I'm a bit confused because you keep referring to humidity in the root zone. Humidity implies water vapor, but I think you are referring to "moisture" which is the water content in the
growing medium (not vaporize, in the air). If I remember correctly your running NFT type system, but have a PVC tube with holes running through the NFT channel to spray water on the growing medium from below. The biggest issue with that is getting enough water pressure to spray the water out of enough holes for good even coverage. But a simple solution is to run a flexible line on top of the NFT channel, directly over the tops of the baskets. Then just poke some holes in the flexible tubing right over the baskets to drip the water directly down on the growing medium. That requires a LOT less water pressure, and making the holes exactly where you want them gives even coverage.
Another simple solution is to close off both ends of the NFT channel, run your fill line in the top end of the tube (like normal). But on the other end install a through hole (bulkhead fitting) in the bottom (for a overflow back to the reservoir). Then on the inside of the NFT channel insert a piece of tubing into the through hole (bulkhead fitting). Basically that temporarily converts the NFT system into a "flood and drain" system, and you can control the water height inside the NFT channel by the height of the piece of tubing you insert into the bulkhead fitting. When the roots reach far enough down, you can easily convert it back to a NFT system by taking out the pice of tubing from the bulkhead fitting, and letting the
pump run 24/7.
Other than getting water to the roots, it dosen't matter how long your roots are for a healthy plant. In fact roots are more likely to grow long when they are lacking water. They grow longer to search out more water sources. But when they have plenty of water, they don't need to search for moisture. Thus they just don't need to grow longer roots, and instead focus energy on growing foliage.