I find it funny that some people are hating on this thread because good intentions didn't go as planned. This thread is titled "I found a secret room in my house" not "I'm building a grow room in the secret room I found in my house". If I where in his shoes wouldn't spend my time and money on building a second smaller grow room just because I found a room.
OP, the first thing I thought of when I saw that room was, DAMN that would make an awesome hotbox/smoking room. Water proof it, drape some fabric on the walls and ceiling, throw some bean bags in there, a shag rug lol, and maybe a small HTPC for music purposes (gotta have Pandora radio lol). You would have the best chill spot ever lol. GL with your future endeavors, stay high.
Edit: Meant to put Shag rug not sag lol.