A vapor barrier is a material or a material assembly which virtually eliminates the movement of water vapor through a structure. Many air barrier products also act as a vapor barrier. The very nature of a vapor barrier is a conflicting one as a good vapor barrier should block out water in vapor form but also to allow the building materials to dry out if they do become wet. Vapor Barriers should not be confused with air barriers which work to stop air permeance or infiltration. Vapor Barriers should also not be confused with moisture-resistive barriers. Vapor barriers should also not be confused with vapor retarders which slow the movement of water vapor by diffusion. Lastly, vapor barriers should not be confused with weather barriers which are most commonly used as a building wrap to ‘resist’ air and moisture passage into the wall assembly/building. For a material to be considered a vapor barrier, ASTM E-96 test method requires a vapor permeance (or passage) of 0.1 perm or less which is also referred to as a Class I vapor retarder. Class II vapor retarders allow 0.1-1.0 perm and Class III vapor retarders allow 1.0-10 perms.

Typically water vapor will move from the warm side of a building assembly to the cold side. ‘Double’ Vapor Barrier installation (or installation on both sides of the assembly) should typically be avoided to allow one side to dry the assembly out. It’s important to note that installation of a vapor barrier on the interior side of an air conditioned building can lead to mold issues as moisture gets trapped between the finish material and vapor barrier. Placement of vapor barriers can be a confusing proposition as in many climates the warm and cold ‘sides’ of an assembly can vary significantly based on seasons. But typically in warm climates the barrier is installed on the exterior and vice versa for cold climates.

Vapor Barriers come in many different material types;

-plastic sheets (typically polyethylene or similar)

-self-adhered membranes

-fluid-applied materials

-insulation boardstock


-medium density spray polyurethane foam


It is important to note than many manufacturers produce materials which serve more than one or all of the following purposes; vapor-retarder, moisture-resistant barrier, air-barrier. There are different types of applications where vapor barriers are utilized and the material type used is often based on this application;

-horizontal under concrete slab application (plastic or synthetic sheets)


-interior vertical/horizontal, wall/ceiling applications (foil faced batt insulation, plastic or synthetic sheets, spray foam, etc)


-exterior walls (plastic or synthetic sheets, self adhered membranes, fluid-applied materials, insulation boardstock, spray foam)

-Crawl Space floors (plastic sheets)


Plastic Sheet Vapor (and Air) Barrier


Commonly installed between the stud and the wall on exterior, above grade walls but this can vary depending on the climate and the wall assembly. A continuous barrier is the goal during installation and specialized plastic tape should be used at all seams. Interior walls rarely get plastic vapor barrier sheets except some situations when they are placed between the drywall and studs in a high-moisture bathroom or kitchen area.


Plastic Sheet Vapor Barrier is commonly used on grade or under aggregate in crawl spaces to keep ground moisture from making it’s way up into the structure. Plastic vapor barrier is also commonly used under concrete slabs on grade to keep moisture from passing through the concrete to the interior floor of the structure. 6 Mil, 10mil, 15mil, 20mil are used depending on the strength and performance specifications required.

 Self-Adhered Membrane Vapor (and Air) Barrier


Self-sticking sheet rolls are commonly installed on building exteriors. Most products do not require a primer and can be installed on sheathing, aluminum, metal, drywall, cast-in-place concrete (7 day cure typically required), concrete block (stuck flush joints), brick (stuck flush joints), etc. For difficult to adhere surfaces spray applied or brush applied primer is usually available (some products however require primer on anything other than wood sheating). Material is typically lapped 2″ on sides and ends and has a ‘starting’ adhesive strip placed on the bottom of the roll. The material can be adhered by hand or with a rubber roller to promote consistent adhesion. Weather ranges for install are typically fairly wide at 0 – 150 degrees fahrenheit. Detail work around openings and penetrations requires great care to avoid compromising the complete sealed building envelope effect. Rough openings are often flashed with cut strips of membrane if other material is not specifically required. Common roll widths are 36″ and weigh 20-25 pounds but heavy duty versions can weigh more. Most membrane vapor barriers can resist UV exposure for up to 6 months.


 Fluid-Applied Vapor (and Air) Barrier


Fluid-Applied Vapor/Air Barrier characteristics are:

-single (polymer) or two-component (rubber)

-solvent based or water based

-spray or brush/roll applied

-hot or cold-applied

It forms a strong, seamless, elastomeric membrane which resists vapor. The product commonly comes in 5 Gallon pails or 55 gallon drums and will usually cover 15-25 square feet per gallon (for smooth substrates) at an 80 mil (polymer) or 60 mil (rubber) application thickness. Most products can be installed down to 10 degrees fahrenheit but should not be installed on ‘green’ concrete (14 day cure minimum typical, some rubber products only 3 days). Most products can’t be installed onto polystyrene boards. When being installed on concrete block walls, all cracks/voids/irregularities/etc must typically be repaired with a specialty patching product a couple hours prior to fluid-applied barrier installation. Standard prep work to produce a clean, void free, protrusion free, hole free, consistent, dry substrate is required. Some products (particularly rubber-based) may require ‘detailing’ openings, penetrations, and vulnerable areas which includes joint sealants, self-adhered flashings, or mastics to ensure maintenance of building envelope. When installing on exterior gypsum or fiberglass paneling, joints will typically require reinforced or mesh wallboard tape prior to installation of vapor barrier. If the product is to be spray installed it must typically be passed through a heat exchanger to lower the viscosity before spraying. A ‘Big Rig’ (Hydraulic Airless) Sprayer (with a 0.035″ heavy duty switch tip) is typically recommended in terms of spray-application equipment to most effectively/efficiently apply the product.


Roller application may require two coats (1-2 hours between coats) as 40mil is a typical thickness expectation when rolling.


A ‘Wet Mil’ Gauge is often used to inspect the thickness during application.


Porous block walls may require more than one coat to achieve the needed thickness. The material is typically tack free in a couple hours. Temperature, huminidity, airflow, sunlight, etc will effect drying time but 48 hours is typically recommended as a minimum before subsequent material application (except rubber product on which insulation boards can be placed when still slightly tacky). Xylene is commonly used to fluch the sprayer unit of any excess material. 30-60 days is the typical max UV exposure time before degradation will occur. Rubber based products can be applied to wood sheathing, concrete block, concrete, exterior gypsum board, fiberglass sheathing, and brick veneer.