Env. Controllers, at their very simplest, can be as simple as a thermostat that turns on an exhaust fan when temperature hits a particular setpoint. Most folks find that they outgrow that sort of rig pretty quickly.

Let's think about the temperature and humidity cycle of a room over a 24 hour lighting cycle. We'll assume that you're flowering at 12/12, and that the lights come on at midnight, which just makes the math easier. We'll use 24 hour military time.

At 0:00, your light(s) kick on and the room begins to warm. (If you just have a fan running on the same timer as your light, you may find that the coldest moment of the day is actually right after the light goes on, as the fan is sucking cold outside air into a room that's not hot yet).

By 1:00, the room is hopefully pretty warm, ideally at 70-75F @ an RH of 40-60%. Lovely.
If your fan is perfectly sized, it might even be kinda stable. More likely though, a slightly oversized fan is causing the room to run cool, or a slightly undersized fan is causing the temperature to slowly rise as the ?? day? goes on. The warmest point in the ??day? will probabley come near the end of the ??day?, around 12:00, when the light has been running for a long time and the outside temperature is going up.

Now we're in the dark part of the cycle, and temps are going down. Maybe.
Another scenario is that the fan has turned off, but the bulb and reflector are still hot. As the room's no longer being cooled, it's common to get a temperature spike before temps start going down.

As warmer air will hold more water vapor than cooler air per cubic foot, this temperature spike will cause a water uptake into the room's air- which is fine, until the room starts cooling down. As the room cools, the relative humidity goes up- and if it get's high enough, you can get dew.

The combination of high humidity and lower temperature will often cause mold problems.

So now it looks like we need to control both temperature and humidity, and we need to control them seperately from the lighting cycle.

The easiest, and pretty much cheapest, way to do this is with a purpose built environmental controller, like the CAP Air-1 or the Greenair CT-DH1. These units contain a thermostat and a humidistat that are each wired to apply power to an outlet when either temperature or humidity exceed preset conditions. They're designed to run an exhaust fan.

When you look at purchasing these, you'll be shocked at the cost- I was. Being a bit of a yahoo about making my own controllers ( I once rebuilt a Holley choke plate so that it could be controlled by a bicycle brake cable from inside the cab, as I was too cheap to replace the automatic choke element), I decided to build my own. A quick visit to the local electrical supply left me about $140 lighter in parts (humistat elements are wicked pricey), and then I spent a day figuring out how to paralell wire the two units. I should've just gone the $150-$175 for the pre-built, and I have ever since.

These units are the bare basics, and it's worth looking in the $200 range for feature sets that you think you'll be able to use. For instance, if you think that you're ever going to use C02, you'll want a unit that has an out let that gets switched off when the fan outlet gets switched on. You can plug your c02 timer or ppm controller into this outlet and be assured that you aren't releasing expensive gas when your fan is running. If you're using a cycle timer to control the gas, it'll reset to beginning of cycle when the fan goes off. Neat, huh?

I really love the Sentinal stuff- look at their EVC-1 at a street price of $200- very nice unit.

Anyway, let's go through the cycle again, but this time with a basic controller in place.

At 12:00, the light kicks on. The fan is off, so the room warms quickly to operating temperature. When it gets to the top end of operational range, the fan kicks on and cools the room to the low end of operational range. ( Thermostats use a built in hysterisis, or swing- the fan will come on a few degrees above setpoint, and go off a few degrees below- like your home heating thermostat in revere.)

The room continues this slight swing until the lights go off. If the room is warmer than setpoint, the room is cooled until it hits the low end of the range- then the fan goes off. The room will continue to cool until you hit the high end of your range for humidity- then the fan kicks on and flushes the damp air from the room. No dew- less chance of mold.

When you get into fancier controllers, they'll often have a ??night time on? outlet- a built in photocell controls an out let that's only powered when the lights are off. Perfect place for a heater, if you need to use one.

If you have seperate temperature and humidity control outlets, you may choose to dehumidify with a dehumidifier rather than a fan, which doesn't cool the room. This makes it very easy to control low ??lights off temps?, or to keep inside humidity at appropriate levels when the outside humidity is very high. ( Think Alabama in August).

Your controller can also be used to run an AC instead of a fan- just watch out that your plants are getting enough fresh air or enough supplemental c02. Also, make sure that your AC isn't blowing unfiltered (stanky) air out into the street- most units can be set up so that they don't actually purge air, as it's more efficient not too.

An environmental controller is one of those investments where you get more from the third hundred that you spend than you do for the first two, and they scale up to larger spaces very well- think through your purchase, and buy all that you think you'll need for at least a couple of years. Going cheap and then buying better is often more expensive than just buying best to begin with.

When you mount the controller, make sure that it's exposed to good airflow- I like to actually have a fan blow over it. You want air from plant level, and blown over the canopy if possible. Mount it someplace where you can see it well and you won't jostle the knobs.

If you get a humistat wet, it's useless- either dead or so far out of calibration that it's effectively dead. Up out of the splash zone, as well as all associated power cords.