A vapor barrier is often used to refer to any material for damp proofing

Damp proofing

Damp proofing in construction is a type of waterproofing applied to building foundation walls to prevent moisture from passing through the walls into interior spaces....

, typically a plastic or foil sheet, that resists diffusion of moisture through wall, ceiling and floor assemblies of buildings and of packaging. Technically, many of these materials are only vapor retarders as they have varying degrees of permeability

Permeation

Permeation, in physics and engineering, is the penetration of a permeate through a solid, and is related to a material's intrinsic permeability...

.

Materials have a moisture vapor transmission rate

Moisture Vapor Transmission Rate

Moisture vapor transmission rate , also water vapor transmission rate , is a measure of the passage of water vapor through a substance....

 that is established by standard test methods. One common set of units is g/m²/day or g/100in²/day. Permeability can be reported in perm

Perm (unit)

A perm is a unit of permeance or "water vapor transmission" given a certain differential in partial pressures on either side of a material or membrane.-Definitions:US permMetric permEquivalent SI unit...

s, a measure of the rate of transfer of water vapor through a material (1.0 US perm = 1.0 grain/square-foot·hour·inch of mercury

Inch of mercury

Inches of mercury, ' is a unit of measurement for pressure. It is still widely used for barometric pressure in weather reports, refrigeration and aviation in the United States, but is seldom used elsewhere....

 ≈ 57 SI perm = 57 ng/s·m²·Pa). American building codes have classified vapor retarders as having a water vapor permeance of 1 perm or less when tested in accordance with the ASTM E96 desiccant, or dry cup method. Vapor retarding materials are generally categorized as:

impermeable (≤1 US perm, or ≤57 SI perm) (Materials as:asphalt-backed kraft paper, vapor retarding paint, oil-based paints, vinyl wall coverings, extruded polystyrene, plywood, OSB),

semi-permeable (1-10 US perm, or 57-570 SI perm) (Materials as: unfaced expanded polystyrene, fiberfaced isocyanurate, heavy asphalt impregnated building papers, some latex-based paints),

permeable (>10 US perm, or >570 SI perm) (Materials as: unpainted gypsum board and plaster, unfaced fiber glass insulation, cellulose insulation, unpainted stucco, cement sheathings, spun bonded polyolefin or some polymer-based exterior air barrier films).

Materials

Vapor diffusion retarders are normally available as coatings or membranes. The membranes are technically flexible and thin materials, but sometime includes thicker sheet materials named as "structural" Vapor diffusion retarders. The vapor diffusion retarders varies from all kinds of materials and keep updating every day, some of them nowadays even combined the functions of other building materials.

Materials used as vapor retarders:

• Aluminum foil
  Aluminium foil
  Aluminium foil is aluminium prepared in thin metal leaves, with a thickness less than , thinner gauges down to are also commonly used. In the USA, foils are commonly gauged in mils. Standard household foil is typically thick and heavy duty household foil is typically .The foil is pliable, and...
  
  , 0.05 US perm (2.9 SI perm).
• Paper-backed aluminum.
• Polyethylene
  Polyethylene
  Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...
  
   plastic sheet, 4 or, 0.03 US perm (1.7 SI perm).
• Advanced Polyethylene vapor retarders that pass the ASTM E 1745 standard tests ≤0.3 US perm (17 SI perm).
• Asphalt-coated kraft paper
  Kraft process
  The kraft process describes a technology for conversion of wood into wood pulp consisting of almost pure cellulose fibers...
  
  , often attached to one side of fiberglass batts, 0.40 US perm (22 SI perm).
• Metallized film
  Metallised film
  Metallised films are polymer films coated with a thin layer of metal, usually aluminium. They offer the glossy metallic appearance of an aluminium foil at a reduced weight and cost...
  
  
• Vapor retarder paints (for the air-tight drywall system, for retrofits where finished walls and ceilings will not be replaced, or for dry basements: can break down over time due to being chemically based).
• Extruded polystyrene or foil-faced foam board insulation.
• Exterior grade plywood
  Plywood
  Plywood is a type of manufactured timber made from thin sheets of wood veneer. It is one of the most widely used wood products. It is flexible, inexpensive, workable, re-usable, and can usually be locally manufactured...
  
  , 0.70 US perm (40 SI perm).
• Most sheet type monolithic roofing membranes.
• Glass and metal sheets (such as in doors and windows).

Building construction

Moisture or water vapor moves into building cavities in three ways:
1 With air currents, 2 By diffusion through materials, 3 By heat transfer.
Of these three, air movement accounts for more than 98% of all water vapor movement in building cavities.
A vapor retarder and an air barrier

Air barrier

Air barriers control air leakage into and out of the building envelope. Air barrier products may take several forms:*Mechanically-attached membranes, also known as housewraps, usually a polyethylene-fiber or spun-bonded polyolefin , such as Tyvek is a generally accepted moisture barrier and an air...

 serve to reduce this problem, but are not necessarily interchangeable.

Vapor retarders slow the rate of vapor diffusion into the thermal envelope of a structure. Other wetting mechanisms, such as wind-borne rain, capillary

Capillary action

Capillary action, or capilarity, is the ability of a liquid to flow against gravity where liquid spontanously rise in a narrow space such as between the hair of a paint-brush, in a thin tube, or in porous material such as paper or in some non-porous material such as liquified carbon fiber, or in a...

 wicking of ground moisture, air transport (infiltration

Infiltration (hydrology)

Infiltration is the process by which water on the ground surface enters the soil. Infiltration rate in soil science is a measure of the rate at which soil is able to absorb rainfall or irrigation. It is measured in inches per hour or millimeters per hour. The rate decreases as the soil becomes...

), are equally important.

Usage

The industry has recognized that in many circumstances it may be impractical to design and build building assemblies which never get wet. Good design and practice involve controlling the wetting of building assemblies from both the exterior and interior. So, the use of vapor barrier should be taken into consideration. Their use has already been legislated within the building code

Building code

A building code, or building control, is a set of rules that specify the minimum acceptable level of safety for constructed objects such as buildings and nonbuilding structures. The main purpose of building codes are to protect public health, safety and general welfare as they relate to the...

 of some countries (such as the U.S., Canada, Ireland, England, Scotland & Wales). How, where, and whether a vapor barrier (vapor diffusion retarder) should be used depends on the climate. Typically, the number of Heating Degree Days in an area is used to help make these determinations. A Heating Degree Day is a unit that measures how often outdoor daily dry-bulb temperatures fall below an assumed base, normally 18°C (65°F).
For building in most parts of North America, where winter heating conditions predominate, vapor barrier are placed toward the interior, heated side of insulation in the assembly. In humid regions where warm-weather cooling predominates within buildings, the vapor barrier should be located toward the exterior side of insulation. In relatively mild or balanced climates, or where assemblies are designed to minimize condensation conditions, a vapor barrier may not be necessary at all.

An interior vapor retarder is useful in heating-dominated climates while an exterior vapor retarder is useful in cooling-dominated climates. In most climates it is often better to have a vapor-open building assembly, meaning that walls and roofs should be designed to dry: either to the inside, the outside, or both, so the ventilation of water vapor should be taken into consideration.
A vapor barrier on the warm side of the envelope must be combined with a venting path on the cold side of the insulation. This is because no vapor barrier is perfect, and because water may get into the structure, typically from rain. In general, the better the vapor barrier and the drier the conditions, the less venting is required.

In areas below foundation level (or, subgrade areas), particularly those formed in concrete, vapor retarder placement can be problematic, as moisture infiltration from capillary action can exceed water vapor movement outward through framed and insulated walls.

A slab-on-grade or basement floor should be poured over a cross-laminated polyethylene vapor barrier over 4 inches (10.2 cm) of granular fill to prevent wicking of moisture from the ground and radon gas incursion.

Inside a steel building, water vapor will condense whenever it comes into contact with a surface that is below the dew point

Dew point

The dew point is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into liquid water. The condensed water is called dew when it forms on a solid surface. The dew point is a saturation temperature.The dew point is...

 temperature. Visible condensation on windowpanes and purlin

Purlin

In architecture or structural engineering or building, a purlin is a horizontal structural member in a roof. Purlins support the loads from the roof deck or sheathing and are supported by the principal rafters and/or the building walls, steel beams etc...

s that results in dripping can be somewhat mitigated with ventilation; however insulation is the preferred method of condensation prevention.

Confusion between Vapor barrier and Air barrier

The function of a vapor barrier is to retard the migration of water vapor, which are not typically intended to retard the migration of air. This is the function of air barriers. Air is mixed with water vapor. When air moves from location to location due to an air pressure difference, the vapor moves with it. This is a type of migration of water vapor. In the strictest sense air barriers are also vapor barriers when they control the transport of moisture-laden air.
An discussion about the differences between vapor barriers and air barriers can be found in Quirouette.

Packaging

The ability of a package to control the permeation

Permeation

Permeation, in physics and engineering, is the penetration of a permeate through a solid, and is related to a material's intrinsic permeability...

and penetration of gasses is vital for many types of products. Tests are often conducted on the packaging materials but also on the completed packages, sometimes after being subjected to flexing, handling, vibration, or temperature.

External links

• Consumer's Guide to Vapor Barriers at the U.S. Department of Energy
• Vapor Barriers Under Laminate Floors
• Green Building Advisor: Vapor Retarders and Vapor Barriers
• Building Science Fundamentals: Air Barriers vs. Vapor Barriers Green Building Advisor
• Vapor Barriers Are a Good Thing, Right? Fine Homebuilding