Welcome, T! If your Dendrobium is a nobile type it should be fairly easy to grow in a windowsill. I have a number of species and hybrids in the same section as nobile that I grow in windowsills without much difficulty. Humidity shouldn't be an issue with these unless it is exceptionally low. The biggest challenge you may have will be providing enough light; it'll definitely like a good bit more than your Phals and Oncidium intergenerics. Here's a link to a website that has good culture information for this type of Dendrobium:
Yamamoto Dendrobiums Hawaii
Hope this helps, and have fun!
--Nat