What Is Vapor Permeation in Plastics? (Barrier Specs)
Vapor permeation is the movement of water vapor through a plastic layer, measured under stated test conditions. To choose a plastic for a wall, roof, or floor, compare its reported permeance or water-vapor transmission rate (WVTR), test method, thickness, temperature, and humidity. Then check the layer’s location against the whole assembly and your locally adopted building code.
If someone in your home has allergies, damp materials and mold may be a concern. A plastic sheet can affect how quickly moisture dries, but its permeance rating alone does not show whether an assembly will stay dry or resolve an indoor-air concern. I start with the wall or floor as a system: where moisture can enter, where it can dry, and which layers control air, water, and vapor.
Planning: Define the Vapor-Barrier Requirement
Vapor permeance describes how readily water vapor passes through a material. In construction, a “vapor barrier” is often used loosely; building codes instead classify vapor retarders by permeance. The right rating depends on the assembly, climate, code, and drying paths, not just the plastic’s trade name.
Permeance, permeability, and WVTR
Permeance is the vapor flow through a material of a given thickness under a stated vapor-pressure difference. Permeability is a thickness-normalized property of the material. That distinction matters: two products with different thicknesses cannot be fairly compared by WVTR alone.
WVTR is the mass of water vapor that passes through a defined area over time under test conditions. Permeance relates that rate to the vapor-pressure difference across the specimen. A report that lists WVTR without the test conditions and specimen thickness is not enough to select a product for an assembly.
In US units, 1 perm equals 57.2 ng/(Pa·s·m²). For a simple illustration, a permeance of 1 perm under a vapor-pressure difference of 1,000 Pa corresponds to 57,200 ng/(m²·s), or about 4.94 g/(m²·day). That is a calculation from the stated conditions, not a universal product rating.
What do IRC vapor-retarder classes mean?
The International Residential Code (IRC) groups vapor retarders by permeance. These thresholds help describe a product, but they do not, by themselves, tell you where it belongs. Check the IRC edition and amendments adopted where the work will be done.
| IRC class | Permeance | General meaning |
|---|---|---|
| Class I | ≤0.1 perm | Very low vapor flow |
| Class II | >0.1 to ≤1.0 perm | Low vapor flow |
| Class III | >1.0 to ≤10.0 perms | Moderate vapor flow |
Some plastics may fall into different classes depending on product and thickness. Do not assume that all polyethylene, foam, or film has the same rating. Ask for the tested value for the product you plan to install.
Access and Clearances: Check Assembly Drying Paths
A drying path is a route for moisture to leave an assembly by evaporation and vapor movement. Before adding a low-permeance plastic layer, identify existing layers on both sides, likely moisture sources, and the direction or directions in which the assembly can dry. A low rating can reduce drying through that layer.
Survey the assembly before choosing plastic
Record the indoor and outdoor conditions that matter to the project, along with the assembly’s layers and any known moisture sources. Consider rain leakage, ground moisture, indoor humidity, plumbing leaks, and construction moisture. The aim is not to diagnose every building problem from a product label; it is to avoid blocking a needed drying route.
For a wall or roof, note where the plastic would sit relative to insulation, sheathing, interior finishes, and exterior water-control layers. For a floor, check the layers below and above it, and whether the plastic is intended for ground moisture control or another purpose. Those are different functions.
A vapor retarder is not automatically an air barrier, waterproofing layer, or drainage plane. Air leaks can carry moisture through gaps, while bulk water needs a suitable drainage or water-control strategy. Plan the layers for their intended jobs and coordinate their connections.
Watch for low-drying assemblies
A plastic layer with very low permeance can trap construction moisture or leakage if it is placed between two other low-drying layers. That can raise the risk of concealed moisture damage. The risk depends on the full assembly and its exposure; the plastic’s rating alone cannot predict the outcome.
For example, adding polyethylene over an existing low-permeance layer may leave little room for drying. Before enclosing such an assembly, identify the layers, moisture sources, and code requirements. If you cannot establish a safe drying plan, pause and seek a building-science or design professional’s review.
Execution: Specify and Install the Plastic Layer
A useful product specification names the test method, measured result, units, specimen thickness, temperature, and humidity conditions. Installation also needs a clear plan for seams, edges, and penetrations. This keeps the selected material’s test result tied to the actual product and helps the installed layer perform as intended.
Ask for a complete test report
ASTM E96/E96M reports water-vapor transmission or permeance using a specified procedure and test conditions. Results can differ when the procedure or temperature and humidity conditions differ. ASTM F1249 measures WVTR through plastic film or sheeting with a modulated infrared sensor. Its result should also state test conditions.
When requesting product information, ask for:
- The test standard and procedure used
- The reported WVTR or permeance, with units
- The tested specimen thickness
- Test temperature and relative humidity, or the vapor-pressure conditions
- Whether the value applies to the exact product being supplied
Do not treat an E96/E96M result and an F1249 result as interchangeable without checking their conditions and basis. Nor should you compare two WVTR values as if they describe equal performance if the test conditions or specimen thickness differ.
Plan the joints and transitions
Follow the product maker’s approved instructions for attachment, seams, and compatible tapes or sealants. Detail how the plastic meets adjacent layers and passes around openings or penetrations. A good material rating cannot make up for open seams, torn film, or an unplanned gap.
Before installation, confirm that the chosen layer meets the local code for the project and assembly. Building codes are adopted and amended locally, so a requirement in one jurisdiction may not apply unchanged in another. Keep the product documentation with the project records.
Verification: Confirm Test Conditions and Continuity
Verification means checking both the paper specification and the installed layer before it is covered. Match the test result to the product, thickness, and conditions you specified. Then inspect the actual work for gaps, damaged areas, and incomplete transitions. This is the last practical chance to correct defects before they become concealed.
Use a short pre-cover check
I use a simple document-and-installation check rather than relying on a label such as “vapor barrier.” First, confirm the test report matches the supplied product. Then inspect the whole run, including edges and penetrations, before finishes conceal it.
- Compare the product name and thickness with the report.
- Confirm the test method, result, units, temperature, and humidity conditions.
- Check laps, seams, edges, and penetrations against the installation plan.
- Repair damage using the product maker’s approved method.
- Record photographs and keep the test report and product instructions.
If a report gives only a WVTR number, ask for the missing conditions before using it to compare products. If the installed plastic is intended to serve more than one control function, verify that the product and details are suitable for each function; do not infer that one rating proves all of them.
Practical Examples: Match the Rating to the Assembly
These examples are planning scenarios, not reports of specific completed projects. They show how the same plastic rating can lead to different choices when the assembly, moisture sources, or code requirements change. Use them to frame questions, then verify the actual design and local rules.
Example 1: Comparing two film reports
Suppose Film A and Film B each list a WVTR. If Film A was tested at a different temperature or humidity from Film B, the numbers do not provide a direct comparison. If their thicknesses differ, WVTR alone still does not give a like-for-like material comparison. Ask for permeance or comparable test data with the conditions stated.
Example 2: Adding plastic during a retrofit
Suppose a planned interior plastic layer would sit behind a finish, while an existing exterior layer also has low permeance. Before adding the plastic, map both layers and check whether the wall can dry in either direction. Look for likely leaks and construction moisture, too. If drying is restricted, do not proceed on the assumption that a lower permeance is always better.
Conclusion and FAQ
A sound plastic barrier specification links a measured result to a real assembly. Confirm the test method and conditions, understand the difference between WVTR and permeance, and check code and drying potential before installation. Keep that evidence with the project and inspect continuity before covering the work.
What is vapor permeation in plastic?
It is the movement of water vapor through plastic. A test measures how much vapor passes under defined conditions. The result helps describe the material, but does not alone determine whether it suits a wall, roof, or floor assembly.
What is the difference between WVTR and permeance?
WVTR is the amount of vapor passing through an area over time during a test. Permeance relates vapor flow to the vapor-pressure difference across the material. Compare results only when the test basis and conditions are understood.
What does 1 US perm equal?
One US perm equals 57.2 ng/(Pa·s·m²). A permeance value should still be read with its test method and conditions, since those details affect how the result applies to a project.
What is the difference between permeance and permeability?
Permeance applies to a tested material at its tested thickness. Permeability is normalized for thickness. This is why a WVTR number by itself may not fairly compare plastic products of different thicknesses.
What are the IRC vapor-retarder classes?
Class I is 0.1 perm or less. Class II is above 0.1 and up to 1.0 perm. Class III is above 1.0 and up to 10.0 perms. Verify the locally adopted code edition and amendments.
Are ASTM E96 and ASTM F1249 results interchangeable?
Not automatically. ASTM E96/E96M and ASTM F1249 use different test approaches. Check each report’s method, temperature, humidity conditions, units, and specimen thickness before comparing results.
Does a lower-permeance plastic always work better?
No. A low-permeance layer can restrict drying. If moisture is trapped between low-drying layers, or if leakage occurs, the assembly may be at risk. Evaluate the full assembly and its moisture sources.
Is a vapor retarder also an air or water barrier?
Not necessarily. A vapor retarder limits vapor diffusion according to its tested permeance. Air control and bulk-water control need suitable materials and continuous details designed for those functions.
What information should a plastic barrier specification include?
State the test method, reported WVTR or permeance and units, specimen thickness, test temperature, and humidity or vapor-pressure conditions. Also identify the intended assembly and confirm code compliance and installation details.
What should I check before covering the plastic?
Confirm the product matches its test documentation. Inspect seams, edges, penetrations, and damage, and verify repairs follow the product instructions. Keep photographs and records so the installed work can be reviewed later.
(This article was written by one of our staff writers, Garrett Vance. Visit our Meet the Team page.)
