Dew Point Calculator

The substrate has to be at least 5°F (2.8°C) warmer than the dew point of the air around it before you apply a coating. SSPC-PA 3 and ASTM D3276 both state it, and most epoxy and polyaspartic data sheets repeat it.

The reason for a margin rather than simply staying above the dew point is that conditions move. Air temperature, humidity and slab temperature all drift over the hours a floor takes to lay, and a slab sitting exactly at the dew point is one small change away from being wet.

Because condensation forms on the cold surface, not in the air. Concrete is a large thermal mass and lags well behind the air above it, so a slab is routinely several degrees colder than the room, especially first thing in the morning, on ground floors, and anywhere the sun has not reached.

This is the single most common way the check goes wrong. The air reads comfortably warm, the job looks fine, and the floor itself is sitting below the dew point the whole time. Use an infrared or contact thermometer on the concrete, and take readings in several places, because a shaded bay and a sunlit one are not the same slab.

Both work, because both widen the same gap, but they do not work at the same speed. Dehumidification lowers the dew point and shows up in your readings within the hour. Heating a slab is slow, because you are warming a lot of thermal mass, and heating the air alone barely moves the concrete at all in the short term.

One warning about heat: an unvented gas or propane heater puts a large amount of water vapour into the space as it burns. That raises the dew point at the same time as it raises the temperature, and it can leave the spread no better, or worse, than when you started. Use indirect fired or electric heat if the space is sealed.

No, and passing one tells you nothing about the other. They are two different failures with two different tests.

The dew point check is about water condensing onto the slab from the air, right now, on the day you coat. Concrete moisture testing (an RH probe to ASTM F2170, or calcium chloride to ASTM F1869) is about water already inside the slab that will push up through the coating for months afterwards. A slab can be bone dry inside and still be wet on top on a cold humid morning, and a warm dry surface can sit on top of a slab that is soaking. You need both checks before you coat.

The air reading, from a hygrometer in the space. The in slab RH probe number is a different measurement entirely and putting it in here gives a meaningless dew point.

That confusion is easy to fall into because both are quoted as a percentage RH. The probe reports the humidity of the air inside a hole drilled to 40% of slab depth, which is why it can read 80% or more on a slab whose surface feels perfectly dry. Use the probe reading in the concrete moisture calculator, and the ambient hygrometer reading here.

It uses the Magnus formula, the standard approximation used across coatings, HVAC and meteorology, applied to your air temperature and relative humidity. It then subtracts the dew point from your substrate reading to get the spread, and judges that spread against the 5°F minimum.

Under 5°F it returns a do not apply. From 5°F up to 9°F it passes the reading and flags the margin as thin. At 9°F or more it reads as safe to apply. That 9°F line is our own caution threshold rather than a published standard, and the page says so wherever it appears.