We are planning on keeping our small outdoor pool open until Jan 1in Cleveland.
We purchased is a Thermaltech ULTRA insulated cover from a company called Prairie Tubs. The cover seems well made and if fairly heavy, but not so heavy that it's difficult to take off. We just fold it up like a blanket. Cost was $430 for our 21x11 kidney shaped pool. You could probably use this over the entire surface of the water, both inside and out.
We're using the cover now and the temps have dipped into the mid-30s at night. We're only losing 4-5 degrees in the evening in those temps, which is easily recouped after running the heater for 3-4 hours. We're supposed to get temps in the 20s this week, so I'm keeping my fingers crossed that all will continue to run well.
Thanks, magicman. We already have a retracting cover, so I don't think the thermaltech Ultra will add enough insulation for year-round use (plus the cover would then no longer be functionally able to retract). With this cover we are only losing 3-4 degrees (degF) at night as well right now, but I'd prefer less!
I think we're going to end up with something like: