Since you described my challenges I have to say; fighting the heat and humidity problems was almost hopeless. I have been learning to grow for two years now. Been trying to pull this off on a shoestring budget. It hasn't been until this attempt I finally got my cool tube and now have risen the humidity from 16% to 55%. I had a exhaust fan sucking out the hot air. This remedied the heat problem but at the same time it was taking out the co2 and humidity. The cool tube ended that problem within an hour. I have stabilized the temperature to recommended levels. I live in a similar climate, cold winters. So a small space heater works well for lights out. I have a 2x5 area with four healthy and full plants. I'm running out of room. Should I be concerned. I am looking to find some info on pruning. Whether its necessary or if it will do more harm then good. There are huge different schools of thought here. I have some of the conditions you mentioned and on disability. My goal is to grow my own, because it's so expensive to purchase. I am at a real impasse, I need to get a constant flow going and have some successful harvest. I can't keep taking these hits by the cost of growing and not having a worthwhile yield in the end.
Alright, we have a few things to look at here.
First, you only want humidity during the vegetation stage and not during flowering. Keep your humidity around 50% during your veg and I think around 25% during flower (anyone can chime in here). Too much humidity during flowering will cause mold and bud rot. If you're having a problem keeping it humid during veg, go to your local GoodWill and get a cheap humidifier for a few bucks. I always liked using the ultrasonic humidifiers because they run cool and I could usually drop my temps 5 or 10 degrees.
If you have heat issues and you live in a cold state, try reversing your grow schedule so that your lights are on mostly during the cooler parts of each day (meaning overnight) and off during the warmer parts during the day. This will not only prevent you from wasting electricity with a heater, but you can exhaust the warm air into your house making it warmer.
If you're on a shoestring budget, I wouldn't worry about co2. There's plenty in the air and co2 is too much of a challenge and too expensive to do it properly when there are other things to worry about. You really need to have two inline fans (and not the inline duct fans from home depot) to do this correctly. You need to have one primarily for your hood that totally isolates the heat caused by the lamp from your grow area and draws the air in from a separate intake outside the grow, and then you need to have another that runs on a 15 minute timer to exhaust and refresh the air in your grow space. If you can draw the air in from underneath your house then that will help keep your hood cooler and ultimately your grow space cooler.
Definitely prune, but do it correctly. You can read all about pruning in these forums. Pruning will do many positive things like get light to lower leaves, allow better air flow, avoid mold and fungus issues from leaves sitting on top of one another, and get your plants to put more energy into the more important upper parts of your plants where your light will have the most impact. You'll do more pruning at the bottom and ultimately get your plants to look like an upside down Christmas tree. Don't get super-stoned and prune, take your time, don't be over-zealous, and use clean pruning shears to avoid disease. You're not going to hurt your plants if you do it right.
If you're running out of vertical space, look at how to do SOG (sea of green), SCROG (screen of green), LIT (low-impact training), and topping. All of these methods will give you more in the end. You won't have one big cola at the top, instead you'll have a whole bunch of good sized buds at the same distance from the light, creating a canopy. Last thing you want is to have a cola touching your hood because it grew too tall.
Hope that helps. Everything I learned I learned on these forums. Look at what other people are doing and read about how others have solved issues of their own. You'll get some great ideas.
Just thinking aloud. Thanks for the help Jiji