Just thought I'd share what I thought was a pretty surprising success. I decided to brew a 10gal Oktoberfest on Tuesday of this week (first time I did a single decoction, btw), even though it's summer and I don't have a temp-controlled fermentation chamber. I have been able to maintain low-50s with a simple water-filled bin w/ ice addition setup. I just add large ice blocks morning, early afternoon (SWMBO is a saint), and before going to bed. Setup is in the basement, which does not get above 70*F. I think I could probably get away with just morning and night additions if I brought the water down to upper 40s in the morning.
Anyway, I thought that this probably wasn't doable, but gave it a shot. FWIW, with a little effort you can get ferm temp 20 degrees below ambient air temp if you want. Not sure if it would be possible with most ale yeasts because of the greater heat they generate, but you wouldn't need to get them down 20 degrees unless you live in a really warm home.