Moisture Vapor Barrier: When Is It Necessary Under Epoxy?
When installing an epoxy floor, success begins beneath the surface. One of the most critical—yet often overlooked—factors in epoxy flooring failure is moisture vapor transmission (MVT) from the concrete slab. If not addressed properly, this invisible force can lead to delamination, bubbling, and costly repairs.
In this article, we'll explain:
- What a moisture vapor barrier is
- When it is necessary under epoxy coatings
- How to test for moisture vapor
- The best types of barriers for your application
What Is a Moisture Vapor Barrier?
A moisture vapor barrier is a specially formulated coating or membrane applied to a concrete substrate to block the upward migration of water vapor. These barriers create a seal that prevents moisture from disrupting the adhesion and performance of resin-based floor coatings like epoxy, polyurea, or polyaspartic systems.
Why Is This Important?
Concrete is porous—it absorbs and releases moisture continuously, especially in below-grade or ground-level environments. Even if a slab looks dry, vapor may be rising through it due to groundwater, ambient humidity, or lack of a vapor retarder under the slab.
When Is a Moisture Vapor Barrier Necessary?
Not every epoxy project requires a moisture vapor barrier, but certain conditions absolutely demand it.
✅ You need a vapor barrier if:
- Moisture test results exceed 3 lbs/1,000 sq ft/24 hours (per ASTM F1869 calcium chloride test)
- Relative humidity in the slab exceeds 75% (per ASTM F2170 in-situ probe test)
- The project is on grade or below grade
- The building lacks a polyethylene vapor retarder beneath the slab
- You're working in a region with high water tables or humidity
- The space is used for warehousing, manufacturing, or healthcare, where floor failure is unacceptable
❌ You might not need a vapor barrier if:
- Moisture readings are well below 3 lbs MVT or 75% RH
- The slab is elevated (e.g., upper floor of a commercial building)
- A sub-slab vapor retarder is confirmed
- The space is climate-controlled and dry
How to Test for Moisture Vapor in Concrete
Before applying any epoxy or vapor barrier system, always perform at least one of the following concrete moisture tests:
1. Calcium Chloride Test (ASTM F1869)
Measures moisture vapor emission rate (MVER). A result above 3 lbs/1,000 sq ft/24 hrs suggests a barrier is needed.
2. Relative Humidity Test (ASTM F2170)
Involves placing humidity sensors inside drilled holes in the slab. Above 75% RH typically indicates risk.
3. Plastic Sheet Test (ASTM D4263)
A simple, qualitative check where a clear sheet is taped down for 24 hours. Condensation or darkening signals MVT.
Pro tip: Conduct multiple tests in different slab locations for accuracy.
Choosing the Right Moisture Vapor Barrier System
PolyTitan Coatings offers industrial-grade moisture vapor barrier systems compatible with all major epoxy and polyurea systems. These are typically:
- 100% solids epoxy moisture mitigation primers
- Low-permeability urethane sealers
- Moisture-tolerant base coats for high-MVT substrates
Features to look for:
- Meets or exceeds ASTM E96 permeability standards
- Designed for compatibility with subsequent epoxy layers
- Resistant to pH variations and salt efflorescence
What Happens If You Skip the Vapor Barrier?
Ignoring high moisture vapor transmission can lead to:
- Peeling, blistering, or bubbles under the coating
- Adhesion loss leading to delamination
- Mold growth and trapped moisture
- Costly recoating or removal projects
In short: skipping the barrier is a gamble—especially in moisture-prone environments.
Final Thoughts
Installing a moisture vapor barrier under epoxy flooring isn't just a precaution—it’s often a critical requirement. Whether you're coating a commercial warehouse, a garage, or an industrial facility, proper testing and moisture control ensures the long-term performance of your resin flooring system.
Need help choosing the right barrier system? Contact PolyTitan Coatings for expert product support and moisture-control solutions tailored to your site.