Under-Slab Vapor Barrier
I’m looking at City of Oakland California bulletin B19-001, which applies to conversion of non-habitable space (read basements, garages).
I’m trying to understand the fundamental reasons here. In lieu of the 6 mill polyethene that was never installed under the slab, they want a 4-mill poly on top, or an ASTM E96 topical vapor barrier of .06 perms or less.
In the past I’ve used a vented insulated floor system (like DriCore R+) without any special attention to a vapor barrier. In fact DriCore prohibits adding a second vapor barrier. DriCore R is rated at .79 perms, far below the .06 perm requirement.
Could you help me understand why the City wants a tight vapor barrier, and what ways that might be better or worse than a vented system between permeable slab and subfloor?
Vapor barrier BELOW the slab means a dry slab most of the year in our climate.
Vapor barrier ABOVE the slab means a wet slab basically all of the year.
Is that a good thing?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part