from Greenhouse Gardener's Companion
Another way to cool down your greenhouse is to use the cooling effect of evaporation. This is the same cooling effect that we feel when wind blows across sweaty skin. As water evaporates, it cools the air around it. Unfortunately, if you live in a humid part of the world, the air is already wet and evaporation just doesn't work very well, thus evaporative cooling is not as effective. But in many places, you can cool down a greenhouse considerably by just watering down the floors. The process can be as simple as air moving across a wet greenhouse floor; the water evaporates and thus causes a cooling effect.
A fine mist near an air-intake vent is also a way to increase evaporation (and cooling) in a greenhouse. Misting nozzles are commonly used in propogation areas and are available in greenhouse-supply catalogs. The downside to many misting nozzles is that the droplets of water are relatively large, and you end up with pools of water nearby, which can be messy. There is one misting system called Mist & Cool that seems to create smaller droplets than most mist systems because of a pressurized line. The manufacturer claims to be able to provide cooling of 15 degrees F to 25 degrees F (around 8 degrees C to 12 degrees C).