Dew Point Table: Minimum Slab Temperature to Coat
The whole psychrometric question in one grid. Find your air temperature and humidity, read off how warm the slab has to be, and compare it with your thermometer.
Find your air temperature down the side and your relative humidity across the top. The cell is the minimum slab temperature that clears the 5°F rule. Compare it against what your infrared thermometer reads on the concrete: at or above the cell you may coat, below it you may not. The dew point itself is always exactly 5°F below the cell. For your exact readings rather than the nearest cell, use the dew point calculator.
Minimum slab temperature, Fahrenheit
| Air °F | 30% RH | 40% RH | 50% RH | 60% RH | 70% RH | 80% RH | 90% RH |
|---|---|---|---|---|---|---|---|
| 45°F | 20 | 27 | 32 | 37 | 41 | 44 | 47 |
| 50°F | 25 | 32 | 37 | 42 | 46 | 49 | 52 |
| 55°F | 29 | 36 | 42 | 46 | 50 | 54 | 57 |
| 60°F | 33 | 41 | 46 | 51 | 55 | 59 | 62 |
| 65°F | 38 | 45 | 51 | 56 | 60 | 64 | 67 |
| 70°F | 42 | 50 | 56 | 60 | 65 | 69 | 72 |
| 75°F | 46 | 54 | 60 | 65 | 70 | 73 | 77 |
| 80°F | 51 | 59 | 65 | 70 | 74 | 78 | 82 |
| 85°F | 55 | 63 | 69 | 75 | 79 | 83 | 87 |
| 90°F | 59 | 67 | 74 | 79 | 84 | 88 | 92 |
Minimum slab temperature, Celsius
| Air °C | 30% RH | 40% RH | 50% RH | 60% RH | 70% RH | 80% RH | 90% RH |
|---|---|---|---|---|---|---|---|
| 5°C | -8.4 | -4.7 | -1.8 | 0.7 | 2.8 | 4.6 | 6.3 |
| 10°C | -4.0 | -0.2 | 2.8 | 5.4 | 7.6 | 9.5 | 11.2 |
| 15°C | 0.3 | 4.3 | 7.4 | 10.1 | 12.4 | 14.4 | 16.2 |
| 20°C | 4.7 | 8.8 | 12.0 | 14.8 | 17.1 | 19.2 | 21.1 |
| 25°C | 9.0 | 13.2 | 16.6 | 19.5 | 21.9 | 24.1 | 26.0 |
| 30°C | 13.3 | 17.7 | 21.2 | 24.2 | 26.7 | 28.9 | 31.0 |
The shaded column is the point of the table
Look at what happens on the right hand side. Once relative humidity passes roughly 85%, the dew point sits less than 5°F below the air temperature, so the minimum slab temperature ends up above the air temperature. There is no way to satisfy the rule by warming the concrete, because the concrete would have to be hotter than the room it is in.
That is not an edge case. It is the ordinary condition on a humid morning, and it is the single most useful thing this grid shows you: on those days heat is the wrong lever entirely. Dehumidification lowers the dew point directly and shows in your readings within the hour, while heating a slab is slow work on a large thermal mass. The companion guide goes through both fixes, including the unvented gas heater that raises the dew point faster than it raises the slab.
What the table cannot tell you
It covers two of the three readings. Air temperature and humidity give you the dew point, and that is what the grid is. The third reading, the temperature of the slab itself, is the one that decides the answer, and no table can supply it: it depends on the ground under the floor, what the building did overnight, and how long the roller door has been open.
So use this to see the shape of the day before you load the van, and take a real thermometer to the concrete before you open a kit. Take several readings, and go looking for the coldest part of the floor rather than a representative one.
One last boundary worth restating: none of this says anything about moisture inside the slab. That is a different failure with a different test, and it is covered by the concrete moisture calculator. A floor can pass every cell in this table and still be far too wet to coat.
Frequently Asked Questions
What number in the table do I compare my thermometer against?
The slab reading. Each cell is the minimum substrate temperature that clears the 5°F rule at that combination of air temperature and relative humidity. If your infrared thermometer reads below the cell, you are under the rule and should not coat.
The dew point itself is always exactly 5°F below the number in the cell, so you can recover it by subtracting five if you need it for a report.
Why does part of the table ask for a slab hotter than the air?
Because at high humidity the dew point is very close to the air temperature, so a 5°F margin below it does not exist. Once relative humidity passes roughly 85%, the dew point sits less than 5°F under the air temperature at ordinary working temperatures, and the only way to satisfy the rule by heating would be to make the concrete hotter than the room.
Those cells are marked in the grid. They are not a mistake and they are not an edge case: they are the ordinary condition on a humid morning, and they are telling you that heat is the wrong lever. Dehumidify instead, which lowers the dew point directly.
Is this table specific to epoxy?
No. Dew point is physics rather than chemistry, so the grid is the same whatever you are applying. What changes by product is the margin your data sheet asks for.
This table is built on the 5°F minimum from SSPC-PA 3 and ASTM D3276, which is the figure most epoxy, polyaspartic and polyurea data sheets repeat. If your product asks for more, add the difference to every cell.
How accurate is it, and can I use it instead of measuring?
The dew points are the Magnus approximation, which is the standard used across coatings, HVAC and meteorology, and it is well within the precision of any thermometer or hygrometer you will have on site.
But the table cannot replace measuring, because it only covers two of the three readings. It tells you what the air is doing; it cannot tell you how cold your slab is, and that is the number that decides the answer. Use the grid to see the shape of the problem before you load the van, and take real readings on the floor before you open a kit.