Commercial Building Inspector B2Floor ConstructionMedium
A commercial building inspector is examining the installation of a new concrete floor slab on grade in a warehouse. The plans specify a vapor retarder directly beneath the slab. According to the IBC, what is the minimum perm rating required for this vapor retarder?
- A1.0 perm
- B0.01 perm
- C0.3 perm
- D0.1 perm
Show answer & explanationAnswer & explanation
Correct answer: B. 0.01 perm
The IBC, referencing ASTM E1745, requires a vapor retarder beneath concrete slabs on grade to have a maximum permeance of 0.01 perm (Class A) when tested in accordance with ASTM E96.
Why the other options are wrong
- A. 1.0 perm is typically considered a vapor retarder but not a vapor barrier for critical applications like under slabs.
- C. 0.3 perm is too high for a vapor retarder required beneath a concrete slab on grade.
- D. 0.1 perm is a common requirement for vapor barriers in some wall assemblies, but not sufficiently low for beneath concrete slabs.
Slab-on-Grade Vapor Retarder
A vapor retarder beneath a concrete slab on grade is essential to prevent moisture migration from the ground into the building, protecting floor finishes and indoor air quality.
- Required by IBC for slabs on grade.
- Minimum Class A material per ASTM E1745.
- Maximum permeance of 0.01 perm.
- Installed directly under the slab.
Memory trick: Moisture's foe, a perm so low, under the slab, where roots don't grow.