing to create a capillary break and to allow drainage and evaporation. The rainscreen is the cladding or siding itself but the term rainscreen implies a system of building. Ideally the rainscreen prevents the wall air/water barrier from getting wet but because of cladding attachments and penetrations water is likely to reach this point, and hence materials are selected to be moisture tolerant and integrated with flashing. In some cases a rainscreen wall is called a pressure-equalized rainscreen wall where the ventilation openings are large enough for the air pressure to nearly equalize on both sides of the rain screen, but this name has been criticized as being redundant and is only useful to scientists and engineers.

Dynamic insulation is a form of insulation where cool outside air flowing through the thermal insulation in the envelope of a building will pick up heat from the insulation fibres. Buildings can be designed to exploit this to reduce the transmission heat loss (U-value) and to provide pre-warmed, draft free air to interior spaces. This is known as dynamic insulation since the U-value is no longer constant for a given wall or roof construction but varies with the speed of the air flowing through the insulation. Dynamic insulation is different from breathing walls. The positive aspects of dynamic insulation need to be weighed against the more conventional approach to building design which is to create an airtight envelope and provide appropriate ventilation using either natural ventilation or mechanical ventilation with heat recovery. The air-tight approach to building envelope design, unlike dynamic insulation, results in a building envelope that provides a consistent performance in terms of heat loss and risk of interstitial condensation that is independent of wind speed and direction. Under certain wind conditions a dynamically insulated building can have a higher heat transmission loss than an air-tight building with the same thickness of insulation. Often the air enters at about 15 °C.

Interstitial condensation is a type of condensation that may occur within an enclosed wall, roof or floor cavity structure, which can create dampening.

Building airtightness can be defined as the resistance to inward or outward air leakage through unintentional leakage points or areas in the building envelope. This air leakage is driven by differential pressures across the building envelope due to the combined effects of stack, external wind and mechanical ventilation systems.

<span class="mw-page-title-main">Ice dam (roof)</span>

An ice dam is an ice build-up on the eaves of sloped roofs of heated buildings that results from melting snow under a snow pack reaching the eave and freezing there. Freezing at the eave impedes the drainage of meltwater, which adds to the ice dam and causes backup of the meltwater, which may cause water leakage into the roof and consequent damage to the building and its contents if the water leaks through the roof.

References

  1. ↑ Cleveland, Cutler J., and Christopher G. Morris. Building envelopergy. Expanded Edition. Burlington: Elsevier, 2009. Print.
  2. 1 2 Syed, Asif. Advanced building technologies for sustainability. Hoboken, N.J.: John Wiley & Sons, Inc., 2012. 115. Print.
  3. 1 2 Straube, J.F., Burnett, E.F.P. Building Science for Building Enclosures. Building Science Press, Westford, 2005.
  4. ↑ 11. Straube, J.F. and Burnett, E.F.P., "Rain Control and Design Strategies". Journal of Thermal Insulation and Building Envelopes, July 1999, pp. 41–56.
  5. ↑ various authors. Guideline for condition assessment of the building envelope. Reston, Va.: American Society of Civil Engineers, 2000. 4. Print.
  6. ↑ Hens, Hugo S. L. C. Performance Based Building Design 2: From Timber-framed Construction to Partition Walls. Berlin: Ernst, William & Son, 2012. 10. Print.
  7. ↑ Harrje, D. T, G. S. Dutt and K. J. Gadsby, "Convective Loop Heat Losses in Buildings". Oak Ridge National Laboratory. 1985. Print. Archived November 2, 2013, at the Wayback Machine
  8. ↑ Lstiburek, Joseph W., and John Carmody. Moisture Control Handbook: Principles and Practices for Residential and Small Commercial Buildings. New York: Van Nostrand Reinhold, 1993. 88. Print.
  9. ↑ Asaee, S. Rasoul; Sharafian, Amir; Herrera, Omar E.; Blomerus, Paul; Mérida, Walter (May 2018). "Housing stock in cold-climate countries: Conversion challenges for net zero emission buildings". Applied Energy. 217: 88–100. Bibcode:2018ApEn..217...88A. doi:10.1016/j.apenergy.2018.02.135.
  10. ↑ Canada, Natural Resources (2014-03-06). "Keeping The Heat In - Section 4: Comprehensive air leakage control in your home". www.nrcan.gc.ca. Retrieved 2022-03-26.
  11. ↑ Vliet, Willem. The Encyclopedia of Housing. Thousand Oaks, Calif.: Sage, 1998. 139. Print.
  12. ↑ Hunaidi, Osama. Leak Detection Methods for Plastic Water Distribution Pipes. Denver, Colo.: AWWA Research Foundation, 1999. 57. Print.
  13. ↑ Faulkner, Ray. Infrared Building Surveys. Portsmouth, United Kingdom: iRed, 2017.
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