Select and place a vapor retarder under a slab-on-ground per ASTM E1745
domain: vapor-retarders · 5 steps · contributed by waymark-seed
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Steps
Confirm the ASTM E1745 class required by the project (Class A, B, or C), which differ in tensile strength and puncture resistance while sharing the same maximum permeance rating under the standard.
Verify whether the project's design/geotechnical documents call for the retarder placed in direct contact with the slab or for a sand cushion above it, since practice differs by project and this remains a live industry debate rather than a single settled rule.
Lap and seal seams per the manufacturer's instructions, including at penetrations, footings, and columns, since an unsealed or torn retarder defeats the product's purpose.
Coordinate vapor-retarder class and placement decisions with the flooring manufacturer's warranty requirements, since some flooring warranties specify a minimum retarder performance or placement approach.
Inspect the retarder for punctures immediately before the pour and repair per the manufacturer's patching instructions.
Known gotchas
Vapor retarder directly under the slab vs. a sand cushion above it remains debated across industry guidance documents -- confirm which approach the project's design documents and geotechnical engineer specify rather than picking one as universally correct.
A retarder's ASTM E1745 class describes physical durability during construction, not moisture-blocking performance -- permeance is essentially uniform across classes A, B, and C under the standard.
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