Imagine two buildings in New England. They are located next door to each other and have similar low-slope roofing systems. Contractor A applies a silicone coating to one. Contractor B applies an acrylic coating to the other. The two crews finish their work at the same time. The silicone coating cures far more quickly than the acrylic coating.
Change the setting to Arizona, and the exact opposite happens. The acrylic coating cures much more rapidly than the silicone coating.
Why? Humidity.
The impact that relative humidity has on the cure times of roof coatings confuses (and frustrates) many contractors, but understanding it is vital to delivering successful coatings projects. Rain will wash away freshly applied coating, and even a heavy dew can dilute it and compromise its performance. Applying a second coat before the first has sufficiently cured can result in adhesion problems and coating failure.
Understanding the relationship between humidity and cure times requires recognizing the different ways in which acrylic and silicone coatings cure, and thinking back to those middle or high school science lessons about relative humidity.
Silicone coatings are moisture-cure; the coating absorbs moisture from the atmosphere to fuel the chemical reaction that turns the coating into a protective film. Acrylic coatings work in the opposite way; they cure as moisture in the coating evaporates into the atmosphere.
So, in the humid coastal climate of New England, a silicone coating will feast on the moisture-laden atmosphere and cure quickly. An acrylic coating, however, will have trouble shedding its moisture, resulting in a much longer cure time.
In the desert of Arizona, the opposite is true. There is virtually no atmospheric moisture for a silicone coating to pull in, inhibiting the curing process. On the other hand, the parched air sucks up the moisture in an acrylic coating at warp speed, allowing it to cure rapidly.
When working with acrylic coatings, contractors also must understand how atmospheric moisture moves at different air temperatures. Water evaporates more quickly in warm temperatures and more slowly in cool temperatures, so cure times will generally be shorter during the summer months and longer in the spring and fall.
And when the air temperature is within five degrees of the dew point (the temperature below which water droplets in the air begin to condense and dew can form), contractors should monitor the situation carefully and be prepared to halt work. It can be tempting to push ahead; contractors are on a schedule and need to finish a job so they and the crew can be paid and everyone can move on to the next job.
But Mother Nature will do what Mother Nature does, and pushing the limits she sets can prove costly. If the temperature dips below the dew point and condensation forms on the newly applied coating before it has sufficiently cured, it may be compromised and the time, effort and materials spent will be wasted. If the weather is sketchy, it may be wiser to go home and wait for more favorable conditions to wrap up the job.
Monitoring the weather is second nature to roofing contractors. When applying coatings, humidity and dew point should join temperature, rain, snow and hail on the watch list.