Moisture Vapor Transmission in Arizona Concrete Floors
“It’s a desert — how can we have a moisture problem?” That question comes up in nearly every pre-job consultation on a Phoenix-area slab that blistered or failed. The short answer: Arizona soil can have significant moisture below grade even when the surface looks dry, and concrete is permeable. Water vapor migrates upward through the slab continuously. When a coating blocks that migration path, hydrostatic pressure builds at the coating-to-concrete interface and the coating pops.
What MVT Is and Why It Matters
Moisture vapor transmission (MVT) is the rate at which water vapor moves through a concrete slab, measured in pounds per 1,000 square feet per 24 hours. ASTM E1907 testing involves a calcium chloride test kit placed on a prepared surface for 60 to 72 hours — the weight gain of the kit measures the vapor emission rate. Most coating manufacturers specify a maximum of 3 to 5 lb per 24 hours. Phoenix-area slabs sitting over clay soils, near irrigation infrastructure, or above high water table zones can exceed 10 lb per 24 hours, even in the summer months.
Why Arizona Slabs Have MVT Issues
Several factors make Phoenix-metro slabs prone to elevated MVT:
- Irrigation and landscaping. Turf, drip-irrigated landscaping, and pool decks adjacent to the slab keep soil moisture elevated year-round, regardless of ambient humidity.
- Clay soil. Maricopa County has substantial clay soil zones that retain moisture below grade even through dry surface conditions.
- New construction slabs. Slabs poured less than 12 months ago have residual construction moisture that has not fully dissipated — particularly relevant for commercial buildouts on new shells.
- Vapor barrier failures. Older construction may have inadequate or punctured vapor barriers below the slab, allowing ground moisture direct access.
What Happens Without Testing
A contractor who applies epoxy or polyaspartic over a high-MVT slab without testing and addressing it produces a floor that blisters within six to eighteen months, often faster in summer when thermal cycling accelerates the process. The blistering looks like a product failure. It is a prep failure. The coating was never going to hold without addressing the underlying vapor emission rate. We test before we coat. If MVT is elevated, we address it with a vapor mitigation primer before any finish system. See our epoxy garage coating page or commercial polished concrete page for how this fits into the full installation process. Call (602) 292-2849 or contact us with questions.
MVT Questions
Is MVT testing required on every project?
We test on any project where we intend to apply a coating system. Polished concrete without a coating is less affected by MVT — the densifier treatment actually improves the slab’s resistance to vapor transmission. For coating applications, testing is non-negotiable in our process.
How do you fix high MVT before coating?
For MVT under about 8 lb/24hr, a moisture-tolerant epoxy primer bridges the gap. Above 8 lb, we use a dedicated vapor mitigation product — a two-component epoxy specifically formulated to block vapor transmission. These products add to project cost and cure time but prevent the blistering failure mode.
Does polishing reduce MVT?
Partially. Lithium silicate densification reduces surface porosity and slows vapor transmission somewhat, but does not eliminate it. For floors being polished and left without a coating, MVT is a reduced concern. For polished floors receiving a guard sealer, we still assess vapor emission before applying the sealer.
How do I know if my floor has an MVT problem before calling?
Warning signs: white powder deposits or efflorescence on the slab surface, a damp smell in an otherwise dry space, or a previous coating that blistered or peeled without obvious mechanical damage. These all suggest elevated MVT. Call (602) 292-2849 and describe what you’re seeing.
Can MVT be tested on an existing coated floor?
Yes, but it requires removing the coating from a test area first. Testing over an existing intact coating produces meaningless results since the coating itself is blocking the vapor. If you’re evaluating a previously coated floor that failed, we test on a cleared patch.