HCA: Hermit Crab Association

It's been colder here recently (highs in the mid-60s and lows in the low 30s), and this has caused some trouble with my crabitat.

Our family chooses to keep the heat off upstairs during the winter. During the coldest parts of January, it can get down to the upper 50s at night. It's actually quite cozy under all the blankets, and since we don't spend a lot of time upstairs except for sleeping, it works out great.
My crabs are located in my bedroom, and I don't forsee moving them anytime soon except as a last resort.
My current heating setup is a 6x23 Ultratherm with a thermostat (set to 83) on the back of my tank (20 gal tall with 6 in of substrate). There is insulation directly over the heat mat (foil and whatnot), and I also have a small towel taped so it hangs down on the back and side but doesn't cover the top.

Yesterday morning, I woke up to their thermometer reading 68. In a half-awake panic, I grabbed several towels and blankets and covered the tank completely. It did get back up to 79, so I removed all but one because the humidity was spiking (99 and foggy) under so many layers. When I went to bed, it was back down to 74-ish, so I added a ridiculous amount of blankets (about 5) and also stuck a large pillow between the tank and the wall. This morning, it was still only about 75 (which I know is in the 'safe' range, but still not ideal.)

I am considering getting another heat mat or two for the sides, but would really rather wait for Black Friday or possibly Christmas.

Is there anything I can do? Are nightly temp drops okay? Is having blankets/towels on the tank going to mess with their biorhythms since I don't have a tank light? I know this problem is only going to get worse, not better.

You said you have insulation, "foil and whatnot" but what does that look like? My suggestion, if not done so already, is to get some foam board insulation. A sheet of it is relatively inexpensive since you don't need much. A 1 in thick, 4ft by 8ft sheet is $22 at my home depot. Just cut it to length to cover the back of your tank and any sides / top. If that isn't enough to raise your tank, double it up. You will have plenty left to just put another sheet ontop of the other. You should be able to leave the front of your tank uncovered to let light in.
If it comes down to it, a small light for your tank would be pretty cheap and you can cut the foam to fit around your light, assuming you are placing it right on the lid.

I have cardboard wrapped in aluminum foil, with it covering my uth on the back, the sides, and the lid of my tank.Very helpful. Front of the tank drops to 75* when the room is in the 60's. I've heard of people doing a double layer, too.

I believe the lowest we should have it is 72*F, so getting it bumped up a bit to that minimum is important. Hope some of that helps

Thanks, guys!
I may try repositioning my heating pad--it's a bit lower than I intended. And redoing the insulation wouldn't hurt either.

So I keep my bedroom in the 50s and 60s year round, it's a converted sun porch so it gets chilly naturally in winter. I have a 29 gallon tank and two pads, a 29x11 and a 6x23. The entire back is covered! And then I also insulated over those and the screen lid with foil wrapped cardboard. It's always toasty, and double layers of insulation actually made it too warm. I would suggest trying the pad without the thermostat first, just in case it's not working correctly, and if it's not warming up enough then for sure go with a larger or second UTH. You can heat below the substrate line safely, but it may cause humidity spikes at first and then dry areas after as the moisture evaporates. I just have to add in a bottle of water every few weeks in the back when it gets too dry.