Indeed. Add some blue foam insulation board to the outside back and sides of the cages and you might see some better results. I had a 4x2 RBI cage that wouldn't really hold the heat either since I keep my home rather cool as well, but insulating it really helped. (Sold that cage after I decided to build a rack)

I used to keep my winter temps at 68...but made the "sacrifice" when I got snakes and raised that to 72. All my enclosures, from tubs to tanks, have insulation around them and I have an oil-filled space heater that runs near the enclosures during winter. It's one of the few things about keeping snakes that my (rather skinny) BF likes.

As for racks, you might be able to boost efficiency if you wrapped the outside of the rack with insulation and covered the front with a blanket or something similar at night. Just some off the top ideas.