Applying a coating onto a surface that is too close to the dew point is one of the quickest ways to ruin a floor. Moisture condenses on the surface, the coating blushes or cures cloudy and sticky, and adhesion is poor. The fix is simple: measure, calculate and check again.
This guide explains the dew point rule used for most epoxy, polyurethane and similar coatings, shows worked examples, and says when to recheck. The data sheet for your product has the final say on temperature and humidity limits.
What dew point means
The dew point is the temperature at which the water vapour in the air turns to liquid on a surface. If the surface is at or below that temperature, it gets a film of moisture you cannot always see. A coating applied onto it traps the moisture or reacts with it.
The rule: at least 3 °C (5 °F) above dew point
Most manufacturers require the substrate temperature to be at least 3 °C (5 °F) above the dew point during application and while the coating cures. Check the data sheet, because some products ask for more.
To work it out, measure three things:
- Air temperature
- Relative humidity
- Substrate (floor) surface temperature
Calculate the dew point from the air temperature and humidity. Then compare it with the substrate temperature.
Worked examples
| Air temperature | Relative humidity | Dew point (approx.) | Minimum substrate temperature |
|---|---|---|---|
| 20 °C | 60% | 12 °C | 15 °C |
| 25 °C | 70% | 19 °C | 22 °C |
| 30 °C | 80% | 26 °C | 29 °C |
| 15 °C | 80% | 12 °C | 15 °C |
In the second row the air is 25 °C at 70% humidity. The dew point is about 19 °C, so the floor needs to be at least 22 °C. A cool slab at 20 °C in a warm, humid room fails the check even though the air feels comfortable.
When to recheck
Conditions change during the day, so measure again:
- Before mixing
- During application on long jobs
- After a break
- In the evening and early morning, when floors cool faster than the air
- After rain or opening doors to humid air
Keep the substrate above the limit during curing as well as application.
Common problems when it goes wrong
- A cloudy, hazy or greasy-looking film on the surface (often called blushing)
- A sticky coating that does not harden properly
- Poor adhesion between coats
- Pinholes and surface defects
Frequently asked questions
What does the 3 °C rule mean?
The surface must stay at least 3 °C (5 °F) above the dew point while the coating is applied and curing, so condensation cannot form on it.
Does the rule apply to every coating?
Most epoxy, polyurethane and similar coatings use it, but limits differ by product. Follow the data sheet.
Do I use air temperature or floor temperature?
Use both. The air temperature and humidity give you the dew point, and the substrate temperature is what you compare against it.
How often should I measure?
Before starting, at intervals during application, and again as the day cools. Record the readings with your job records.
Dew point values are approximate. The manufacturer's data sheet and the project specification set the limits for the product you are applying.