Quick answer: There are three kinds of "moisture barrier" for concrete, and they solve different problems. A below-slab vapor retarder (plastic sheet under the concrete, installed before the pour) stops ground moisture for the life of the slab. A surface-applied moisture mitigation system (usually a two-part epoxy, ASTM F3010) seals an existing slab whose moisture is too high for the flooring. An underlayment with a vapor barrier (for example 6-mil polyethylene) protects floating floors such as laminate. Test the slab first (ASTM F2170 or F1869) and pick the barrier that matches the flooring manufacturer's limit.
Quick answer
- New slab: vapor retarder below the slab (ACI recommends at least 10 mil under moisture-sensitive flooring).
- Existing slab too wet for glue-down flooring: epoxy mitigation system meeting ASTM F3010.
- Floating laminate or vinyl plank: underlayment with a built-in vapor barrier or 6-mil poly.
The three types compared
| Below-slab vapor retarder | Surface moisture mitigation system | Underlayment vapor barrier | |
|---|---|---|---|
| Where | Directly under the concrete, on the prepared base | Applied to the top of the cured slab | Between slab and floating floor |
| When | Before the pour | Before flooring on an existing slab | When installing floating floors |
| Stops | Ground moisture rising into the slab | Moisture leaving the slab into the flooring and adhesive | Vapor reaching a floating floor |
| Does not | Remove water already in the concrete | Dry the slab; fix liquid water or hydrostatic pressure | Protect glue-down floors |
| Typical spec | ASTM E1745 vapor retarder; at least 10 mil under moisture-sensitive flooring (ACI 302.1R) | ASTM F3010 two-component resin system, rated to the measured RH or MVER | 6-mil polyethylene, taped and overlapped, or as the floor maker specifies |
ACI's guide to concrete floor and slab construction discusses vapor retarder placement and moisture-sensitive floor coverings (ACI 302.1R-15, chapter 5); ASTM F3010 is the standard practice for resin-based moisture mitigation membranes (ASTM F3010).
Step 1: Find the moisture source
| Source | Sign | Fix |
|---|---|---|
| No vapor retarder under the slab | Constant high readings, slab on grade | Surface mitigation system |
| Construction water in a new slab | High readings that fall over months | Time, heat and dehumidification; then test |
| Recent leak or flood | Readings high near the leak | Dry first; see drying a concrete floor after a leak |
| Liquid water (high water table, drainage) | Standing water, efflorescence, damp patches | Drainage and waterproofing; barriers are not enough |
| Condensation on a cold slab | Wet surface in humid weather, spring | Dehumidify and warm; see spring warehouse floor condensation |
Related: drying a concrete floor after a leak, spring warehouse floor condensation.
Step 2: Test
| Test | Measures | Typical flooring limits |
|---|---|---|
| ASTM F2170 in-situ RH probes | RH inside the slab at a specified depth | Often 75–85 % RH; mitigation systems may be rated to 95–100 % |
| ASTM F1869 calcium chloride | Moisture vapor emission rate from the surface | Often 3–5 lb per 1,000 ft² per 24 h |
Sources: ASTM F2170, ASTM F1869. Test with the building at service conditions. Use the flooring and adhesive manufacturer's test method and limit.
Step 3: Choose
| Test result | Choose |
|---|---|
| Within the flooring maker's limit | Install as specified; underlayment barrier for floating floors |
| Slightly above, slab still drying | Wait and dry with heat and dehumidification, then retest |
| Well above, or no below-slab retarder | Surface mitigation system rated for the measured value |
| Liquid water or hydrostatic pressure | Fix drainage first |
Where dehumidification helps
A dehumidifier dries the air above an exposed slab, so construction water or leak water leaves faster, and it prevents condensation on cold slabs in humid weather. It does not stop ground vapor; that needs a barrier. On fit-outs, running dehumidifiers with the building heated to service temperature is often the fastest way to get a new slab to the flooring limit. Our commercial portable dehumidifiers and industrial dehumidifiers are used for this.
Common failures
- Laying flooring on a slab that "looks dry" without testing.
- A mitigation coating applied over a dirty or weak surface, which then debonds.
- Polyethylene under a glue-down floor, which prevents bonding.
- Ignoring liquid water or drainage problems.
- Sealing a slab that is still wet from a leak.
FAQ
Frequently asked questions
What is the best moisture barrier for a concrete floor?
For new slabs, a vapor retarder under the slab. For existing slabs too wet for flooring, an epoxy mitigation system meeting ASTM F3010. For floating floors, an underlayment with a vapor barrier.
Do I need a moisture barrier on concrete under laminate?
Yes, almost always: a 6-mil polyethylene sheet or an underlayment with a built-in barrier.
Can I put a moisture barrier on a wet concrete floor?
Not if it is wet from a leak or flood; dry it first. Mitigation systems are for slabs with high vapor emission, not standing water.
How do I test concrete for moisture?
ASTM F2170 in-situ RH probes or the ASTM F1869 calcium chloride test, as the flooring maker specifies.
Sources
- ACI 302.1R-15, Guide to Concrete Floor and Slab Construction, Chapter 5.
- ASTM F2170, F1869, F3010; ASTM E1745.
