What Is Vapor Permeation Resistance? (Perm Ratings)
Vapor permeance measures how readily water vapor diffuses through a material or assembly. Its US perm rating is determined by a stated ASTM E96/E96M test method; a lower rating means greater resistance to vapor diffusion. Perms do not measure air leakage. Choose and place a vapor retarder based on the adopted code, climate, assembly, and drying paths.
A suitable perm rating can help control moisture movement without trapping water inside a wall or roof. The key is not to choose the lowest number automatically. A layer that limits vapor movement in one climate or assembly may slow drying in another.
I start by treating the rating as one part of an assembly decision. The code, product test data, layer position, air control, water control, and expected drying directions all matter. The examples below are illustrative scenarios, not reports of specific projects.
Plan the Assembly and Select the Required Perm Class
A perm class groups materials by vapor permeance under code definitions. Lower permeance means slower vapor diffusion through the tested material. Before choosing a product, identify the applicable code edition and the assembly it will serve; a rating alone cannot show whether the full construction is moisture-safe.
Permeance is the rate at which vapor passes through a material under a vapor-pressure difference. In US building practice, it is expressed in perms. One US perm equals one grain of water vapor per hour, per square foot, per inch of mercury pressure difference. That is approximately 57.2 ng/(Pa·s·m²).
The International Residential Code (IRC) defines three vapor-retarder classes:
| Class | Vapor permeance | General meaning |
|---|---|---|
| Class I | ≤0.1 perm | Very low permeance |
| Class II | >0.1 to ≤1.0 perm | Low permeance |
| Class III | >1.0 to ≤10 perms | Moderate permeance |
| Not a Class I–III retarder | >10 perms | Generally vapor-permeable |
These are classification thresholds, not a universal product-selection chart. The locally adopted IRC or International Building Code (IBC), including local amendments, determines what a specific wall or roof assembly needs. Requirements can depend on climate zone, framing, insulation, and the assembly’s interior and exterior layers.
Before comparing products, record:
- The adopted code edition and climate zone.
- Whether the project is a wall, roof, ceiling, or floor.
- The intended insulation type and location.
- Indoor humidity conditions, such as those in a pool room or other high-moisture space.
- Which directions the assembly can dry.
Takeaway: Find the code rule for the actual assembly first. Then use the perm rating to check whether a product meets that rule and supports a safe drying strategy.
Check Layer Placement, Clearances, and Drying Paths
Layer placement affects whether an assembly can dry after construction moisture, a leak, or condensation. A low-perm layer on the wrong side can restrict drying, and low-perm layers on both sides can limit drying in either direction. Code compliance alone does not prove that an assembly is moisture-safe.
A vapor retarder controls diffusion, but it does not stop bulk water from a leak or rain intrusion. Nor does the perm value predict where condensation will form. That depends on conditions such as indoor and outdoor temperature, humidity, insulation, and the location of materials with high or low permeance.
For each assembly, trace the layers from indoors to outdoors. Note which layers resist vapor, which allow drying, and where water can drain or ventilate. Check the adopted code’s placement rules for that assembly; do not assume the same location works in every climate or building type.
Use extra care when:
- A wall has foam sheathing or another low-perm exterior layer.
- Interior finishes or coatings also have low permeance.
- The assembly includes dense insulation that may reduce heat flow and change condensation conditions.
- The building has high indoor humidity or limited ventilation.
- New construction moisture or a known water leak could be present.
If low-perm materials occur on both sides, pause before closing the assembly. Review the design for moisture accumulation and drying potential with a qualified building-envelope professional. A code official can clarify the adopted code requirements, but a code check does not replace assembly-specific moisture analysis.
Takeaway: Confirm both the required retarder location and the assembly’s drying paths. Avoid adding low-perm layers by habit.
Install and Coordinate Vapor, Air, and Water-Control Layers
A vapor retarder works as intended only when its placement and installation match the assembly design. It must also be coordinated with separate air and water controls. The layers may overlap in a product, but their functions are different and should be checked independently.
A perm rating measures vapor diffusion through a test specimen. It is not an airflow rating, and it does not show whether seams or penetrations leak air. Air leakage can carry moisture through gaps, so a continuous air barrier must be designed and detailed separately.
Likewise, a vapor retarder is not automatically a water-resistive barrier or drainage plane. Follow the wall or roof design for water control, drainage, and ventilation. Do not seal a detail in a way that blocks a required drainage path or ventilation opening.
A practical installation sequence is:
- Confirm the design. Identify the correct product, location, substrate, thickness, and any required code approval.
- Read the instructions. Check compatible tapes, sealants, primers, and fastening methods. Product systems differ.
- Plan transitions. Identify seams, corners, edges, service penetrations, and changes in material before installation.
- Maintain continuity. Seal or connect the retarder as the design requires, while preserving drainage and ventilation paths.
- Coordinate control layers. Make clear which material provides vapor control, which provides air control, and which manages water.
- Inspect before concealment. Check seams, damage, edges, and penetrations while they remain visible.
Some products combine more than one control function. Do not assume they do so based on appearance or marketing language; verify the product data and details for the intended use.
Takeaway: Detail the vapor layer as part of the whole enclosure, not as a stand-alone sheet. Keep air and water control in the plan.
Verify Product Ratings, Continuity, and Code Compliance
A product’s perm rating is useful only when its test basis and intended use are clear. ASTM E96/E96M results can vary with the specified procedure and test conditions. Compare ratings only when the test basis is stated and relevant to the assembly.
ASTM E96/E96M includes methods for measuring water vapor transmission. Product data may report results from different test conditions, often described as dry-cup or wet-cup methods. These results are not automatically interchangeable. Look for the test standard, method or conditions, tested thickness, and whether the value applies to the product as installed.
| What to check | Why it matters |
|---|---|
| Perm value and units | Confirms the stated vapor permeance |
| ASTM E96/E96M test basis | Shows how the reported result was measured |
| Product thickness or substrate | The tested configuration may affect the result |
| Code acceptance and use | Confirms suitability for the specified assembly |
| Seams and penetrations | Installation gaps can undermine continuity |
| Other control layers | Helps identify restricted drying or conflicting functions |
Keep product data with the project records. Before concealment, document the product name, rating, test basis, installation location, and any approval required by the local code. If substitutions occur, check the new product’s data and compatibility rather than relying on a similar name or appearance.
Takeaway: Verify the rating, test conditions, and installed configuration. Then inspect and record the work before covering it.
Illustrative Assembly Checks
These examples show how to apply the decision process, not universal construction prescriptions. Actual code requirements and moisture behavior depend on the project’s location, materials, and design. Use the examples to identify questions to resolve, not to copy a layer arrangement without review.
Example 1: Exterior wall with low-perm layers. A proposed wall has a low-perm exterior sheathing and a low-perm interior finish. I would flag the restricted drying potential before installation, then check the adopted code and the assembly’s climate-specific condensation and drying behavior. I would not add another retarder solely because it is familiar.
Example 2: Product comparison. Two membranes list different perm values, but one data sheet states the test method and thickness while the other does not. I would not treat the numbers as a direct comparison. I would request the missing test basis and confirm that the tested product matches the intended substrate and use.
Example 3: Penetrations at inspection. A continuous membrane has an unsealed service opening. Its published rating does not describe the air leakage at that opening. I would check the specified detail, repair the discontinuity with compatible materials, and inspect it before finishes cover the area.
Takeaway: The rating informs material selection; the full assembly and its details determine whether the control strategy works.
Frequently Asked Questions
These short answers define common terms and clarify what a perm number can and cannot tell you. Use them as a starting point, then confirm the adopted code and product data for your project. Ratings should always be read with their test basis and intended assembly in view.
What does a perm rating measure?
It measures vapor permeance: how readily water vapor diffuses through a material under specified test conditions.
Does a lower perm mean more vapor resistance?
Yes. A lower rating means less vapor passes through the tested material under the stated conditions.
How much is one US perm?
One US perm equals one grain of water vapor per hour per square foot per inch of mercury pressure difference, or about 57.2 ng/(Pa·s·m²).
What are the IRC vapor-retarder classes?
Class I is ≤0.1 perm; Class II is >0.1 to ≤1.0 perm; Class III is >1.0 to ≤10 perms.
Is a material above 10 perms a vapor retarder?
It is generally considered vapor-permeable and does not fall within IRC Classes I, II, or III.
Does a perm rating measure air leakage?
No. Perms measure vapor diffusion, not airflow. Air-barrier performance must be addressed and verified separately.
Can I compare any two published perm values?
Not reliably unless the test basis and conditions are stated and relevant to the intended use. Check ASTM E96/E96M details and tested thickness or substrate.
Should I install a vapor retarder on both sides of a wall?
Not without checking the code, climate, and drying potential. Low-perm layers on both sides can restrict drying and trap moisture.
Does code compliance guarantee a moisture-safe assembly?
No. Code compliance is necessary, but assembly-specific condensation risks and drying paths still need review.
Where should the vapor retarder go?
Its location depends on the adopted code and the assembly. Confirm the required class and placement for the specific climate, wall or roof design, and materials.
Conclusion
A perm rating tells you how readily vapor diffuses through a tested material; it does not choose the right layer position or prove that an assembly is safe from moisture. Check the adopted code, climate, test basis, and drying paths, then coordinate vapor, air, and water controls. Inspect and document the work before concealment.
(This article was written by one of our staff writers, Garrett Vance. Visit our Meet the Team page.)
