Originally Posted by Nina

Billy said:

Sure they do, usually it is a water adsorbent pad (can be made of Sphagnum
moss) that is placed in one end wall of the green house. Water is dripped over
the pad, collected at the bottom and reused. The vent fan at the other end
pulls air through the pad.

My understanding of those things is that they serve to cool
greenhouses, not humidify them. As the water passes over a wall of
sphagnum or rock wool, it evaporates, increasing humidity, but more
importantly, cooling the air without resort to an expensive air
conditioner. Almost no outdoor greenhouse (except in arid climates)
needs increased humidity very badly! A cold frame, on the contrary,
needs good air circulation, otherwise in early spring it can get pretty
moldy in there.

Nina