Specify a compliant vapor barrier/retarder beneath a concrete slab (ASTM E1745 classes)

Steps

1. Check the applicable code edition's requirement — the IRC has required water vapor retarders beneath concrete floor slabs to conform to ASTM E1745, commonly specifying Class A as a minimum — confirm against the specific edition adopted by the jurisdiction.
2. Understand the class system: ASTM E1745 (Plastic Water Vapor Retarders Used in Contact with Soil or Granular Fill under Concrete Slabs) defines three classes (A, B, C) differentiated by tensile strength and puncture resistance, with Class A the most robust on both — permeance is held at the same maximum (0.1 perms) across all three classes, so class is a durability distinction, not a vapor-permeance distinction.
3. Match class to job-site conditions: rougher sub-base (crushed stone vs. sand), heavier foot/equipment traffic, or rebar/mesh-heavy pours that raise puncture risk generally call for Class A rather than defaulting to the cheapest compliant class.
4. Verify installation per the manufacturer's instructions and any local code amendments layered on top (ACI 302 guidance is commonly referenced): seams lapped and taped, penetrations sealed, and placement matching the specific slab design detail (some designs place a sand cushion above or below the retarder).