Here is where I am now. In the coldest months, the darkroom can go as low as 40F/5C. Right now it is about 50F/10C. A few days ago, I ran a small fan heater to get the temperature up to 68F/20C and I was able to maintain that temperature with an oil radiator. The fan from the small heater is probably not going to produce as much dust as duct from the furnace so I may try this for a while.
I would have to keep all of my liquid chemistry out of the darkroom but that is not such a huge problem: film developer, paper developer, fix, stop, and a few other things. I don't think it is good for the temperature of the chemistry to fluctuate so much. I have a lot of dry chemistry too since I mix my own developers but I don't think that it is affected as much as liquid chemistry. I also wonder about the effect on my equipment (Jobo, enlargers, etc.) of wide temperature fluctuation. For this reason, I am also consulting with a heating guy about running a small duct into the darkroom which would run all the time and thus keep a constant temperature in the darkroom. I already have a vent extraction point.
Insulating the darkroom is not the issue. The rest of the basement is just fine, heated by the furnace ducts. The darkroom simply doesn't have a source of heat so there is nothing for the insulation to keep in.
Andrew, just for the record, the average daytime temperature in Chicago in January is 29F/-2C. At night, the average is 14F/-10C. You are right: this is very cold. But the same data for Ottawa is 21F/6C down to 7F/-14C. The darkroom is not this cold (thankfully!) or I would truly freeze my arse off down there.
Thanks again for all the replies. This has been very helpful.