The 5°F Dew Point Rule for Coating Concrete
The slab is colder than the air, and it is the slab that gets wet. Here is what the dew point rule actually asks of you, how to measure it, and which fix works fastest when a morning fails.
The slab has to sit at least 5°F (2.8°C) above the dew point of the air around it before you coat. Measure three things: air temperature, relative humidity, and the slab surface itself. The first two give you the dew point; the third is the one that decides the answer, because it is the slab that gets wet, not the air. Put your readings into the dew point calculator and it returns the spread and the verdict.
What the rule actually says
SSPC-PA 3 and ASTM D3276 both state it, and nearly every epoxy and polyaspartic data sheet repeats it: the substrate must be a minimum of 5°F above the dew point at the time of application. Some manufacturers ask for more. None of them ask for less.
It is worth being precise about what that sentence does not say, because the two most common ways of getting this wrong are both misreadings of it. It is not a rule about air temperature, so a warm room proves nothing. And it is not a rule about humidity on its own, so a reasonable looking hygrometer reading proves nothing either. It is a rule about the gap between one specific surface and one specific calculated number, and both halves have to be measured on the day.
Why the slab, and not the air
Condensation forms on cold surfaces. That is the entire mechanism. Air holds a certain amount of water vapour, and the dew point is the temperature at which it can hold no more and starts giving it up. Anything colder than the dew point pulls water out of the air onto itself, which is why a cold drink sweats in a warm room while the room stays dry.
Concrete is a large thermal mass and it lags the air badly. A slab holds last night’s cold well into the morning, and a ground floor pour sits on earth that stays cool all year. The result is a floor that is routinely several degrees colder than the room above it, and that gap is largest exactly when contractors want to start: early, before the day has warmed anything up.
So the failure mode is quiet and it looks like nothing at all. The air is comfortable, the humidity reading seems fine, the floor looks dry to the eye, and there is a film of condensation on the concrete that is far too thin to see. The coating goes down onto water. Adhesion is compromised from the first minute, and the job comes back weeks later as peeling or a blushed, milky finish.
The three readings, and how to take them
Air temperature
- Taken in the space, at roughly working height
- Away from heaters, open doors, and direct sun through a window
- Feeds the dew point calculation together with humidity
Relative humidity
- A hygrometer reading of the air in the space
- This is not the in slab RH probe number, which measures something else entirely
- Both are quoted as a percentage, which is why they get mixed up
Substrate temperature
- The concrete surface itself, by infrared or contact thermometer
- Several readings, deliberately including the coldest parts of the floor
- The reading that decides the verdict, and the one a forecast can never give you
On the substrate reading, go looking for the worst spot rather than a representative one. Perimeter strips against external walls, anywhere permanently shaded, slabs over unheated voids or crawl spaces, the area inside a roller door that has been open all morning, and the far end away from whatever heat is running. A single reading in the middle of a warm bay is the easiest way to pass a check the floor as a whole fails, and a coating that fails in one bay has failed.
Run your readings through the calculator
Enter the three site readings and it returns the dew point, the spread, the verdict, and the slab temperature you would need to clear the rule at today's conditions.
Returns dew point 55.8°F, a spread of 6.2°F, and a thin margin verdict
Open the live calculator →Reading the verdict
The calculator sorts the spread into three bands. Two of them come straight from the standard; the third is our own, and the page says so wherever it appears.
Under 5°F: do not apply
- Below the industry minimum, whether or not the slab is visibly wet
- Includes the case where the slab is already under the dew point and condensing now
- Fix the conditions rather than the paperwork
5°F to 9°F: thin margin
- Clears the rule. You may coat
- Little room for the drift that happens over a working day
- Re-check between coats rather than trusting the morning reading
9°F and above: safe to apply
- Comfortable margin for normal conditions
- Our own caution threshold, not a published standard
- Still worth a second look if the weather is turning
Why have a middle band at all, when the standard gives a single pass or fail line? Because a floor is not laid in an instant. Air temperature, humidity and slab temperature all move over the hours a coat takes to go down and start curing, and a job that cleared the line by half a degree at eight in the morning may not still clear it at eleven. The margin also absorbs instrument error: a hygrometer reading a few percent high and a thermometer reading a degree low can between them put you under the rule while your paperwork says you passed.
What to do when a morning fails
There are only two levers. Raise the slab temperature, or lower the dew point by drying the air. Both widen the same gap. They do not work at anything like the same speed, and picking the wrong one costs you the day.
Dehumidification is the fast lever. Pulling water out of the air lowers the dew point directly, and because you are changing the air rather than the concrete it shows up in your readings within the hour. If you need a decision this morning, this is the one that can give you one.
Heating the slab is the slow lever. You are warming a very large thermal mass, and warming the air above it barely moves the concrete in the short term. Heat is the right answer when you can plan a day ahead and leave it running overnight. It is close to useless as a same morning fix.
The heater that makes it worse
One trap is worth stating on its own, because it catches people who did everything else right. An unvented gas or propane heater burns a hydrocarbon, and burning a hydrocarbon produces water. A direct fired heater puts all of that water into the space it is heating.
So the air warms, which helps, and the humidity climbs at the same time, which raises the dew point and eats the gain. In a sealed space the dew point can climb faster than the slab does, and you end up with a spread no better than when you started, or worse, having spent two hours on it. Use indirect fired heaters that vent their exhaust outside, or electric.
This is not the concrete moisture test
Passing the dew point check tells you nothing about whether the slab is dry enough to coat, and treating the two as one check is a genuinely expensive mistake.
The dew point check is about water condensing onto the slab out of the air, on the day, in the hour you are working. Concrete moisture testing is about water already inside the slab that will keep pushing 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. Different mechanism, different instrument, different verdict.
Run both before you coat. The concrete moisture calculator handles the in slab side, and the vapor barrier guide covers what to do when that reading is the one that fails.
Keep a lookup on the van
Once you have done this a few times you stop needing the arithmetic and start needing the pattern: at this air temperature and this humidity, roughly how warm does the slab have to be? The dew point reference table lays that out across the conditions this work actually happens in, so you can see at a glance which mornings are going to be a problem before you load the van.
Frequently Asked Questions
What exactly is the dew point rule?
The substrate must be at least 5°F (2.8°C) above the dew point of the surrounding air before you apply a coating. SSPC-PA 3 and ASTM D3276 both state it, and most epoxy and polyaspartic data sheets repeat it.
Note what it does not say. It is not a rule about air temperature, and it is not a rule about humidity on its own. It is a rule about the gap between one specific surface and one specific calculated number, and both halves have to be measured.
Why a 5°F margin instead of just staying above the dew point?
Because a floor is not laid in an instant and conditions do not hold still. Air temperature, humidity and slab temperature all drift over the hours a coat takes to go down and start curing, and a slab sitting exactly at the dew point is one small change away from being wet.
The margin is there to absorb that drift. It also absorbs instrument error: a hygrometer that reads a few percent high and a thermometer that reads a degree low can between them put you under the line while your paperwork says you cleared it.
Can I use the weather forecast instead of measuring?
No. A forecast dew point describes the outdoor air at a weather station, and you are coating an indoor slab whose temperature depends on the ground under it, the doors that have been open, and what the building did overnight.
The slab reading is the one a forecast can never give you, and it is the one that decides the answer. Concrete lags the air by hours, so a warm afternoon forecast tells you nothing about a floor that is still holding last night’s cold.
Does passing the dew point check mean the slab is dry enough to coat?
No, and treating it that way is a genuinely expensive mistake. They are two different failures.
The dew point check is about water condensing onto the slab out of the air, on the day. Concrete moisture testing is about water already inside the slab that will keep pushing up through the coating for months. 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. Run both checks.
If I fail, is it faster to heat the slab or dehumidify?
Dehumidify, almost always. Dropping the humidity lowers the dew point and shows in your readings within the hour, because you are changing the air, and air responds quickly.
Heating a slab is slow. Concrete is a large thermal mass, and warming the air above it barely moves the concrete in the short term. Heat is the right answer for a job you can plan a day ahead, not for a decision you need to make this morning.
Why did my heater make the reading worse?
Almost certainly because it was an unvented gas or propane heater. Burning a hydrocarbon produces water, and a direct fired heater puts all of it into the space it is heating.
So the air warms, which helps, but the humidity climbs at the same time, which raises the dew point and eats the gain. In a sealed space the dew point can rise faster than the slab does, leaving your spread no better than when you started, or worse. Use indirect fired heaters that vent their exhaust outside, or electric.
Where on the floor should I take the substrate reading?
In several places, and deliberately in the worst ones. A single reading in the middle of a warm bay is the easiest way to pass a check that the floor as a whole fails.
Go after perimeter strips against external walls, anywhere shaded, slabs over unheated voids or crawl spaces, areas near roller doors that have been open, and the far end away from whatever heat is running. The coldest spot on the floor is the one that condenses first, and a coating that fails in one bay has failed.