A couple of nights this winter my heat pump* couldn't keep up with the low temperature outside and the house temp dropped below 65*F. I put a wool blanket over the racks on those nights, which did the trick nicely at keeping the ambient temp in the tubs warm enough. I removed the blanket as soon as the house warmed up during the day.
*Tip: don't get a heat pump if your outside temps can regularly drop below 20*F.