Effects of Condensation and Dew Point on Sealer Performance

Clcik table to enlarge

Question:
How do humidity levels and dew points affect sealer application over architectural concrete?

Answer:
Humidity is the measure, in percent, of how much moisture is in the air. When humidity reaches 100%, it rains. Dew point is the temperature at which humidity in the air begins to condense, changing from a gas to a liquid that can settle on concrete and other solid surfaces. We also need to consider surface temperature when talking about humidity and dew point, especially when the air is warm but the concrete surface is cool. This is especially true during spring and fall, the seasons of temperature transition.

A great way to get a handle on the concepts of humidity, dew point, and condensation is to look at your bathroom mirror the next time you shower. Run a hot shower for 10 minutes, and the bathroom will fill up with steam (water as a gas) due to high humidity. When that hot steam hits a cold mirror, it condenses as a liquid (dew point) on the mirror’s surface.

This same process can happen with cold concrete, whether indoors or outdoors. But unlike a mirror, concrete acts like a sponge and the condensed water will soak in. If the surface has been sitting open to the air for hours or days, the concrete may look dry and show no outward signs of moisture. However, if applicators go ahead and apply a sealer, they will trap in all that moisture, causing the sealer to turn white or hazy.

The accompanying chart uses air temperature and relative humidity to determine the dew point of a concrete surface. Sealing or applying overlays is not recommended any time the concrete surface temperature is within 5 degrees F above the dew point at a given air temperature and humidity level.

When sealing exterior concrete surfaces, remember that humidity and condensation are most often higher in the morning. Consider applying the sealer in the late afternoon or evening. When sealing interior surfaces, make sure there is enough air movement through the room or building to allow the floor to give up its moisture. Damp or enclosed environments obviously will hold more moisture, and pose a bigger potential sealing issue. Bring the floor up to temperature slowly before sealing.