How do I know if my concrete is too wet for epoxy?

You test it, and there are two accepted ways. A calcium chloride kit under ASTM F1869 measures what is coming off the surface, reported in pounds per 1,000 square feet per 24 hours. An in-situ probe under ASTM F2170 measures humidity inside the slab, reported as a percentage. Both Euclid Chemical and Paramount Coatings put the calcium chloride limit at 3 pounds. On humidity they disagree: Paramount says 80 percent, Euclid says 85.

What happens if I coat a slab that is too wet?

The water keeps coming and the coating has nowhere to let it go, so pressure builds under the film and lifts it. It rarely happens on day one, which is what makes it dangerous: the floor looks perfect through the first summer and then bubbles, and by the time it shows the whole coating has to come off. This is the failure mode a moisture vapour barrier exists to prevent, and at $2.00 to $4.50 per square foot it is cheap next to doing the floor twice.

Can I just tape plastic to the floor overnight?

It will tell you something and it will not tell you enough. A sheet taped down for a day or two showing condensation underneath is a clear signal there is moisture moving, but a dry sheet is not evidence of a dry slab, because the method has no threshold attached to it. None of the manufacturers quoted on this page accepts it as a measurement. If you are spending four figures on a floor, a calcium chloride kit costs a small fraction of that and produces a number a contractor will act on.

How old does a new slab have to be before I coat it?

The sources on this page disagree by a factor of four, which is worth knowing before you plan around either. Sherwin-Williams states 28 days at 24 degrees for ArmorSeal 1000 HS. The Westcoat specification says a minimum of 7 to 10 days. This tool takes the stricter figure, because coating a slab that is still releasing its own mix water is the one mistake that no product on top can fix, and waiting three more weeks costs nothing but patience.

Why do the manufacturers not agree on the humidity limit?

Because it is a judgement about a specific formulation and its tolerance, not a law of concrete. Paramount will not use its PCE No. 120 as a primer over 80 percent RH; Euclid says use a mitigation system at 85 percent or above. Between those two numbers, one company would proceed and the other would not, so this tool reports that as a disputed band rather than picking a side. If your reading lands there, the deciding vote belongs to whoever is warrantying your floor.

Is there a reading too high for a barrier to fix?

Yes. Westcoat specifies its EC-15 moisture vapour barrier for floors up to 15 pounds per 1,000 square feet per 24 hours, or 95 percent RH. Above that you are outside what these named products are rated for, and the problem has stopped being about coatings. A slab pushing that much water usually has no vapour retarder under it, or has groundwater against it, and both are solved below the concrete rather than on top of it.

Concrete Moisture Test: Will Epoxy Stick to This Slab?

Last updated: August 26, 2026By EpoxyFloorIndex Research Team

Moisture is the reason coatings peel in this climate, and it is measurable before anybody mixes anything. Run a calcium chloride kit or an in-situ probe, put the number below, and this page grades it against the limits three manufacturers actually publish, including the one place where two of them contradict each other.

Grade your own moisture reading

Which test did you run?

Anhydrous calcium chloride test ASTM F1869Moisture leaving the surface of the slab, measured by how much a sealed dish of calcium chloride gains in weight.In-situ relative humidity probe ASTM F2170Humidity inside the slab, measured by a probe in a drilled hole rather than at the surface.

Your reading (lb / 1,000 sq ft / 24 h)Floor area (sq ft)

How old is the slab, in days? (optional)The strictest sheet here wants 28 days before anything is coated.

3.5 lb / 1,000 sq ft / 24 h · ASTM F1869

Over every published limit. Budget for a moisture mitigation system

Euclid Chemical EUCOPOXY TUFCOATover 3 lb / 1,000 sq ft / 24 h
“If the relative humidity is 85% or greater, or the MVER is 3 lbs/1000 ft2/24 hrs or greater, use a moisture mitigation system”
Paramount Coatings PCE No. 120 Epoxy Moisture Mitigation Barrierover 3 lb / 1,000 sq ft / 24 h
“Do not use as a primer when concrete slab exceeds ASTM F1869 3 lbs. or ASTM F2170 80% RH.”
Westcoat EC-15 Moisture Vapor Barrierthe highest reading any barrier here is specified forup to 15 lb / 1,000 sq ft / 24 h

A moisture vapour barrier over 400 sq ft would add $800 to $1,800 at $2 to $4.50 per sq ft (SurfacePro Epoxy Flooring; Armour Guard), before 13% HST.

These are acceptance limits published by coating manufacturers, not the ASTM test methods themselves. Whatever product goes on your floor has its own limits on its own sheet, and those are the ones that govern the warranty.

The two tests, and what each one actually measures

They are not interchangeable and they do not measure the same thing, which is why a slab can pass one and fail the other.

Anhydrous calcium chloride test

ASTM F1869

Moisture leaving the surface of the slab, measured by how much a sealed dish of calcium chloride gains in weight.

Reported in lb / 1,000 sq ft / 24 h

In-situ relative humidity probe

ASTM F2170

Humidity inside the slab, measured by a probe in a drilled hole rather than at the surface.

Reported in % RH

The surface test can read low on a slab that is wet inside, because a dense trowelled surface slows the moisture down without stopping it. That is why specifications increasingly ask for the probe, and why a contractor who runs both is being careful rather than difficult.

Every published limit, quoted

Three manufacturers, three documents, and one genuine disagreement. All read in August 2026.

Euclid Chemical EUCOPOXY TUFCOAT
Mitigate at or above 3 lb / 85% RH
“If the relative humidity is 85% or greater, or the MVER is 3 lbs/1000 ft2/24 hrs or greater, use a moisture mitigation system”
Euclid Chemical EUCOPOXY TUFCOAT product data
Paramount Coatings PCE No. 120 Epoxy Moisture Mitigation Barrier
Mitigate at or above 3 lb / 80% RH
“Do not use as a primer when concrete slab exceeds ASTM F1869 3 lbs. or ASTM F2170 80% RH.”
Paramount Coatings PCE No. 120 technical data sheet, October 9, 2020
Westcoat EC-15 Moisture Vapor Barrier
Specified up to 15 lb / 95% RH
“EC-15 Moisture Vapor Barrier is recommed for use on floors with moisture up to 15 lbs./1000 square feet / 24 hours (ASTM F1869) or 95% RH (ASTM F2170).”
Westcoat EC-15 Moisture Vapor Barrier, CSI specification 09 05 61.13

How long a new slab has to sit first

These two disagree by a factor of four. Both are quoted here rather than reconciled, and the calculator uses the stricter one, because coating a slab still releasing its own mix water is the failure nothing on top can fix.

28 days (at 75F / 24C)
“Concrete and mortar must be cured at least 28 days @ 75F (24C).”
Sherwin-Williams ArmorSeal 1000 HS product data sheet
7 days (no temperature stated)
“Concrete must be cured for a minimum of 7-10 days before coating is applied.”
Westcoat EC-15 Moisture Vapor Barrier, CSI specification 09 05 61.13

What this page will not tell you

It will not estimate your moisture level. There is no input on this page that produces a reading, because there is no honest way to guess one: a slab's moisture depends on what is under it, how it was poured, how old it is and what the weather has done since, and none of that can be inferred from a form. The reading has to come from a kit or a probe on your actual floor.

It will not reproduce the ASTM methods. F1869 and F2170 are purchased standards with detailed procedures about conditioning the building, placing the kit and how long to leave it, and summarising them here from second-hand descriptions would be the wrong kind of confident. What is here instead is the part manufacturers publish openly: the number at which they change what they will do.

And it will not tell you your floor is fine. A reading under every limit on this page means those three products' published thresholds are satisfied. Whatever is actually going on your slab has its own sheet with its own numbers, and that sheet is what a warranty is written against.

Frequently asked questions

How do I know if my concrete is too wet for epoxy?: You test it, and there are two accepted ways. A calcium chloride kit under ASTM F1869 measures what is coming off the surface, reported in pounds per 1,000 square feet per 24 hours. An in-situ probe under ASTM F2170 measures humidity inside the slab, reported as a percentage. Both Euclid Chemical and Paramount Coatings put the calcium chloride limit at 3 pounds. On humidity they disagree: Paramount says 80 percent, Euclid says 85.
What happens if I coat a slab that is too wet?: The water keeps coming and the coating has nowhere to let it go, so pressure builds under the film and lifts it. It rarely happens on day one, which is what makes it dangerous: the floor looks perfect through the first summer and then bubbles, and by the time it shows the whole coating has to come off. This is the failure mode a moisture vapour barrier exists to prevent, and at $2.00 to $4.50 per square foot it is cheap next to doing the floor twice.
Can I just tape plastic to the floor overnight?: It will tell you something and it will not tell you enough. A sheet taped down for a day or two showing condensation underneath is a clear signal there is moisture moving, but a dry sheet is not evidence of a dry slab, because the method has no threshold attached to it. None of the manufacturers quoted on this page accepts it as a measurement. If you are spending four figures on a floor, a calcium chloride kit costs a small fraction of that and produces a number a contractor will act on.
How old does a new slab have to be before I coat it?: The sources on this page disagree by a factor of four, which is worth knowing before you plan around either. Sherwin-Williams states 28 days at 24 degrees for ArmorSeal 1000 HS. The Westcoat specification says a minimum of 7 to 10 days. This tool takes the stricter figure, because coating a slab that is still releasing its own mix water is the one mistake that no product on top can fix, and waiting three more weeks costs nothing but patience.
Why do the manufacturers not agree on the humidity limit?: Because it is a judgement about a specific formulation and its tolerance, not a law of concrete. Paramount will not use its PCE No. 120 as a primer over 80 percent RH; Euclid says use a mitigation system at 85 percent or above. Between those two numbers, one company would proceed and the other would not, so this tool reports that as a disputed band rather than picking a side. If your reading lands there, the deciding vote belongs to whoever is warrantying your floor.
Is there a reading too high for a barrier to fix?: Yes. Westcoat specifies its EC-15 moisture vapour barrier for floors up to 15 pounds per 1,000 square feet per 24 hours, or 95 percent RH. Above that you are outside what these named products are rated for, and the problem has stopped being about coatings. A slab pushing that much water usually has no vapour retarder under it, or has groundwater against it, and both are solved below the concrete rather than on top of it.

Sources

Read August 2026. Manufacturer acceptance limits, not the ASTM standards themselves. Published technical data is subject to change without notice, so check the sheet for the product going on your floor.