In getting back into brewing after a few years off, I'm intending to improve on several aspects of my approach, one being to better control fermentation temperatures. I realize there are several approaches and I used to use a heating mat over the carboy I was fermenting in. I also used a large insulated bag with various amounts of water frozen in plastic jugs when I made lagers. For adding heat, I have no problem with what I've been doing, but I do have a question -- how do you determine the temperature of fermentation? I used a thermo sensor housed in a chunk of styrofoam taped tightly to the outside wall of the carboy, usually plastic, but sometimes glass. I'm not so sure this is a good approach, but I didn't have a sensor to place in the fermenting beer and didn't want one more potential source of contamination.
As for controlling temperature to keep it from getting too warm, I know people often use a kegerator, whether a converted fridge/freezer or a fridge for storing and serving keg beer. I still have the same question regarding the temp probe, but I also wonder if there could be some approach better than an insulated bag (and I do hook up the heating mat in tandem with a "thermostat" of sorts to ensure the fermentation doesn't get too cool) but not as pricey and bulky as a fridge/freezer with a temp regulator. Also, if you were to ferment in a room or in a kegerator at, say 55 degrees, does the fermentation temperature remain at 55 degrees or can the yeast still generate enough heat to, at least for some period of time, keep the fermentor contents at a higher temp? That is, can you really control fermentation temperature if you've got some aggressive yeast strain in a high SG wort?
I'm focused on fermentation temp since it seems like keeping the temps controlled is critical even for some Belgian beers -- despite that some brewers just "let the yeast go", and temps can get up to 90. So maybe the last question is whether controlling fermentation temps, other than obvious cases with lager strains, is as important as I'm tending to believe?