Hi Stephen
Air temps don't correlate directly with water temps in a space. Ambient heat, whether in air, wood, stone or water has momentum (not literally, but it's a way to think about it). It takes time for it to change, whether up or down and thus, for example, 12 hours where the air temps are 20F. might lower a 40gal tank of water from 45F. to 35F. If, during night, the tank lost 10F. and ended up at 35F when the sun came up and then heated 12F. so it was 47F by sundown... That would make a sustainable system for the tank.
The above doesn't speak to individual pipes where the volume of water is much less and thus more subject to cooling quicker. But, it does present an important way to think about sustaining temps. And it may indicate that the tank is not the problem and you can concentrate your thought on the pipes alone. There are just broad strokes of thought but they help prioritize and point out what's important and what's not. The _average_ temperature is very useful information.
Air leaks and most especially convective currents of air which carry the heat up and down (the pipes are "down", right?) are a _major_ problem when trying to maintain temps w/out heating. Major. So sealing all your air leaks would carry a huge ROI (ReturnOnInvestment) or BangForBuck. The ideas above about creating a tiny sealed space about the vulnerable pipes is spot on, too. First, stop that air from moving around those pipes - iow, seal your tiny pipe enclosure (blankets, tarp, plastic sheeting...) tight to the walls. Gorilla tape comes to mind, but gravity using boards or such to capture the edges of your membrane against something would work well also.
But if you had more exact information you would be able to think about this more productively. Do you have an IR laser thermometer? They are relatively cheap, less than $100 and often much less. For your purposes, accuracy isn't critical - "close" is good enough. What are the temps of the walls? Floor? Ceiling (bottom of the roof)? When you "step in" first thing after opening it up, check immediately, starting with the ceiling (that's the one that will change fastest). If they are all above 30F, you may be fine and IAC, you're way better than if they're 20-25F. If they're low, as in outside air temp... They shouldn't' be. As was mentioned above, soil temps are normally well above air temps in winter, especially deeper below the surface. If the floor temp and lower wall temp is 25F, there is probably really major air leakage through that space. And _that_ tells you where real effort is needed and will pay back quickly.
Eventually, a drain back solar "water" system may provide heat using a very small pump to move the fluid. A heat pump. But those require careful planning and some technical skills and so are not an immediate fix.
Best luck
Rufus
