Ok, anything over 81 degrees can become a problem on the leaf temp... 84 degrees in the room is pretty high. I have heard that if running C02 in the right PPM level you are at the top of the margin however, its right on the borderline. I have a temp and humidity meter that also does the last 24 hrs so i take a glance at it and a few times the room became a little too warm and the temp/humidity device recorded it. Also, I noticed the plants were drooping and some of the leaves were noticeably affected.. It happened during my current grow before I went into flower... I would show you a pic but I just defoliated and lollipooped so those leaves were part of the stuff I just tossed out. I have a freonic portabe ac unit in the room to keep that from happening but I cant believe I had turned it off the night before and was about to drain it and forgot to push the power button and resume it back on. What a mistake... however it did only creep up to 84 degrees for part of one full day. and yes the plants were affected by the heat stress. Since then I have been diligent on room temps but still make mistakes. For instance I forgot to turn the lights back on for 12 hrs in one of my tents... arghhh so frustrating when I forget something so simple yet so important. Your room temp of 74 quite comfortable... Good luck with your grow...