Namesort descending Definition
UL

abbr. Underwriters Laboratories

An independent product safety certification organization. Underwriters Laboratories provides product safety standards, product testing, training and related services.

UV

abbr. ultraviolet radiation

Radiation from the sun, which has a degrading effect on many membranes, coatings, and sealants.

Vapor Barrier

A material that has a permeance of 0.1 US perm or less. A vapor barrier is a material that is essentially vapor impermeable (e.g., metal, glass, thick plastics, unperforated epoxy paint). A vapor barrier is a Class I vapor control layer. The test procedure for classifying vapor barriers is ASTM E-96 Test Method A—the desiccant or dry cup method.

Vapor Control Layer (or Layers)

The component or components that are designed and installed in an assembly to control the movement of water by vapor diffusion.

Vapor Control Layer Class

The measure of a material or assembly’s ability to limit the amount of water that passes through it by vapor diffusion. The test procedure for determining vapor control layer class is ASTM E-96 Test Method A (the desiccant or dry cup method).

Class I:       Materials that have a permeance of 0.1 perm or less

Class II:      Materials that have a permeance of 1.0 perm or less and greater than 0.1 perm

Class III:     Materials that have a permeance of 10 perms or less and greater than 1.0 perm

 

Vapor Impermeable

Materials with a permeance of 0.1 perm or less (rubber membranes, polyethylene film, glass, aluminum foil). A Class I vapor control layer.

 

Vapor Permeable

Describes materials with a permeance of greater than 10 perms (housewraps, building papers).

Vapor Permeance

A layer property that describes the ease with which vapor molecules diffuse through it. More specifically, vapor permeance is defined as the quantity of vapor flow across a unit area that will flow through a unit thickness under a unit vapor pressure difference. It is to vapor diffusion what conductance is to heat transfer. The unit of measurement is typically the "perm" (gr/h·ft2·in. Hg); in metric/SI units, it is stated in terms of ng/(s·m2·Pa).

Vapor Permeance Classes

Test procedure for determining vapor permeance class is ASTM E-96 Test Method A (the desiccant or dry cup method).

  • Class I: Materials that have a permeance of 0.1 perm or less (Note: This is the definition of a "vapor barrier.")
  • Class II: Materials that have a permeance of 1.0 perm or less and greater than 0.1 perm (Note: This is the definition of a "vapor retarder.")
  • Class III: Materials that have a permeance of 10 perms or less and greater than 1.0 perm
  • Class IV: Materials that have a permeance of greater than 10 perms

 

 

Vapor Retarder

A vapor retarder is a material that has a permeance of 1.0 perm or less and greater than 0.1 perm. A vapor retarder is vapor semi-impermeable and is classified as a Class II vapor control layer. The test procedure for classifying vapor retarders is ASTM E-96 Test Method A (the desiccant or dry cup method).

Vapor Semi-Impermeable

Describes materials with a permeance of 1.0 perm or less and greater than 0.1 perm (oil-based paints, most vinyl coverings).

Vapor Semi-Permeable

Describes materials with a permeance of 10 perms or less and greater than 1.0 perm (plywood, OSB, most latex-based paints).

Veneer

1. A non-loadbearing layer of masonry on the exterior of an enclosure.

2. A thin slice or sheet of wood or other material that is glued onto a base of wood, fibreboard, or other material to create a flat panel (furniture, counter tops, etc.).

Ventilation

The intentional flow of air into occupied spaces or behind cladding/roofing to move heat and moisture in a desirable manner.

VLT

See VT.

VOC

abbr. volatile organic compound

In terms of indoor air quality, a VOC is any organic chemical compound that can evaporate under normal indoor atmospheric conditions of temperature and pressure. VOCs may have short- and long-term adverse health effects. VOCs are emitted by a wide array of products including many building materials.

VT

abbr. visible light transmittance

A metric of the amount of visible light that passes through the glazing material of a window, door, or skylight. VT theoretically varies between 0 and 1, but most values among double- and triple-pane windows are between 0.30 and 0.70. The higher the VT, the more light is transmitted.

Variants: VLT, Tvis

Water Control Layer

The layer in an enclosure assembly that controls the passage of liquid water even after long or continuous exposure to moisture. 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 perfect 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.

Variants: Rainwater Control Layer, Rain Penetration Control Layer

Special note: there is currently not an adequate performance metric or series of performance metrics for water control layers. Anyone interested in this issue is invited to contact Joe Lstiburek at joe@buildingscience.com.

Water Resistant Barrier

A sheet, spray- or trowel-applied membrane or material layer that prevents the passage of liquid water even after long or continuous exposure to moisture.

Variants: WRB, water resistive barrier

Weep Hole

An opening placed in a wall or window assembly to permit the escape of liquid water from within the assembly.  Weep holes can also act as vents.

Whole-Wall R-value

R-value for the whole opaque assembly including all additional structural elements (such as double studs), and typical enclosure interface details, including wall/wall (corners), wall /roof, and wall/floor connections.

Wind-Washing

The phenomenon of air movement driven by wind pressures wind passing through or behind the thermal insulation within enclosures, causing significant loss of heat flow control and potentially causing condensation. Typically occurs at exposed building edges, such as at the outside corners and roof eaves because of the large pressure gradients at these locations. This can be thought of as the “wind blowing through the insulating sweater” effect.

Window

A manufactured assembly of a frame, sash, glazing and necessary hardware, made to fit an opening in a wall. Components of a window may include the following:

  • Window sill: horizontal member at the base of a window opening.
  • Window head: horizontal member at the top of a window opening.
  • Window jamb: either of the vertical members at the sides of a window opening.
  • Mullion: A vertical member between the glazed units.
  • Rail: A horizontal member between the glazed units.
  • Glazing: The glass portion of the window.
  • IGU: Insulated glazing unit. Double or triple panes of glass sealed together to provide insulation value.  The still gas between the panes acts as the insulation.
  • Condensation track: a channel at the interior sill level of the window intended to intercept small amounts of water condensing on the interior surface of the glass.

 

Window Frame

The stationary part of a window unit; the window sash fits into the window frame.

Wood Decay

A specific type of wood fungus damage that occurs at higher moisture levels (~28%+ weight moisture content).

WUFI

abbr. Wärme Und Feuchte Instationär

A modeling program for simulating one-dimensional dynamic heat and moisture transfer through a building enclosure.

Wythe

A single vertical layer of masonry.

Variant: Leaf

Xeriscaping

Climate-tuned landscaping that minimizes outdoor water use while maintaining soil integrity and building aesthetics. Typically includes emphasis on native plantings, mulching, and no or limited drip/subsurface irrigation.

XPS

abbr. extruded polystyrene

See Extruded Polystyrene Insulation.

Zero Energy House

Any house that averages out to net zero energy consumption. A zero energy home produces as much energy over a year as it consumes, typically using photovoltaics, or combined heat and power (CHP) to generate electricity on site. In a zero energy home, efficiencies in the building enclosure and HVAC are great enough that electrical plug loads tend to dominate; therefore, these homes must have the added focus of high efficiency appliances and lighting. Several incompatible definitions are commonly used, including net-zero site energy, net-zero source energy, net-zero energy costs, and net-zero energy emissions. Therefore, terms should be clarified when this name is applied.

ZNE

abbr. zero net energy

A term used to describe buildings that minimize energy consumption and offset it with energy production (for example through solar panels). Several more specific definitions exist; because these conflict, it is recommended that the intended definition of the term “zero net energy” be clarified when it is used.

Related resources: BA-1401: Design Challenges of the NIST Net Zero Energy Residential Test Facility; CP-1102: San Francisco Bay Area Net Zero Urban Infill; CP-1103: New England Net Zero Production Houses

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