Modern heat pumps can heat efficiently at -25 degrees Celsius or even lower. Not sure if they are available in Canada, but in Scandinavia (similar climate) they are pretty common.

I also agree that 66F (about 19 degrees Celsius) is not comfortable during day time. It is fine for sleeping temperature. During winter homes in heating dominated climates typically have higher indoor temperatures. One advantage of lower inside temperature is that relative humidity stays slightly higher when it is very cold outside.

People acclimatize pretty well if you let them. We keep our house at 65F all winter, and set the AC for 85F in the summer and everyone is pretty happy. The payback period on a good sweater is not very long.

“elderly people would be shivering constantly and highly susceptible to respiratory illness.”

At 66 degrees F? That sounds like put a sweater on if you’re chilly, not some near death extreme.

Any evidence that such an ‘extreme’ would cause issues?

I actually live on the same latitude as Ontario so -40F/C is not unusual. Add in windchill, and it gets even more common, given my windy location.

Yeah, I understand I'm probably an outlier at 66F. I was using the numbers more to point out how little a house temperature will drop with good windows and insulation.

>66F is ridiculously cold to me...I would find that very uncomfortable and elderly people would be shivering constantly and highly susceptible to respiratory illness.

I know people who live in the Mediterranean and get by with no heating during the winter with indoor and outdoor tempuratures this low or lower, so it seems that one can be conditioned into doing so.

Perhaps it's the presence of more sunlight on average rather than the temperature that makes the difference.