You'll definitely still want to think about adding at least a small carbon filter. Even though you won't be venting directly outside, it can still smell your entire house up really quick...especially during the last few weeks of flowering. My first grow didn't use anything in the way of odor reduction, and I seriously couldn't have invited anyone over to my house who wouldn't have been perfectly fine with the fact that there was fresh marijuana growing somewhere in the vicinity of my living room. (Actually, fifteen feet straight down in a crawl space.)

I've since switched grow locations, added a much needed ventilation system, and now don't have to worry about unexpected guests coming over. This helps me a great deal, as I have a family of my own, and relatives are prone to dropping by at a moments notice, generally without calling...

Good luck with your grow!:rastasmoke: