Most coating failures on basement floors are not coating failures. They are water, moving up through the slab from the ground underneath, pushing a sealed film off from below.
You cannot see it, and you usually cannot feel it. You measure it, with one of two published tests.
Why the water goes upward
Concrete is porous. Under a slab poured without a vapor barrier, and most older New England slabs were, the soil is damp more or less permanently. Water moves from wet to dry, so it travels up through the concrete and evaporates off the surface into your basement.
On a bare slab that happens invisibly. Put a sealed coating over the top and the vapor has nowhere to go, so pressure builds under the film until it lifts. The coating comes away in sheets with a chalky white residue on the back, and the concrete underneath looks perfectly sound. That last detail is what makes it so confusing.
Test one: calcium chloride, ASTM F1869
A small dish of anhydrous calcium chloride is weighed, placed on the bare slab and sealed under a plastic dome for 60 to 72 hours. Calcium chloride absorbs moisture, so you weigh it again afterwards and the gain tells you how much vapor came out of that area of slab in a day.
The result is a moisture vapor emission rate, expressed in pounds per thousand square feet per 24 hours. Coating manufacturers state a maximum their product tolerates.
What it measures is the surface. That is its limitation: it tells you what is leaving the top of the slab under the conditions on the day, not what is inside the concrete.
Test two: in situ relative humidity, ASTM F2170
Holes are drilled into the slab, to 40 percent of its depth where it dries from one side only, sleeved, and left to equilibrate. A probe then reads the relative humidity inside the concrete.
This measures what the slab actually contains rather than what is coming off it, which makes it far more predictive of what happens once you seal the surface. It has become the preferred method for anything critical.
What we do
We test a basement before we write the price, not after the floor fails.
If the reading comes back inside the coating system's tolerance, the standard build goes down. If it is high, the floor gets a moisture tolerant primer first and that appears as a line item on the quote rather than as a surprise later. If the slab is genuinely wet, we say the floor is not the first job that needs doing.
Garages usually do not need this. They are above grade, they dry through the door, and vapor drive is rarely the issue. Basements are the reverse.
What a high reading does not mean
It does not automatically mean no. Moisture tolerant systems exist and they work. It means the specification changes, and therefore the price changes, and you should know that before you sign rather than afterwards.
Related
- Basement floors and how the test changes what we quote
- Basement coating vs the can of epoxy paint
- Why coatings peel for the bonding side of the same question
Reference
Both methods are published standards: ASTM F1869 for anhydrous calcium chloride vapor emission, and ASTM F2170 for in situ relative humidity in hardened concrete.