Vapor Barrier vs Vapor Retarder: What's the Actual Difference?
Most people use these terms interchangeably. Most flooring manufacturers, building codes, and coating spec sheets do not. A vapor barrier and a vapor retarder are different product classifications with different performance specs. Installing a vapor retarder where a barrier is required is technically compliant with nothing—and the failure mode shows up years later in adhesive bond failures and flooring damage.
This distinction matters most in two contexts: when a flooring manufacturer's installation spec calls for a vapor barrier, and when a building inspector is evaluating code compliance for an enclosed slab. In both cases, the term means something specific.
The Technical Definitions
The classification is based on permeance—how much water vapor passes through a material per unit time per unit area. Moisture moves through materials by vapor diffusion, and permeance measures how easily that happens.
The unit is the perm (grains of water vapor per hour per square foot per inch of mercury pressure difference). Lower numbers mean less vapor gets through.
| Classification | Perm Rating | Meaning |
|---|---|---|
| Vapor barrier | Under 0.1 perms | Near-impermeable to vapor |
| Class II vapor retarder | 0.1–1.0 perms | Reduces but doesn't eliminate vapor flow |
| Class III vapor retarder | 1.0–10 perms | Minimal vapor resistance |
| Not rated | Over 10 perms | No meaningful vapor resistance |
For concrete slab applications, you want a true vapor barrier (under 0.1 perms). Vapor retarders at 0.1–1.0 perms slow moisture transmission but allow enough vapor through to cause problems over time with moisture-sensitive flooring and coatings.
ASTM E1745 Classification System
ASTM E1745 is the standard that governs vapor retarder performance for use under concrete. It classifies products into three grades based on three performance metrics:
| Test | Class A | Class B | Class C |
|---|---|---|---|
| Permeance (ASTM E96) | Under 0.1 perms | Under 0.1 perms | Under 0.1 perms |
| Tensile strength (ASTM D882) | Over 45 lbf/in | Over 30 lbf/in | Over 22 lbf/in |
| Puncture resistance (ASTM E154) | Over 2,200 g | Over 1,700 g | Over 1,200 g |
All three classes meet the vapor barrier threshold (under 0.1 perms). The difference between A, B, and C is physical durability—puncture resistance and tensile strength. Class A is the most durable; Class C is minimum performance.
The term "vapor retarder" in ASTM E1745 is somewhat misleading from a consumer standpoint: all three classes are true vapor barriers by the perm definition, but the standard uses "retarder" as the product category name. This is part of why the terminology is confusing.
What's NOT covered by ASTM E1745: Generic polyethylene sheeting that isn't certified to this standard. A roll of 10-mil poly from a hardware store may meet or not meet E1745 requirements—you'd need the manufacturer's data sheet to verify. Products labeled "E1745 compliant" or "meets ASTM E1745 Class A" have been tested and certified.
What Building Codes Say
International Residential Code (IRC): Section R506.2.3 requires a minimum 6-mil polyethylene vapor retarder (or equivalent) under concrete slabs for residential construction. The code uses "vapor retarder" as the category and specifies 6-mil poly as the minimum. Note that this is a minimum—local codes and project-specific requirements often exceed it.
ACI 302.1R and 302.2R: The American Concrete Institute's slab construction guides recommend an ASTM E1745 Class A vapor retarder placed directly under the slab for any slab intended to receive moisture-sensitive flooring.
Local code amendments: Many jurisdictions have adopted amendments that require 10-mil minimum rather than 6-mil. Some areas with high groundwater or soil moisture issues require Class A compliance. Always check your local code.
Flooring manufacturer specs: These are separate from building codes and often more stringent. A manufacturer may require 15-mil minimum or specifically require ASTM E1745 Class A compliance for warranty coverage. Spec compliance affects warranty claims if the flooring fails.
When a Vapor Retarder Is Enough
A vapor retarder (0.1–1.0 perms) is appropriate for:
- Outdoor slabs with good drainage (patios, driveways, sidewalks)
- Detached, unheated outbuildings where moisture transmission won't affect anything meaningful
- Temporary or utility slabs not adjacent to conditioned space
In these applications, moisture passing through the slab doesn't cause consequential damage—there's nothing to damage.
When You Need a True Vapor Barrier
A true vapor barrier (under 0.1 perms) is required for:
- Any slab that will have flooring installed over it
- Heating systems embedded in the slab
- Any enclosed, conditioned space above the slab
- Any project where the flooring or coating manufacturer specifies vapor barrier compliance
In these cases, vapor that passes through a retarder reaches the adhesive layer, the back of flooring material, or the interior air—causing real damage.
How to Read a Product Label
Many products are sold as "vapor barriers" that are technically vapor retarders. Here's how to verify what you're buying:
- Find the perm rating on the data sheet or spec sheet (not just the product label). It should list permeance in perms, measured per ASTM E96 or equivalent.
- Check for ASTM E1745 certification. If the product is certified, it meets the perm threshold regardless of what the label says.
- Verify thickness actually matters. Some products have different perm ratings at different thicknesses—the spec sheet should list the rating at the labeled thickness.
- Ignore marketing language that uses "vapor barrier" without a perm number. Verify the specification.
Comparison table for common materials:
| Material | Perm Rating | ASTM E1745 | Classification |
|---|---|---|---|
| 6-mil polyethylene (generic) | 0.08–0.12 | Usually not certified | Marginal barrier / retarder |
| 10-mil polyethylene (generic) | 0.04–0.06 | Usually not certified | Vapor barrier |
| 15-mil reinforced poly (certified) | 0.01–0.02 | Class B or C | Vapor barrier |
| Stego Wrap 15 mil | 0.0009 perms | Class A | Vapor barrier (high performance) |
| Roofing felt | 5–15 perms | Not applicable | Not a vapor barrier |
| Kraft paper | 60+ perms | Not applicable | Not a vapor barrier |
Related Topics
Vapor management doesn't stop at the barrier. Two adjacent topics:
- Sealing concrete — topside moisture protection that complements the below-slab barrier
- Concrete floor moisture problems — what happens when vapor makes it through, and how to diagnose and address the damage
For a full comparison of vapor barrier product types and which to specify by project, see vapor barrier types for concrete.