Condensation Can Defeat an Otherwise Good Installation

Concrete may pass the required slab-moisture tests and still be unsuitable for coating at a particular time. If the concrete surface is too close to the dew-point temperature, invisible moisture or visible condensation can form on it.

Applying a primer, coating, adhesive, or mitigation membrane over condensation can interfere with wetting, penetration, adhesion, cure, appearance, and long-term performance.

Environmental testing is therefore not a one-time administrative step. Conditions must remain within the permitted range during preparation, application, cure, and recoating for the periods required by the manufacturer.

Four Measurements the Contractor Must Understand

What Is Dew Point?

Dew point is the temperature at which air reaches saturation when cooled without changing its moisture content. If a surface reaches or falls below that temperature, moisture can condense from the air onto the surface.

Dew point is not the same as relative humidity. Relative humidity changes when air temperature changes, even when the amount of water vapor remains the same. Dew point provides an indication of the actual moisture content of the air.

A Simple Example

A warm warehouse may contain humid air while its concrete floor remains cool from the previous night. When doors open and warm humid air enters, the air next to the cool slab can reach saturation. Moisture then forms on the concrete even though the water did not come through the slab.

The Dew-Point Spread

The dew-point spread is the difference between the surface temperature and the calculated dew-point temperature.

Many coating specifications require the substrate to remain at least 5°F (approximately 3°C) above the dew point. This is a common industry requirement, but it is not universal.

The project specification and coating manufacturer's current technical data govern. Some materials, substrates, or exposures may require a larger margin or impose additional relative-humidity and temperature limits.

Why Condensation May Be Invisible

A surface does not have to appear wet for moisture to interfere with coating adhesion. A thin molecular film can develop before droplets are easily visible.

Dust, profile, porosity, lighting, and surface color can make condensation difficult to see. Relying on touch or appearance alone can therefore allow coating work to begin under unacceptable conditions.

Where Condensation Is Most Likely

• Near exterior doors and loading docks
• Inside refrigerated or cold-storage facilities
• On slabs cooled overnight
• Near uninsulated cold-water pipes
• Below HVAC diffusers or cooling equipment
• At exterior walls and perimeter slab areas
• In unconditioned buildings during changing weather
• Where warm humid air enters a cooler interior
• Near open pits, tunnels, tanks, or below-grade structures
• On metal plates, drains, or embedded components that differ from slab temperature

Environmental Conditions Can Vary Across the Work Area

One reading at the center of a large room may not represent the entire project. Surface temperature can differ near exterior walls, doors, windows, refrigeration equipment, sunlight, air-conditioning outlets, and unconditioned adjoining spaces.

Concrete temperature may also lag behind air temperature. Turning on temporary heat can warm the air rapidly while the slab remains cold. Relative humidity may appear to improve, but the cold surface can still be near the dew point.

Representative Measurement Locations

• The coldest expected substrate area
• The warmest or sun-exposed area
• Exterior doors and perimeter walls
• HVAC supply and return locations
• Refrigerated or mechanically cooled areas
• Low points, pits, trenches, and drains
• Areas with different slab elevations or thicknesses
• Locations where work will begin, continue, and finish

Measuring Environmental Conditions

Digital environmental meters commonly measure air temperature and relative humidity and calculate dew point. Surface temperature can be measured with a contact thermometer, surface probe, or infrared instrument.

Traditional sling or aspirated psychrometers may also be used when permitted. Whatever instrument is selected must be suitable for the required range and accuracy and should have current calibration or verification documentation.

Digital Meter Considerations

• Allow the instrument to acclimate to the work environment.
• Do not hold the humidity sensor directly in your breath.
• Keep the sensor away from wet material, solvent vapor, and direct spray.
• Allow readings to stabilize before recording them.
• Check battery condition and instrument status.
• Record the meter identification and calibration information.

Infrared Thermometer Considerations

Infrared instruments measure emitted energy rather than touching the surface. Their accuracy can be affected by emissivity, distance, spot size, surface angle, reflective materials, and instrument setup.

Concrete generally provides a more reliable infrared target than polished metal, but the operator must still follow the manufacturer's instructions. When a result is close to the application limit, confirm it with an appropriate contact measurement when practical.

Basic Environmental-Monitoring Procedure

1. Review the product limits. Identify permitted air temperature, substrate temperature, relative humidity, dew-point spread, and cure conditions.
2. Inspect the work area. Identify doors, HVAC outlets, refrigeration, exterior walls, sunlight, standing water, and unusual airflow.
3. Check the instrument. Confirm model, condition, battery, calibration, and required settings.
4. Allow the instrument to acclimate. Moving a meter from a vehicle or office directly into the work area can produce unstable readings.
5. Measure air conditions. Record ambient temperature and relative humidity at representative locations.
6. Determine dew point. Use a suitable instrument or approved calculation method.
7. Measure surface temperature. Check the coldest and other representative substrate locations.
8. Calculate the spread. Subtract the dew point from each applicable surface-temperature reading.
9. Compare with all limits. Confirm that temperature, humidity, and dew-point separation meet the complete system requirements.
10. Continue monitoring. Repeat readings during application and cure at the required frequency and whenever conditions change.

When Measurements Should Be Taken

Environmental conditions should be measured before work starts and as often as required during preparation, mixing, application, recoating, and cure.

Additional readings are necessary when weather changes, doors open, HVAC equipment cycles, temporary heat starts or stops, rain approaches, daylight changes, or work moves into another area.

Relative Humidity Can Affect Cure

High relative humidity does more than increase condensation risk. It can affect the cure, appearance, and recoat behavior of some coatings. Certain materials may develop blush, haze, surface film, slow cure, or other defects.

Very low humidity can also affect waterborne and moisture-cured products. Rapid evaporation may interfere with flow and film formation, while some moisture-cured materials require a defined humidity range.

Do not assume that every epoxy, urethane, polyaspartic, cementitious product, or moisture-mitigation membrane responds the same way. Follow the instructions for the specific material.

Temperature Affects the Material and Substrate

Cold material can become viscous and difficult to mix, spread, atomize, or roll. Cure may slow, and the coating may remain vulnerable to contamination or moisture for longer than expected.

High temperatures can shorten pot life, working time, induction time, and recoat intervals. The material may set too quickly to wet the concrete properly or release trapped air.

Product temperature, air temperature, and substrate temperature may all be different. Measure the conditions required by the manufacturer rather than assuming that one temperature represents all three.

Temporary Heating and Dehumidification

Temporary environmental controls can make coating work possible, but they must be designed and operated carefully.

Unvented fuel-fired heaters can introduce water vapor and combustion products into the work area. Rapid air heating may leave the concrete cold, increasing condensation risk. Direct airflow across wet material can change evaporation and cure.

Dehumidification can lower the moisture content of the air, but equipment capacity, room volume, air leakage, temperature, moisture load, and operating time all affect performance.

Environmental-Control Questions

• Is the heating equipment vented appropriately?
• Will the equipment add moisture or contamination?
• Is air distributed evenly throughout the work area?
• Will direct airflow disturb the coating?
• Has the concrete had enough time to change temperature?
• Will doors remain closed during application and cure?
• Is the dehumidifier sized for the enclosure and moisture load?
• Can the required conditions be maintained continuously?
• Are exhaust, electrical, and fire-safety requirements satisfied?

What to Do When Conditions Are Outside the Limits

Environmental Documentation

Environmental records protect the installation team and help explain later coating performance. Write down the actual readings rather than simply checking a box marked “acceptable.”

Record the Following

• Project name, location, date, and work area
• Time of every reading
• Air temperature
• Relative humidity
• Dew-point temperature
• Surface temperature at each representative location
• Calculated surface-to-dew-point spread
• Weather and exterior conditions
• HVAC, heat, and dehumidification status
• Doors, windows, and enclosure condition
• Instrument manufacturer, model, serial number, and calibration status
• Product being applied and its environmental limits
• Corrective action taken when conditions changed
• Name of the person taking the readings

Contractor Field Checklist

• Review environmental limits for every material in the system.
• Confirm instrument condition and calibration.
• Allow instruments to acclimate before recording readings.
• Measure air temperature and relative humidity.
• Determine the dew-point temperature.
• Measure the coldest and representative surface locations.
• Calculate and record the dew-point spread.
• Check conditions before preparation and before mixing material.
• Continue monitoring throughout application and cure.
• Retest whenever weather, HVAC, doors, or work locations change.
• Stop work when any required limit is exceeded.
• Document corrective action and the conditions before restarting.

Knowledge Check

Technical References

Use the editions required by the project specification and follow the current written instructions issued by the specified system manufacturer.

• ASTM D3276 - Standard Guide for Painting Inspectors, including guidance related to environmental conditions and coating application.
• ASTM E337 - Standard Test Method for Measuring Humidity with a Psychrometer.
• AMPP SSPC-Guide 12 - Guide for Illumination of Industrial Painting Projects, where visual evaluation conditions are applicable.
• AMPP guidance for monitoring and controlling ambient conditions during coating operations.
• U.S. Environmental Protection Agency - Moisture Control Guidance for Building Design, Construction and Maintenance.
• Current technical data sheets, application instructions, and safety data sheets issued by the specified primer, coating, adhesive, membrane, and flooring manufacturers.

These references provide technical guidance but do not replace the project specification, applicable regulations, manufacturer requirements, or evaluation by a qualified professional. Final environmental limits and application decisions must be based on current documents and actual site conditions.