Definitions for Building Performance

Building enclosure: The portion of a building that provides a continuous environmental separation between interior space and the exterior environment. A building enclosure comprises a coordinated set of construction assemblies and the connections between those assemblies in order to protect occupants and their property from the harmful and uncomfortable effects of local climate and other ambient conditions. Control of the passage of heat, air, and moisture through the building enclosure determines the critical performance requirements for building enclosure design. We therefore refer to control layers in describing assemblies that comprise the building enclosure. We hope this glossary will aid in the clear and consistent communications between all members of the design and construction team in order to make well-informed decisions of this type.

Building enclosure assembly: An assembly of materials, including control layers, forming a portion of the complete building enclosure, such as roofs, walls, fenestration, doors, foundation walls, floors, slabs-on-grade, and soffits, that provide environmental separation between the interior space and the exterior environment.

Conditioned space: A portion of a building surrounded by the building enclosure or environmental separator, within which air temperature and humidity are controlled to provide conditions that differ from the exterior or other adjacent environments.

Control layer: Materials or assemblies of materials that provide adequate control, as defined below, of the passage of heat, air, and moisture. Control does not imply perfection but instead a level of performance adequate to suit the intended use; exact performance as determined through laboratory testing varies according to building type, exposure, risk, and other factors. Adequate control is that level of performance necessary for the enclosure component to comply with the following: applicable codes, owner’s requirements, project-specific expectations, and quantitative performance as indicated for the individual functions.1

Environmental separator: The portion of a building that provides a continuous environmental separation between two spaces or environments that have different temperature, pressure, and humidity conditions and which comprises a coordinated set of construction assemblies and connections between those assemblies. Separations may be between interior and exterior environments or between two differing interior environments.

References:
Altenhofen, David W., Michael J. Lough, and Drake A. Wauters. 2015_AIA_Rain Water Penetration Wall Assemblies 25 Apr 15 DWA (2). PDF. May 13, 2015.

Air-barrier system: As defined by Air Barriers Association of America (ABAA), this term is commonly used in practice and within the industry to refer to the air-control layer. It is therefore is acceptable, but “air-control layer” is preferred. In common practice, the air-barrier system is commonly and erroneously thought of as the air-control layer located in the exterior walls, as opposed to air-control layers in roofs and foundations.1 2

Air-control layer: The layer in a building enclosure assembly that controls the infiltration and exfiltration of air through construction between any two spaces with differing environmental conditions. It comprises materials and assembly of materials that are impermeable to air, connected with sealed joints and transitions between adjacent materials to form a three-dimensional assembly that encloses the building on all sides. A structurally supported air-control layer is required within each wall and roof assembly in the building enclosure. It is also required as an environmental separator between such spaces as a surgery suite and an office space, or between units in multi-family and apartment construction to control pressure differences and the transfer of moisture and airborne pollutants. Air Barriers Association of America (ABAA) sets performance criteria for air barriers that are useful to establish minimum performance.3

Air leakage: Defined by the Air Barrier Association of America (ABAA) as "the flow of air through unintended openings in the building enclosure, which is driven by either or both positive (exfiltration) or negative (infiltration) pressure differences." Pressure differences can be due to wind, mechanical system operation, and to the stack effect that results from buoyancy of warm air.4

Breathe or breathable: As in "a building needs to breathe." This is an incorrect term to describe an outdated strategy to deal with water vapor and gas accumulation in buildings. While some enclosure components are more or less permeable to air and vapor, "breathe" does not specify which is being exhausted or in which direction. "Breathe" does not address the intended or unintended outcome of the strategy. The air exchange between the exterior environment and enclosed space must occur through engineered ventilation and intentional openings through a building enclosure that incorporates heat-, air-, and moisture-control layers that allow for drying of the building enclosure to the exterior while preventing the uncontrolled movement of air through the enclosure. In addition, wall assemblies should be designed to dry via progressive vapor permeance, moving toward the exterior from the location where the most vapor-impermeable layers are concentrated.

House wrap: Same as building wrap. Use the term “building wrap” instead of “house wrap.”

Vapor-control layer: Interconnected materials or assemblies of materials and sealed joints that control the movement of water by vapor diffusion. Hygrothermal analysis can be used to determine the appropriate vapor permeance of the air-control layer. It can also be used to determine if one is needed at all and where a separate vapor-control layer should be placed within the assembly. If necessary, the vapor-control layer is typically a membrane that has been selected for its water vapor permeance, depending on its location within the assembly relative to the potential location of the dewpoint under anticipated temperature and relative humidity conditions. The most effective vapor-control layers are continuous and have taped seams. In some cases, assembly materials with other purposes may also function as vapor-control layers. In assemblies with more than one vapor-retarding material, analysis is required to avoid trapping moisture between those materials.

Water vapor permeability: Theoretical property that measures the diffusion of moisture through a material in response to a difference in water vapor pressure from one face of the material to the other. Typically measured for a one-inch thickness and reported in perm-inches, permeability can generally be divided by the thickness of a material of a particular thickness to estimate its permeance, although surface conditions will have an additional effect.

Water vapor permeance: A measure of the diffusion of moisture through a particular solid material of a given thickness in response to a difference in water vapor pressure from one face of the material to the other. Permeance is expressed in perms and is the basis of the following classification, in accordance with the International Building Code, Chapter 14: class I vapor retarder—less than 0.1 perm; class II vapor retarder—between 0.1 and 1.0 perms; class III vapor retarder—between 1.0 and 10 perms.

References:
ABAA Master Specification SECTION 01 41 00 THE AIR BARRIER SYSTEM. DOC. Air Barrier Association of America, September 4, 2009.

Straube, John Frederick. 2012. High performance enclosures: design guide for institutional commercial and industrial buildings in cold climates. Somerville, Mass: Building Science Press.

Lstiburek, Joseph. "BSI-024: Vocabulary." Building Science Corporation. October 15, 2009. Accessed February 19, 2018. https://buildingscience.com/documents/insights/bsi-024-vocabulary.

Standard Method for Building Enclosure Airtightness Compliance Testing. PDF. Air Barrier Association of America, August 25, 2016.

Air-barrier assembly: An assembly of membrane, transition materials, sealants, and other components required to provide a continuous air-control layer that is capable of accommodating substrate movement. The International Building Code requires an air-leakage value of no more than 0.04 cfm/sq. ft. of surface area at 1.57 lbf/sq. ft. (0.2 L/s x sq. m of surface area at 75 Pa), when tested according to ASTM E2357.

Air-barrier membrane: A sheet-applied or fluid-applied membrane material that is impermeable to air. The International Building Code requires an air leakage value of no more than 0.004 cfm/sq. ft. of surface area at 1.57 lbf/sq. ft. (0.02 L/s x sq. m of surface area at 75 Pa), when tested according to ASTM E2178.

Backup wall: The portion of an exterior wall assembly that transfers external wind loads to the primary structure and provides a continuous surface to which control layers may be applied. The two most common types of backup wall comprise either stud framing with sheathing or concrete unit masonry.

Cladding: The primary component of the rain-shedding layer, which forms a veneer on the building to provide protection of the underlying layers by shedding or absorbing water striking the surface. Commonly used cladding materials include non-load-bearing masonry, thin panels that are supported by brackets, and framing mounted to the backup wall. For effective control of rain, joints between cladding elements incorporate features such as overlap, shingling, shiplap, backup plate, sealant, gasket, spline, or other closure. The International Building Code refers to cladding as a wall covering, but “cladding” or “rain-shedding layer” is preferred, because “wall covering” is a term typically used for interior wall coverings. Mass masonry, poured concrete, and barrier walls are not considered cladding.1

Drainage cavity: The space between cladding and the water-control layer, which serves to collect water from precipitation or condensation. Water that collects in the drainage cavity is drained to the exterior by the water-control layer and flashing. In cavity wall assemblies with open joints or vents, moisture in the drainage cavity may also be removed by evaporation. The thermal-control layer in the form of continuous insulation is usually located within the drainage cavity.

Flashing: A component of a building enclosure assembly that works with gravity to collect and divert bulk rainwater or condensation to the exterior surface of the assembly where it can be drained. Flashing typically extends from an internal water-control layer to the exterior. Flashing materials are waterproof sheet materials with sufficient flexibility to be field-formed or shop-formed and sealed in the sometimes complex configurations needed to provide a waterproof transition between primary façade elements of a building enclosure system. Without soldered joints, it is difficult to make sheet metal flashing permanently waterproof.2

Open-grille cladding: Cladding with openings that make up more than 5 percent of the surface area. Examples of open-grille cladding include perforated panels, grilles, and screens, which are decorative in nature but do not serve as an effective rain-shedding layer. Without the protection of rain-shedding cladding to limit bulk water penetration into the cavity, the water-control layer behind open-grille cladding performs essentially as a face-sealed barrier system. Open-grille cladding can be made more effective by adding a second water-resistive barrier to limit bulk water penetration into the cavity. Ultraviolet light will penetrate joints and adversely affect materials.

Open-joint cladding: Cladding where all or some of the joints between the individual panels or pieces of cladding are open to the cavity behind without overlap, shingling, shiplap, backup plate, sealant, gasket, spline, or other closure. The openings shall be less than or equal to 5 percent of the surface area. This type of cladding can be decorative. Compared to a rainscreen, this type of cladding is less effective at controlling bulk water infiltration, as more water is allowed behind the first layer of cladding and drained through the cavity by the water-control layer. Water-control layer requirements are substantially more stringent in open-joint cladding systems. Ultraviolet light will penetrate open joints.3

Rainscreen: Cladding with joints designed to reduce the amount of rain entering the drainage cavity, with strategically placed openings as required to enhance air movement within the drainage cavity in a DBVC or with compartmentation for a PER rainscreen wall assembly. (See above definitions for DBVC and PER.) Rainscreen cladding is designed to resist the first four forces that drive water across a gap: gravity, kinetic movement, surface tension, and capillary action. These forces are resisted with overlap, shingling, shiplap, backup plate, sealant, gasket, spline, or other closure. Rainscreen is sometimes incorrectly applied to open-joint cladding and open-grille cladding and to a variety of building wraps with some minor provision for drainage.4

Rain-shedding layer: The outermost layer of the building assembly designed to reject bulk water. This layer comprises multiple components. A rainscreen is a specialized form of a rain-shedding layer, as defined below.

Reservoir: An absorptive material in the building enclosure assembly that can retain some amount of water vapor within its bulk at a given temperature and relative humidity without damage to that material. Brick and concrete are common examples of materials that serve as reservoirs within exterior wall assemblies. A reservoir can buffer the effects of vapor movement through an assembly.

Whole-building air leakage: Air leakage measured for an entire building enclosure. The International Building Code requires a whole-building air leakage value of no more than 0.04cfm/sq. ft. of surface area at 1.57 lbf/sq. ft. (2.0 L/s x sq. m of surface area at 75 Pa), when tested according to ASTM E779, but more stringent standards exist. Be aware of other discussions covering the measurement of air leakage during the test.

References:

Altenhofen, David W., Michael J. Lough, and Drake A. Wauters. 2015_AIA_Rain Water Penetration Wall Assemblies 25 Apr 15 DWA (2). PDF. May 13, 2015.

Lemieux, Daniel J, and Paul E Totten. “Wall Systems.” WBDG Whole Building Design Guide, National Institute of Building Sciences, 5 Oct. 2016, www.wbdg.org/guides-specifications/building-envelope-design-guide/wall-systems.

Altenhofen, David W., Michael J. Lough, and Drake A. Wauters. 2015_AIA_Rain Water Penetration Wall Assemblies 25 Apr 15 DWA (2). PDF. May 13, 2015.

Ibid.

The following is a list of the principal wall assembly types for the control of water infiltration. There are a variety of methods to integrate thermal control, air control, and vapor control into the assemblies.

Barrier wall assembly: An exterior wall assembly that relies principally upon the weather-tight integrity of the outermost exterior wall surfaces and construction joints to control water penetration. Because barrier wall assemblies rely on perfectly sealed joints within a single layer, they provide limited protection with no redundancy. Some lower-performance curtain walls are barrier wall assemblies. Interchangeable terms: face-sealed wall assembly and face-sealed barrier wall assembly. 2

Cavity wall assembly: An exterior wall assembly that relies on a series of control layers and an internal drainage cavity to control moisture. Cladding functions as a rain-shedding layer that either sheds or absorbs the majority of rainwater incident on its surface. Behind the cladding, the drainage cavity collects and controls rainwater that penetrates through joints and other openings in the cladding. Thermal insulation is frequently located within the drainage cavity. To drain properly, drainage cavities are typically wide enough to allow for a minimum 3/8-inch-wide air space between the face of the insulation and the back of the cladding. On the exterior face of the backup wall, the water-control layer serves as the primary line of defense against uncontrolled rainwater penetration, incorporating flashing to direct the water to the building exterior. This layer serves functionally as the dividing line between the wet and dry sections, or zones, of the exterior wall assembly. 3

Combined-control layers: A single construction material or set of connected construction materials that performs more than one function in controlling heat, air, and moisture within a building enclosure assembly.

Mass wall assembly: An exterior wall assembly that relies principally upon a combination of wall thickness and moisture storage capacity to effectively control water penetration and provide energy storage. Concrete, either precast or cast-in-place, are modern examples of mass wall assemblies. Traditional multi-wythe masonry walls are mass wall assemblies.4

Open-joint cladding wall assembly: A cavity wall assembly in which all or some of the joints between the individual cladding elements are open to the drainage cavity without overlap, shingling, shiplap, backup plate, sealant, gasket, spline, or other closure to reduce bulk rain penetration. This type of assembly allows significant water into the drainage cavity, which places a higher burden of performance on the water-control layer. The open joints also increase the risk of exposure of the water-control membrane to damaging ultraviolet radiation. An extra water-shedding layer can be located directly behind open-joint cladding to improve the assembly’s moisture-control performance and decrease exposure to ultraviolet radiation. Open-joint cladding wall assemblies are often incorrectly categorized as rainscreen walls.

Rainscreen wall assembly: A type of cavity wall that is designed and detailed to reduce the movement of water through joints in cladding while promoting both drainage and air movement within the drainage cavity. Rainscreen walls are comprised of an exterior cladding, a cavity, and an inner water-control layer. Rainscreens are subdivided into two distinct performance categories: (1) pressure equalized rainscreen wall assembly and (2) drained and ventilated cavity wall assembly. Rainscreen walls are designed to control water at the outer cladding, but they still require a robust water-control layer behind them. There are a variety of methods to resist the penetration of rain and snow. The following is a list of the rainscreen wall types for the control of water infiltration:

• Drained and back-ventilated cavity wall assembly (DBVC): A rainscreen wall with a minimum 3/8-inch-wide cavity in which air movement is promoted by vents through the bottom and top of the cladding. The cladding resists the first four forces that drive water across a gap: gravity, kinetic movement, surface tension, and capillary action. A water-control layer at the face of the backup wall construction provides further control of water that enters the cavity. An air-control layer is critical to reducing infiltration of moisture-laden air through the backup wall, and it also reduces water infiltration across the cladding joints.  
• Pressure-equalized rainscreen wall assembly (PER): A rainscreen wall with the basic characteristics of DBVC and in which the cavity is compartmentalized to control air pressure difference, the fifth force that drives water across a gap. Compartmentalization is achieved by air-tight elements that extend from the back of the cladding to the air-control layer, which divide the cavity into areas related to their proximity to the edge or top of the wall. An air-control layer must also be provided at the face of the backup wall, which is also the drainage plane. Each compartment is ventilated separately with vents through the cladding at the top and bottom of the compartment. Well-constructed PER wall assemblies provide superior performance in terms of heat, air, and moisture control.  

References

2. Lemieux, Daniel J, and Paul E Totten. “Wall Systems.” WBDG Whole Building Design Guide, National Institute of Building Sciences, 5 Oct. 2016, www.wbdg.org/guides-specifications/building-envelope-design-guide/wall-systems.

3. ibid.

4. ibid.

Area weighted U-factor: The thermal performance value for an assembly that considers the relative areas and U-factors of each material as determined by ASHRAE Fundamentals. Area weighting can be used to estimate the net effect of thermal bridging.

Clear field thermal bridge: An area-based thermal transmission associated with elements of a building envelope assembly. Examples of clear field thermal bridges include studs, webs of masonry units, and masonry ties.

Continuous insulation: Insulating material that is continuous across the lateral extent of a building enclosure assembly without thermal bridges other than fasteners and service openings. No reference standard exists to specify allowable amounts of thermal bridging due to fasteners penetrating the continuous insulation. Where thermally conductive penetrations are unavoidable, thermal breaks aligned with the thermal insulation will reduce their impact on the performance of the thermal-control layer. Continuous insulation is most effectively installed exterior to the backup wall construction and cannot typically be located on the interior surface of an exterior wall assembly because the continuity of the insulation would be interrupted by floor slab edges and other structural members.1

Drainage cavity insulation: Continuous insulation located within the drainage cavity of a cavity wall. Rigid or semi-rigid insulation boards or spray-applied foam insulation can be used. It is typically applied to the outer face of the backup wall, exterior to the air- and water-control layers and, thus, is also located exterior to primary structural members. The insulation is typically penetrated by framing members or attachments that support the cladding. Do not confuse drainage cavity insulation with framing cavity insulation.

Framing cavity insulation: Insulation placed between structural framing members within a wall assembly. Because it is interrupted by conductive framing and other structural elements, framing cavity insulation is not continuous. Do not use the term “cavity insulation” as it is confused with “drainage cavity insulation.”2

Linear thermal bridge: A length-based thermal transmission associated with horizontal, vertical, or diagonal elements or construction details within the building envelope; the length is measured along the exterior surface of the building envelope. Examples of linear thermal bridges include slab edges, balconies, columns and beams in the plane of an assembly, parapets, other roof-wall-floor intersections, fenestration interfaces, and similar conditions not otherwise addressed as a clear field thermal bridge or point thermal bridge.3

Overall effective R-value: This is “the combined effects on R-value of thermal bridging, convective loops, and wind washing of an assembly in the field,” as described in High Performance Enclosures by Dr. John Straube. This is an important scientific concept but is not commonly used when describing common architectural solutions. NOTE: Dr. Straube recognizes this term as undefined but uses "overall effective R-value" and "overall apparent R-value" to describe the combined effects on R-value of thermal bridging, convective loops, and wind washing of an assembly in the field.

Overall effective R-value, as used by Dr. Straube, should generally be reserved for building scientists in high-level discussions regarding the differences between in-place performance versus modeled performance. This issue is not commonly relevant to typical architectural project needs. Be aware of misuse of the term.4

Point thermal bridge: An element-based thermal transmittance associated with a discrete element that penetrates the building envelope. Examples of point thermal bridges include three-way corners, a beam penetrating a wall, a column penetrating a roof or floor, an anchor or connection used to attach an element to the building, and similar conditions not otherwise addressed as a clear field thermal bridge or linear thermal bridge.5

R-value: A measure of thermal resistance through one or more materials in a series. The use of the term “R-value” is regulated by several laws, codes, and standards. R-values claimed by manufacturers for opaque thermal transmittance should always be determined based on FTC 16 CFR Part 460, Labeling and Advertising of Home Insulation Trade Regulations. R-value is the reciprocal of U-factor. Higher R-values mean better thermal resistance.

Thermal performance of fenestration is expressed as U-factor for the assembly.

“Rated R-value,” “rated U-factors,” “assembly U-factors,” “overall U-factors,” and “effective R-values” are terms found in ASHRAE 90.1 Normative Appendix A to assist in everyday determination of the thermal performance of the building enclosure, typically when insulation is interrupted by framing. Do not use these terms beyond their indicated application in ASHRAE 90.1. Any use of terminology such as “adjusted R-value,” “effective R-value,” or “equivalent R-value” should be scrutinized with the highest degree of skepticism unless working with calculations prescribed under ASHRAE Fundamentals and/or ASHRAE 90.1 Normative Appendix A.6

Radiant barrier: A reflective material used to block radiant heat transfer. Foil surfaces can be used to block radiant heat transfer or to retard water vapor diffusion. In some cases, both qualities are desired. Some foil vapor retarder/barrier products are not intended to be radiant barriers and should not be confused as such. Radiant barriers have no effect on R-value and deliver no conductive thermal control unless tested as defined for R-value above. Be highly skeptical of claims of high "equivalent R-value" for thin radiant barriers. Radiant barriers require an adjacent air gap (minimum 1/2-inch air gap on the warm side) to prevent conduction of heat through the radiant barrier itself via contact with other assembly elements.

Thermal barrier: A thermal barrier is a fire-resistant material that separates foam plastic insulation from the occupied interior space, as defined by the International Building Code, Chapter 26. This term should only be used as the building code defines it. When describing a material that acts as a thermal transition between indoor and outdoor temperature, the preferred term is “thermal-control layer.”7

Thermal break: A rigid insulating material that is used to retard the flow of heat through an element that would otherwise act as a thermal bridge. Thermal break materials can be engineered as integral components of metal brackets, wall girts, attachments, structural framing members such as window and curtain wall framing, and other cladding support elements that penetrate the thermal-control layer. A thermal break can also be a separate material installed in alignment with the thermal-control layer to decrease the heat loss through conductive elements.8

Thermal bridge: A material that conducts heat through a thermal-control layer because it is more thermally conductive than the surrounding materials. For example, a steel stud in a wall with framing cavity insulation will conduct more heat than the surrounding insulation, thus reducing the overall performance of the thermal-control layer. Avoid using the terms “thermal gap” and “thermal short circuit,” which are not well-defined.9

Thermal-control layer: An internal layer in a building enclosure assembly that controls conduction and convection of thermal energy as well as radiant heat transfer through the assembly. A thermal control layer may comprise one or more insulating materials, arranged to create a three-dimensional boundary between interior and exterior spaces or between adjacent interior spaces with different temperature requirements. To provide good thermal performance, materials in a thermal-control layer are continuous, aligned at transitions from one material to another, and are located to provide continuous, intimate contact with an air-control layer. The largest temperature and relative humidity gradients in the building enclosure assembly typically occur within the thermal-control layer, resulting in the possibility of reaching dewpoint conditions within the thickness of the thermal-control layer. To minimize the likelihood of condensation within the insulating materials in the thermal-control layer, hygrothermal analysis can be used to determine the appropriate vapor permeance of the air-control layer and to identify whether and where an additional vapor-control layer may be needed within the assembly. The thermal-control layer is frequently penetrated by thermally conductive elements, referred to as thermal bridges, which can have a significant effect on the overall net thermal performance of the assembly. Multiple thermal-control layers in a building enclosure assembly are acceptable (e.g., continuous insulation and insulation in framing cavities).10

Thermal gap: Avoid the use of this term because it is not well defined. The preferred term is “thermal bridge.”

Thermal short circuit: Avoid the use of this term because it is not well defined. The preferred term is “thermal bridge.”11

U-factor: A measure of thermal transmittance through one or more materials, defined by ASHRAE as “the heat flux under steady-state conditions from the environment on the one side of a body to the environment on the other side, per unit temperature difference between the two environments, in Btu/hr-ft2-F. Thermal transmittance is sometimes called the overall coefficient of heat transfer or U-factor.” While U-value is often used interchangeably with U-factor, the latter is the only term used in ASHRAE Standard 90.1, which is the industry standard for heating, ventilating, and air conditioning. It is also the term used by National Fenestration Rating Council and is the industry standard for the thermal performance of fenestration. Avoid using the term “U-value” for these reasons.12

Wind washing: Air movement driven through or across the surface of thermal insulation due to wind pressure. Wind washing reduces the performance of the thermal-control layer, particularly if it consists of low-density fibrous insulation. Under some conditions, wind washing can also cause condensation. It typically occurs within the thermal-control layer located in a drainage cavity at exposed building edges, such as at the outside corners, parapets, and roof eaves, because of the large pressure gradients at these locations. Alternately, air washing can be used. This is not a preferred term because it is less specific.13

References:

International Code Council. 2017. 2018 IECC: International energy conservation code.
Straube, John Frederick. 2012. High performance enclosures: design guide for institutional commercial and industrial buildings in cold climates. Somerville, Mass: Building Science Press.
Ibid.
Ibid.
Ibid.
American Society of Heating, Refrigerating and Air-Conditioning Engineers. 2001. 2001 ASHRAE handbook: fundamentals. Atlanta, GA.: ASHRAE.
Straube, John Frederick. 2012. High performance enclosures: design guide for institutional commercial and industrial buildings in cold climates.Somerville, Mass: Building Science Press.
Ibid.
Ibid.
Ibid.
American Society of Heating, Refrigerating and Air-Conditioning Engineers. 2009. 2009 ASHRAE handbook: fundamentals. Atlanta, GA.: ASHRAE.
Straube, John Frederick. 2012. High performance enclosures: design guide for institutional commercial and industrial buildings in cold climates.Somerville, Mass: Building Science Press.

Building wrap: A mechanically fastened, non-adhered sheet good used as a water-control layer in building enclosures. Building wraps are easily flawed during construction when fasteners are not well sealed; therefore, they are not recommended for water control. In some cases, building wraps can act as an air-control layer if specifically designed and tested to meet air permeance values and if installation complies with the manufacturer's instructions for an air-control layer. Avoid the use of the term “house wrap,” which is synonymous with building wrap.

Vapor-control layer: Interconnected materials or assemblies of materials and sealed joints that control the movement of water by vapor diffusion. Hygrothermal analysis can be used to determine the appropriate vapor permeance of the air-control layer. It can also be used to determine if one is needed at all and where a separate vapor-control layer should be placed within the assembly. If necessary, the vapor-control layer is typically a membrane that has been selected for its water vapor permeance, depending on its location within the assembly relative to the potential location of the dewpoint under anticipated temperature and relative humidity conditions. The most effective vapor-control layers are continuous and have taped seams. In some cases, assembly materials with other purposes may also function as vapor-control layers. In assemblies with more than one vapor-retarding material, analysis is required to avoid trapping moisture between those materials.1

Water barrier: Avoid the use of this term. The preferred term is “water-resistive barrier.”

Water-control layer: As defined by Joseph Lstiburek in BSI-024: Vocabulary from Building Science Corporation: "The layer in an enclosure assembly that controls the passage of liquid water even after long or continuous exposure to moisture. The water control layer separates the dry zones of the assembly from wet or moist zones. More formally, the water control layer is the continuous layer (comprised of one of several materials and formed into planes to form a three-dimensional boundary) that is designed, installed, or acts to form the rainwater boundary. In face-sealed perfect barrier systems, this is the exterior-most face of the enclosure. In concealed barrier systems it is a plane concealed behind the exterior face. In drained systems the water control layer is the drainage plane behind the drainage gap or drainage layer. In storage reservoir systems the rain penetration control is typically the innermost storage mass layer."2

Water-resistive barrier: A membrane attached to the exterior face of backup wall sheathing that functions as a water-control layer. As defined by the International Code Council in the 2018 International Building Code, Section 1404.2: “A material behind an exterior wall covering that is intended to resist liquid water that has penetrated behind the exterior wall covering [cladding] from further intruding into the exterior wall assembly.” Code sets the minimum performance value at that of 15-pound felt, material historically used to cover houses, which is low and should not be considered appropriate for more challenging applications. Many other materials exceed the code minimum performance. Note that code requires a continuous barrier tied in with flashing. Avoid the use of the following, which are not well-defined terms: “water barrier,” “weather barrier,” and “weather-resistive barrier.”3

Water vapor permeability: Theoretical property that measures the diffusion of moisture through a material in response to a difference in water vapor pressure from one face of the material to the other. Typically measured for a one-inch thickness and reported in perm-inches, permeability can generally be divided by the thickness of a material of a particular thickness to estimate its permeance, although surface conditions will have an additional effect.

Water vapor permeance: A measure of the diffusion of moisture through a particular solid material of a given thickness in response to a difference in water vapor pressure from one face of the material to the other. Permeance is expressed in perms and is the basis of the following classification, in accordance with the International Building Code, Chapter 14: class I vapor retarder—less than 0.1 perm; class II vapor retarder—between 0.1 and 1.0 perms; class III vapor retarder—between 1.0 and 10 perms.

Waterproofing: A type of water-control layer typically used below grade or on horizontal surfaces. Water-control layers for wall assemblies are not typically designed to be completely waterproof as defined by ASTM D 1079. The term “waterproofing” is defined by ASTM D 1079 as “treatment of a surface to prevent the passage of water under hydrostatic pressure.” Hydrostatic pressure is found around buildings where there is a fluid at rest on one side of the building enclosure.4

Weather barrier: Avoid the use of this term. The preferred term is “water-resistive barrier.”

Weather-resistive barrier: Avoid the use of this term. The preferred term is “water-resistive barrier.”

References

Straube, John Frederick. 2012. High performance enclosures: design guide for institutional commercial and industrial buildings in cold climates.Somerville, Mass: Building Science Press.
International residential code 2018: for one- and two-family dwellings. Country Club Hills, IL: International Code Council, Inc., 2018.
Lstiburek, Joseph. "BSI-024: Vocabulary." Building Science Corporation. October 15, 2009. Accessed February 19, 2018. https://buildingscience.com/documents/insights/bsi-024-vocabulary.​
ASTM D1079-16, Standard Terminology Relating to Roofing and Waterproofing, ASTM International, West Conshohocken, PA, 2016, www.astm.org