Site hosted by Angelfire.com: Build your free website today!

Construction deterioration & building durability glossary


Article & glossary hosted by A. Sebastian Engineering and Investigation Services


vapor barrier (American) or vapour barrier (Canadian, Brittish)
A material or combination of materials which form either an impenetrable zone or at least having a high resistance to vapor flow.

Vapor Barrier For many years, designers were taught that the vapor barrier was a major requirement for insulated walls in order to control the diffusion of water vapor into the colder reaches of these walls, where it could condense and stain the finished surfaces or, worse, initiate the deterioration of the affected materials. When it became obvious to researchers in the 1960s that air leakage into the walls and roofs was a more important source of water migration, authorities began calling for a "continuous vapor barrier." This new terminology was supposed to somehow get rid of all the hidden holes causing so many moisture-related problems. The expression "air/vapor barrier" then began to appear in the literature, raising the hope that this component would finally put a stop to all moisture flow through the walls and roofs of a building. This, unfortunately, did not happen, so it has become necessary to take apart this concept of the "continuous air/vapor barrier " and go back to basic principles.
A vapor barrier is a material that offers a high resistance to the diffusion of water vapor. It is used to separate an environment which is at a high vapor pressure from an adjacent one at a lower vapor pressure. For best results, it is important that the vapor barrier be continuous, but it does not have to be perfectly continuous. Unsealed laps or minor cuts do not affect the overall resistance to diffusion significantly. The vapor barrier must also be located on the warm side of the insulation or at least in a location in the wall near enough to the warm side to remain above the dew point temperature of the indoor air during cold weather.
Vapor barriers (a 4 mil polyethylene sheet would make a good quality vapor barrier) are intended to retard the passage of moisture as it diffuses or migrates through a wall. Air barriers, on the other hand, are intended to stop both outside air from entering the building and inside air from exfiltrating through the building envelope. Air leakage is recognized as a greater mode of moisture transfer than is diffusion of vapor. The Difference Between a Vapor Barrier and an Air Barrier identifies the materials and the method of assembly required to build effective air barriers. Vapor barriers (a 4 mil polyethylene sheet would make a good quality vapour barrier) are intended to retard the passage of moisture as it diffuses or migrates through a wall. Air barriers, on the other hand, are intended to stop both outside air from entering the building and inside air from exfiltrating through the building envelope. Air leakage is recognized as a greater mode of moisture transfer than is diffusion of vapor.

Vapor retarder
will function to limit moisture diffusion through the veneer system. The vapor retarder is commonly placed on the interior or "warm in winter" side of the steel stud wall, but its location may vary depending on local practice and the configuration of wall insulation used. A vapor retarder commonly contains points where minor leakage can occur, such as electrical outlets or joints at structural members, but in general these systems still function adequately. The vapor retarder can be provided as a part of the insulation backing, on the surface of the gypsum wallboard or as a separate polyethylene sheet. See illustration in Figure 2.13 for a common configuration. Air pressure from wind infiltration, fan pressurization, and stack effect may be sufficient to dislodge inadequately supported vapor retarders allowing for significant vapor leakage.

veneer/ v&-'nir/
Function: noun
Etymology: German "Furnier", from "furnieren" to veneer, also perhaps from French "fournir" to furnish, equip
Date: 16th century
1. a: thin sheet of a material, such as a layer of wood of superior value or excellent grain to be glued to an inferior looking wood
b: any of the thin layers bonded together to form plywood
2. protective or ornamental facing, such as of brick or stone; in masonry: a single wythe of masonry for facing purposes, not structurally bonded.
3. a superficial or deceptively attractive appearance, display, or effect: gloss, facade (or faade /f&-'sd/)
4. BIA definition: a. "A wall having a facing of masonry units, or other weather-resisting, non-combustible materials, securely attached to the backing, but not so bonded as to intentionally exert common action under load." b. A brick veneer wall consists of an exterior wythe of brick isolated from the backup by a minimum prescribed air space and attached to the backup with corrosion-resistant metal ties.
5. The International Masonry Institute definition: "A single facing wythe of masonry units or similar materials securely attached to a wall for the purpose of providing ornamentation, protection, insulation, etc. but not so bonded or attached as to be considered as exerting common reaction under load." 6. The 1994 Uniform Building Code definition: a. "Veneer is nonstructural facing of brick, concrete, stone, tile, metal, plastic or other similar approved material attached to a backing for the purpose of ornamentation, protection or insulation." b."Anchored Veneer is veneer secured to and supported by approved connectors attached to an approved backing."

VIRTUAL ECCENTRICITY
The eccentricity of a resultant axial load required to produce axial and bending stresses equivalent to those produced by applied axial loads and moments. It is normally found by dividing the moment at a section by the summation of axial loads occurring at that section.

VITRIFICATION
The condition resulting when kiln temperatures are sufficient to fuse grains and close pores of a clay product, making the mass impervious.



Back to glossary index.

Back to Environmental loads and construction degradation article.